research paper on fruits pdf

Fruits, vegetables, and health: A comprehensive narrative, umbrella review of the science and recommendations for enhanced public policy to improve intake

Affiliations.

• 1 Department of Nutrition and Food Studies, George Mason University, Fairfax, Virginia, USA.
• 2 Think Healthy Group, Inc., Washington, DC, USA.
• 3 Department of Nutrition Science, Purdue University, West Lafayette, Indiana, USA.
• 4 Friedman School of Nutrition Science and Policy, Tufts University, Boston, Massachusetts, USA.
• 5 Center for Nutrition Research, Institute for Food Safety and Health, Illinois Institute of Technology, Bedford Park, Illinois, USA.
• 6 Biofortis Research, Merieux NutriSciences, Addison, Illinois, USA.
• 7 Department of Human Nutrition, University of Alabama, Tuscaloosa, Alabama, USA.
• 8 Department of Epidemiology, University of Washington, Seattle, Washington, USA.
• 9 School of Exercise and Nutritional Sciences, San Diego State University, San Diego, California, USA.
• 10 Bone and Body Composition Laboratory, College of Family and Consumer Sciences, University of Georgia, Athens, Georgia, USA.
• 11 College of Education and Human Ecology, The Ohio State University, Columbus, Ohio, USA.
• 12 Department of Nutritional Sciences, Rutgers University, New Brunswick, New Jersey, USA.
• 13 D&V Systematic Evidence Review, Bronx, New York, USA.
• PMID: 31267783
• DOI: 10.1080/10408398.2019.1632258

Fruit and vegetables (F&V) have been a cornerstone of healthy dietary recommendations; the 2015-2020 U.S. Dietary Guidelines for Americans recommend that F&V constitute one-half of the plate at each meal. F&V include a diverse collection of plant foods that vary in their energy, nutrient, and dietary bioactive contents. F&V have potential health-promoting effects beyond providing basic nutrition needs in humans, including their role in reducing inflammation and their potential preventive effects on various chronic disease states leading to decreases in years lost due to premature mortality and years lived with disability/morbidity. Current global intakes of F&V are well below recommendations. Given the importance of F&V for health, public policies that promote dietary interventions to help increase F&V intake are warranted. This externally commissioned expert comprehensive narrative, umbrella review summarizes up-to-date clinical and observational evidence on current intakes of F&V, discusses the available evidence on the potential health benefits of F&V, and offers implementation strategies to help ensure that public health messaging is reflective of current science. This review demonstrates that F&V provide benefits beyond helping to achieve basic nutrient requirements in humans. The scientific evidence for providing public health recommendations to increase F&V consumption for prevention of disease is strong. Current evidence suggests that F&V have the strongest effects in relation to prevention of CVDs, noting a nonlinear threshold effect of 800 g per day (i.e., about 5 servings a day). A growing body of clinical evidence (mostly small RCTs) demonstrates effects of specific F&V on certain chronic disease states; however, more research on the role of individual F&V for specific disease prevention strategies is still needed in many areas. Data from the systematic reviews and mostly observational studies cited in this report also support intake of certain types of F&V, particularly cruciferous vegetables, dark-green leafy vegetables, citrus fruits, and dark-colored berries, which have superior effects on biomarkers, surrogate endpoints, and outcomes of chronic disease.

Keywords: Fruit; health; nutrition; produce; vegetable.

Publication types

• Diet, Healthy*
• Nutrition Policy*
• Observational Studies as Topic
• Systematic Reviews as Topic
• United States
• Vegetables*

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Nutritional and Healthy Benefits of Fruits

2021, Biomed J Sci & Tech Res

Fruits, nuts, dried fruits, and fruit juice are important components for a balanced diet; it is an essential part of the staple diet. Due to their benefits for a healthy body and preventive effect of numerous diseases. Fruits contain different phytochemicals, dietary fiber, protein, carbohydrates, vitamins, carotenoids, flavonoids, and other components. Furthermore, nuts and dry fruits considered a precious source of energy, antioxidants components, higher vitamins content, a source of numerous minerals. Daily fruits consume particularly, Date fruits, citrus fruit, pomegranate, and Cranberries, provide several advantages for the body and different metabolic processes in the human body. It plays an important role in the management of diabetes mellitus, delaying infection with chronic diseases, enhancing the performance of different systems and physiological processing, reducing harmful effects on human health, in addition, it provides nutritional ingredients such as vitamins, dietary fiber, carbohydrates, minerals …etc. Higher content of fruits particularly antioxidants plays an important role in altering the metabolic processing and detoxification of different diseases particularly carcinogens and reducing the growth of tumor cells. In addition to the effects as potent antioxidants and anti-inflammatory agents, which reduce risks of the pathogenesis of cardiovascular disease by counteracting oxidative and inflammation hazards. Furthermore, there are important roles for bioactive components contents of fruits in keeping heart-healthy, preventing cardio-metabolic and non-communicable diseases through different mechanisms.

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In recent years a number of nutritional studies have been devoted to examining specific foods for their putative healthy protective role and disease-preventing potential. Different epidemiological studies have consistently shown that there is a positive association between the intake of fruits alone or in combination with vegetables with a reduced rate of heart disease mortality, and between some common tumors and other chronic diseases such as obesity and diabetes as well as, risk of eye diseases. Fruits may reduce blood lipids and when included in hypocaloric diets may help to lose weight. Recently, fruits have attracted a great deal of attention focusing on their role in some oxidative stress related diseases. This interest is attributed to the fact that these foods may provide an optimal content of phytochemicals such as natural antioxidants, vitamins, minerals, polyphenols and other compounds with healthy properties. Moreover, typical fruit components like fructose and fiber have been suggested to produce specific effects on oxidative stress. In addition, the effects of fruit on weight loss and lipid profile when included in energy restricted diets could be triggered through the antioxidant properties of fruit.

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Impact of Processing Factors on Quality of Frozen Vegetables and Fruits

• Open access
• Published: 27 May 2020
• Volume 12 , pages 399–420, ( 2020 )

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• R. G. M. van der Sman 1  

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In this paper I review the production of frozen vegetables and fruits from a chain perspective. I argue that the final quality of the frozen product still can be improved via (a) optimization of the complete existing production chain towards quality, and/or (b) introduction of some promising novel processing technology. For this optimization, knowledge is required how all processing steps impact the final quality. Hence, first I review physicochemical and biochemical processes underlying the final quality, such as water holding capacity, ice crystal growth and mechanical damage. Subsequently, I review how each individual processing step impacts the final quality via these fundamental physicochemical and biochemical processes. In this review of processing steps, I also review the potential of novel processing technologies. The results of our literature review are summarized via a causal network, linking processing steps, fundamental physicochemical and biochemical processes, and their correlation with final product quality. I conclude that there is room for optimization of the current production chains via matching processing times with time scales of the fundamental physicochemical and biochemical processes. Regarding novel processing technology, it is concluded in general that they are difficult to implement in the context of existing production chains. I do see the potential for novel processing technology combined with process intensification, incorporating the blanching pretreatment—but which involves quite a change of the production chain.

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Introduction

Freezing is a widely used long-term preservation method for foods, where they retain attributes associated with freshness much better than other conventional preservation methods like canning and drying [ 1 ]. But especially the texture of cellular foods, like meat, vegetables and fruits, can be strongly impacted by the freezing process. Above all, vegetables often require blanching prior to freezing to prevent enzymatic browning. Blanching already impacts the vegetable texture. Fruits are seldom blanched, but due to their relative softness they are more severely impacted by freezing than vegetables. Via research projects with industry I have learned that significant improvements to the textural quality of fruits and vegetables can still be made. This is also indicated by the wealth of recent studies on the application of novel processing to improve the freezing process, like ultrasound and high-pressure freezing. There are several review papers on these novel technologies [ 2 , 3 , 4 , 5 , 6 ], but most of these papers focus on freezing as a single unit operation, without considering it as part of the complete frozen food production chain. An exception is the review paper, where novel technological developments in both blanching and freezing unit operations on vegetables are discussed [ 7 ].

Via this review I like to show that improvement of frozen vegetables and fruits can be obtained via (1) optimization of the total production chain of frozen foods and (2) introduction of some promising novel processing into the production chain, combined with process intensification. Literature shows there is very little study towards the optimization of the production chain. Only, a limited number of studies investigate the interaction between blanching and freezing steps [ 7 , 8 ]. The usefulness of novel technology is often evaluated on the basis of the reduction of energy usage or food quality. As food losses and waste have much more impact on sustainability than energy usage [ 9 ], I will focus on the improvement of food quality, and in particular to the textural quality. Still, there is not sufficient knowledge on how freezing impacts on the matrix of tissue-based food materials like vegetables, fruits, and meat, via the freeze concentration and the mechanical stress imparted by the growing ice crystals [ 10 ]. For example, one of the major impacts of freezing on vegetables and fruits is the drip loss occurring after thawing. This drip loss occurs due to changes in water holding capacity of the food imparted by freezing. These changes and their physical causes are hardly discussed in the literature, with a few early exceptions discussing freezing of starch-rich foods [ 11 ], and meat [ 12 , 13 , 14 , 15 ].

Hence, before discussing effects of the different unit operations on the final quality of the frozen vegetables and fruits, I discuss the physics and (bio)chemistry of water holding and ice formation in plant tissue, using insights from the fields of cryopreservation of tissue [ 16 , 17 , 18 , 19 ], and freezing tolerance in plants [ 20 , 21 , 22 , 23 , 24 ]. As freezing and thawing imparts dehydration and subsequent rehydration of the tissue, one can expect many similarities with drying and rehydration of plant tissue [ 25 , 26 , 27 ].

Subsequently, I discuss the production chain of both vegetables and fruits, and the novel technologies, which can be applied in these chains. I have evaluated how each unit operation in the processing chain impacts physicochemical factors of relevance to the final quality of frozen fruits and vegetables. This evaluation is summarized in a so-called causal network linking processing factors via physicochemical factors to final product quality factors. Finally, I discuss modifications of the production chain for improved final product quality.

Physics and Chemistry During Freezing of Plant Tissue

Here, we discuss the physics and (bio)chemistry of water holding and ice formation in plant tissue. For the latter insights will also be obtained from the field of cryopreservation of tissue [ 16 , 17 , 18 , 19 ], and freezing tolerance in plants [ 20 , 21 , 22 , 23 , 24 ]. As freezing and thawing impart dehydration and subsequent rehydration of the tissue, one can expect many similarities with drying and rehydration of plant tissue [ 25 , 26 , 27 ].

Steps in the Freezing Process

Supercooling and nucleation.

Freezing can only happen if the food material is below freezing point, i.e., it has to be supercooled. The freezing does not happen immediately, because first ice crystals have to be nucleated. A cluster of liquid water molecules has to adapt to the ice crystal structure, and only a cluster of sufficient size can outgrow to an ice crystal. A cluster smaller than the so-called critical nucleus size will remelt again. The nucleation process is a balance between two opposing energy contributions: (a) the energy gain via the formation of the ice crystal structure and (b) the energy cost of forming an interface between the nucleated ice crystal and the unfrozen liquid phase. Only at the so-called critical nucleus size the energy gain is larger than the energy cost of forming the interface. It is said that the nucleation process has to overcome the energy barrier imposed by the surface energy. Via classical nucleation theory [ 28 ] one can calculate the critical nucleus size and the change of forming a nucleus, that is larger than the critical size and will outgrow. The change of nucleation depends largely on the amount of supercooling, the difference between freezing point and the actual temperature of the unfrozen phase. For pure water without any impurities, nucleation will happen only at T = − 40 ∘ C. If foreign material is present in the solution, having asperities or hydrophobic properties, ice can nucleate at much smaller degrees of supercooling due to the lowering of the energy cost of forming an interface.

It was commonly assumed that food is full of foreign material, that ice will form quite instantly, already at several degrees of supercooling, as stated in the Recommendation of the International Institute of Refrigeration [ 29 ]. However, scientific literature shows that significant supercooling occasionally can occur. Some vegetables like fresh garlic and shallots show a high capacity for supercooling [ 29 , 30 ]. Few studies have reported a moderate supercooling of other vegetables like broccoli, cauliflower [ 30 ], strawberries [ 31 ], tomatoes [ 32 ], and potato slices [ 33 ]. Also in gel-like food products supercooling is observed like soy tofu [ 34 ], agar gels [ 35 ], tylose [ 36 ], agar/maltodextrin gels [ 37 ] and rice gels [ 38 ]. The amount of supercooling will determine the nucleation rate (the number of ice crystals formed per unit of time). Supercooling is well observed in recordings of product temperature at low heat transfer rates, such that the heat release during nucleation and further ice crystal growth cannot be cooled away fast enough, leading to temperature rise after nucleation [ 38 ].

The homogeneous nucleation temperature of pure water is 40 K lower than its freezing point. Surprisingly, in aerosols with solutes added the difference between freezing point and nucleation temperature remains 40 K [ 39 ]. The nucleation temperature is independent on the type of solute, but only on the water activity, similar to the freezing point (as follows from Clausius-Clapeyron [ 40 , 41 ]). It is also assumed that the nucleation rate is a function of the water activity.

Heterogeneous nucleation can also occur in aerosols, if organic solutes are present in the crystalline state immersed in the aerosol for example [ 42 ]. Dicarboxylic acids are such solutes which are non-volatile, and have low solubility. Also biopolymers, especially proteins, are thought to promote heterogeneous nucleation in food [ 43 , 44 , 45 , 46 ]. Also, for heterogeneous nucleation it holds that the nucleation temperature depends only on water activity. Consequently, there is a fixed difference between equilibrium freezing point and nucleation temperature [ 42 , 46 ]. The heterogeneous nucleation temperature is always higher than the homogeneous nucleation temperature. Similar effects are observed during spray freezing of coffee [ 47 ].

The heterogeneous nucleation temperature does depend on the type of nucleation site (organic/inorganic crystal or biopolymer), and the (partial) wetting of the nucleation site. Importance of the interface on nucleation is shown in a recent study [ 48 ]. If air/water interface is sealed by oil, the nucleation of ice at the interface is highly suppressed. Even, the nucleation cannot be initiated via mechanical shocks or ultrasound. Also, nucleation can be suppressed by alcohols, which form an intricate structure at the interface due to their amphipathic nature.

In the fields of cryopreservation of tissues [ 19 ] and freeze-drying of pharmaceuticals [ 49 ] the phenomenon of supercooling is much more acknowledged and observed. In these fields good control of ice morphology is demanded. Hence, there are serious research activities towards methods of controlling the ice nucleation event, which are based either on formulation or on process conditions. For the control of nucleation via formulation one can obtain much inspiration from organisms showing high cold tolerance [ 50 ]. Furthermore, food science can learn from this field how supercooling and ice nucleation depend on cell volume [ 51 ]. Nucleation is expected to occur earlier for tissues with smaller cells, due to the higher amount of cell surface (wall) area. Furthermore, at slow to moderate freezing rate ice nucleation is expected to happen in the extracellular space [ 52 ] (Fig.  1 ).

Cellular structure of vegetables and fruits

Crystal Growth

As the supercooled state, where ice crystals are nucleated, is not at equilibrium, ice crystals will grow to restore equilibrium. Solutes or biopolymers present in the unfrozen phase are excluded from the growing ice crystals. Hence, the growing ice crystals push the solute and polymers forward in front of the growing ice crystals, leading to gradients of solutes or density in the biopolymeric matrix. For growth of the ice crystals the liquid water has to diffuse through these concentrated boundary layers of solute or biopolymer [ 53 , 54 ].

As stated above, the formation of ice releases energy (latent heat), and consequently the (local) temperature will increase and approach the equilibrium freezing point. This release of latent heat after nucleation is called recalescence [ 55 ]. If the freezing rate is sufficiently slow, the average temperature and solute/biopolymer concentration will follow closely the freezing line until the temperature of the cooling medium is reached. Food is commonly frozen at temperatures around − 18 ∘ C, which is the common storage temperature for frozen food. The changes in the state of the unfrozen phase during the freezing operation is illustrated in Fig.  2 . Here we have used the phase diagram of sucrose, which is a good model system for fruits rich in sugar. Observe that initially the temperature drops below the freezing line, where the unfrozen liquid enters the supercooled state. At a sufficient degree of supercooling ice crystals are nucleated (event 1), and the temperature will rise due to recalescence. Simultaneously, the mass fraction of water in the unfrozen phase decreases due to the freeze concentration effect. After a brief moment, the state of the food will commonly follow the equilibrium freezing line, until the outside temperature of cooling medium is reached (here − 18 ∘ C).

Different steps in freezing process as depicted in the supplemented state diagram of sucrose, showing the freezing line, sucrose solubility, and glass transition line. Sucrose is viewed as model system for fruit. Path taken by state of the system during freezing is shown by the dashed gray line

Depending on the process conditions and the food material properties, the ice crystal growth rate is either determined by the heat transfer or by the diffusion rate. The resulting ice crystal size is often the resultant of the interaction between ice crystal growth and the diffusion of the excludes solutes and/or biopolymers, as we have discussed earlier [ 53 , 54 ]. At slow freezing rates the ice crystal size is independent of the number of initial nuclei, but depends only on the freezing rate, and initial solute concentration [ 54 ]. Only at quite fast freezing rates, where the state of the food remains significantly below the equilibrium freezing line, ice crystal size can depend on the number of initial nuclei.

The freezing operation is commonly stopped if a minimal product temperature is reached. The frozen food is transferred to storage facilities, where temperatures around − 18 ∘ C are maintained. As one can observe in Fig.  2 , this storage temperature is likely to be above the glass transition temperature of the food matrix. This means that moisture still has sufficient mobility to diffuse through the unfrozen phase, and allow changes in the morphology of the frozen phase. Consequently, the physical process of ice coarsening (or Ostwald ripening [ 56 , 57 , 58 ]) can still proceed. The coarsening leads to further growth of the largest ice crystals, at the expense of smaller ice crystals.

Temperature fluctuations around the storage temperature lead to ice crystal melting and regrowth [ 59 , 60 ], which is assumed to be a different mechanism than coarsening via Ostwald ripening. Hydrocolloids are assumed to have a large impact on this melting/regrowth mechanism, as shown for ice creams [ 59 ]. One can assume that cell wall material in frozen fruits and vegetables act in a similar way [ 61 ]. Hydrocolloids are thought to have a stabilizing effect on the ice crystal distribution, thus limiting the coarsening via temperature fluctuations. The hypothesis behind the stabilizing effect of hydrocolloids is that they hinder the diffusion of solute towards/away from the interface of a melting/growing ice crystal [ 59 ]. However, in carrots it is found that ice crystals grow due to temperature fluctuations during storage [ 60 ].

Interaction of Ice Formation with Plant Tissue

In the discussion of the interaction of ice formation with plant tissue, we follow the classification in damage as used by Reid [ 62 ]. He distinguishes (1) dehydration of the cell due to the location of ice nucleation, (2) solute damage induced by freeze concentration of the intracellular fluid, and (3) mechanical damage via the stress imparted by the expanding ice phase. To understand the damage induced by dehydration and loss of turgor, it is important to have an understanding of water holding capacity of foods. This is first discussed below.

Water Holding Capacity of Vegetables and Fruits

The water holding capacity of cellular tissue of plants like vegetables and fruits strongly depends on their structure, which is depicted in Fig.  1 . Plant cells comprise of vacuole embedded in cytoplasm. Both vacuoles and cytoplasm are surrounded by lipid membranes, which are impermeable to most of the solutes, but they are permeable to water. The vacuole contains simple solutes like sugars, while the cytoplasm contains mainly biopolymers like proteins. In some vegetables like potatoes the cytoplasm is also rich in starch. The cell membrane is enveloped by the cell wall, which gives the plant tissue strength. The individual cells adhere to each other via the middle lamella, which is rich in pectin. There is little adherence in the three-way junctions, which often contains air and provides pathways for gas diffusion [ 26 , 27 , 63 , 64 ].

Due to the solute impermeability of the vacuole membrane an osmotic pressure is generated, which attracts fluid from the extracellular space, the apoplast. The cell walls are viewed part of the apoplast, and connect to the vascular tissue—allowing for transport of water, nutrients and assimilates. The inflation of the vacuole leads to expansion of the cell, and stretching of the cell wall, which can be regarded as a biopolymer network. This deformation of the cell wall leads to mechanical stress, which will counteract the osmotic pressure. If the mechanical stress balances the osmotic pressure, the cell is at full turgor [ 26 , 27 , 63 , 64 , 65 ].

In a recent paper we have elucidated the physical theory behind the water holding of vegetables [ 65 ], which extends our earlier papers on Flory-Rehner theory [ 66 ]. In fresh vegetables water is held in three compartments: (a) in the intracellular space, both the vacuole and the cytoplasm, (b) the extracellular cell wall material, and (c) the capillary space, which can be filled with fluid after vacuum impregnation [ 67 ]. The water holding in the intracellular space critically depends on the integrity of the cell membranes, otherwise leading to the loss of osmotic pressure and turgor. The cell wall material (CWM) can be regarded as a hydrogel, containing hemicellulose and pectin, embedded with cellulose fibers [ 65 ]. The water holding of the CWM can theoretically be described by Flory-Rehner theory and its extensions. The theory describes the amount of water holding of the gel as function of its hydrophilicity (as indicated by the Flory-Huggins interaction parameter) and the cross-link density. During food processing the cross-link density of CWM is often modulated, especially during blanching [ 64 ]. During conventional blanching the thermal solubilization of pectin via beta-degradation lowers the cross-link density, whereas during long-time-low-temperature blanching the cross-link density is increased via the activation of the PME enzyme, which de-esterifies pectin, and the subsequently cross-linking via calcium ions. As pectin is a polyelectrolyte, extensions of Flory-Rehner theory learns that its water holding is also influenced by ionic strength and pH of the fluid the CWM is immersed in [ 68 , 69 ].

If the integrity of the cell membrane is lost, also the osmotic pressure and turgor pressure are lost. The stretched network of the CWM will relax back to its zero stress state, thereby absorbing a little water [ 65 ]. Due to the relaxing of the CWM the intracellular space will shrink in volume, which cannot be compensated by the increase of water holding of the relaxed CWM. The excess fluid will be located in the extracellular capillary space, which is enlarged compared with the full turgor state. Now the water in the intracellular space and the extracellular capillary space is only held by capillary forces, and consequently it can easily be pressed out by external forces, as occur during eating [ 65 , 70 ] or it can drain easily via gravity, for example appearing as drip loss after thawing or slicing.

Location of Nucleation

One of the important factors for the interaction of ice and the plant tissue is the location of nucleation with respect to the cell, which imparts the integrity of the cell membrane. In cryopreservation it is crucial that tissue survives the freezing operation, for which it is critical to control or even prevent intracellular ice formation [ 71 ]. If the size of the intracellular ice approaches that of the cell size, it will puncture the cell membrane of the vacuole. Consequently, the membrane looses its integrity, and solutes diffuse to the extracellular space. The osmotic pressure and turgor pressure are lost. The stretched cell wall material will return to its relaxed state, and will squeeze out liquid from the intracelluar space. For foods like fruits it is important to maintain cell membrane integrity and turgor.

In plant tissue it is energetically favorable that ice nucleation happens in the extracellular space. The extracellular space is thought to have quite a number of heterogeneous nucleation sites [ 72 ], while intracellular ice formation often requires homogeneous nucleation [ 73 ]. The air/liquid interface in the junctions between cells (if filled with air) can be a preferred nucleation site [ 74 ]. Extracellular nucleation will indeed happen at sufficiently slow freezing rates. Extracellular ice growth extracts water from the cell. There will be only extracellular ice growth if the water extraction rate from the cell can keep up with the ice formation rate [ 72 ]. Otherwise, the cell gets sufficiently supercooled to ice nucleation to happen. Thus, at relatively fast freezing rates both extracellular- and intracellular ice formation will happen. If the tissue is suddenly immersed in liquid nitrogen, the nucleation is mostly intracellular, because the temperature gets quickly below the homogeneous nucleation temperature (around − 40 ∘ C [ 73 ]). However, sufficiently rigid cell wall can prevent intracellular ice nucleation, due to the development of negative pressures, which is a consequence of the deformation of the stiff cell wall [ 73 ].

If nucleation happens extracellular the cell membrane need not to loose its integrity. Growth of extracellular ice will draw water also from the intracellular space, which will freeze concentrate and shrink (and gets in the state known as cytorrhysis [ 72 ]). As the volume of the cell membrane cannot shrink, as it contains lipids only, the shrinkage of the cell can lead to buckling and damage of the cell membrane. Some of the invaginations of the cell membrane can bud off as endocytic vescicles, leading to reduction of the cell membrane area. Hence, upon thawing the melt water of extracellular ice returns to the cell (via osmosis), which will expand again. Due to the reduced area of the membrane due to endocytosis, it will burst just before it is fully hydrated [ 72 ].

Freeze Concentration

The intracellular space and vacuoles also contain sugars, salts and possibly some acids [ 75 ]. Hence, freeze concentration will lead to increase of the ionic strength, and change in pH via the decrease of water activity. These altered conditions may lead to protein denaturation or permeabilization of the cell membrane. Freeze concentration also has influence on the biochemistry, as it increase the substrate concentration for enzymes [ 76 ]. During freezing of vegetables enzyme activity can lead to undesired browning due to loss of the integrity of the vacuole, via which substrates get into contact with membrane-bound enzymes like PPO, leading to discoloring [ 77 , 78 ].

Consequently, many vegetables are blanched before freezing. Yet, blanching also imparts the textural quality of the vegetables due to (1) loss of membrane integrity via the enhanced temperature, (2) the solubilization of pectin, and (3) enhanced cross-linking of cell wall material with calcium made possible via the modification of pectin by the PME enzyme [ 79 , 80 , 81 , 82 ]. Hence, for the final textural quality of frozen vegetables the conditions for blanching and freezing should be well balanced.

Freeze concentration can also enhance oxidation processes [ 83 ], especially to the lipid cell membranes [ 72 ]. Furthermore, retrogradation of starch can happen in cooked products like par-fried potato products [ 84 , 85 ]. During retrogradation recrystallization of amylose is enhanced. For products like frozen rice gels it is shown that retrogradation leads to syneresis, which is released from the food after thawing as drip loss [ 86 ].

Note that the low product temperature slows down the diffusion and reaction kinetics, and thus counteracts the effect of freeze concentration. At temperatures of the frozen storage the slowed-down diffusion and reaction kinetics commonly overwhelms the effect of freeze concentration [ 87 , 88 ]. But at temperatures just below freezing point, effects of freeze concentration can be considerable. Mind that, these conditions reoccur during thawing. Consequently, also the rate of thawing should be taken into account during the design of the frozen food production chain.

Mechanical Stresses

As the density of ice is smaller than liquid water, the food will expand and its matrix will be subject to tensile stresses upon ice formation [ 89 , 90 ]. Later if coolant temperatures are reached the stresses will be compressive as the density of ice increases with the lowering of temperature [ 91 ]. The simultaneous occurrence of compressive and tensile stresses make the food especially vulnerable to freeze cracking. Freeze cracking occurs particularly at quite high freezing rates as occur during cryogenic freezing. During freeze cracking cells will be ruptured straight through the cells.

During slower freezing rate the growth of adjacent ice crystals can lead to localization of stresses [ 92 , 93 , 94 ], which will rupture the food matrix at weak points. If the vegetable has been blanched the weak spots are in between cell walls, where pectin has been solubilized. This will lead to separation of cells. Also, in frozen living plants mechanical stresses are thought to be a major cause in freezing damage [ 72 ].

The growth of adjacent ice crystals towards each other compresses the cell wall material in between. We hold the hypotheses that due to the compression extra cross-links or hydrogen bonds will be made in the cell wall material, which will reduce its water holding capacity, but it will also hinder the swelling back to its original density. Reduction of the cell wall thickness after freezing has been observed in living plants [ 72 ]. This is also apparent in the structure of rehydrated vegetables which have been subject to slow freezing and subsequent (freeze) drying [ 70 ]. The compression of a starch matrix as in freezing (par-fried) potato products will probably enhance the retrogradation, which also lowers the water holding of the starch matrix [ 84 , 85 ].

It is also claimed that freezing and frozen storage leads to breakdown of pectin (depolymerization). For papaya it is shown that multiple freeze/thaw cycles increases to amount of soluble pectin, which indicates indeed the breakdown of pectin. This results in further weakening of the cell wall matrix, and lowers its water holding capacity [ 95 ]. For (non-blanched) apples also decreased cell adhesion after thawing is observed [ 96 ]. This has been explained due to the action of enzymes on the cell wall material, which have come into contact due to rupture of the cell membranes.

Impact of Processing Steps in Production Chain

A typical production chain of frozen vegetables or fruits is shown in Fig.  3 , which is line with production chains described in several patents [ 97 , 98 , 99 ] and textbooks [ 100 ]. The production chain is also consistent with the typical production chain for frozen fries as given by Scanlon [ 101 ], but I have not included par-frying explicitly, but it can be viewed as just another means for dehydration. The main difference between vegetable and fruit processing is that vegetables are commonly blanched, while it is absent for fruits. For the soft fruits it is important to retain the full turgor state, which will otherwise lead to detrimental loss of texture. Below, I discuss how each step in this production chain impacts the final quality, and the potential improvement of that, as offered by novel technology. Promising novel technologies are summarized in Tables  1 and  2 .

Process flow sheet for frozen vegetables and fruits. Prior to freezing the produce may either be blanched, brined and/or dehydrated (as in dehydrofreezing)

Cold Storage

After harvest fruits and vegetables are stored at low, non-freezing temperatures for buffering before further processing like freezing. Optimal long-term chilled storage conditions are found in literature [ 102 ]. In general fruits and vegetables are stored at low temperatures, provided that chilling injury or freezing injury is prevented [ 103 ]. Due to the applied temperatures and storage time, this cold storage will have little impact on the freezing quality. However, there is some potential in inducing cold acclimation , which is the physiological adaptation of plants to freezing conditions [ 10 , 104 , 105 , 106 ], This will be discussed below.

Many overwintering plants will show cold acclimation , if ambient temperature declines slowly below 5 ∘ C [ 23 , 107 ]. The physiological adaptations are targeted at the survival of the dehydrated cell after extracellular ice formation. The primary freezing injury to living tissue is the permeabilization of the cell membrane. The physiological adaptation comprises of (1) changes in lipid composition of membranes, making them less vulnerable to chilling injury, (2) increased accumulation of solutes in the vacuole, (3) synthesis of special proteins for protection of cell membrane, and modulation of the ice growth similar to antifreeze proteins, and (4) synthesis of anti-oxidants. Anti-oxidants are required because the freeze concentration of the cell can promote oxidation reactions. Even, other proteins can be synthesized, that promote ice nucleation in the extracellular space [ 74 ].

The cold acclimation is thus primarily targeted at protection of the cell membrane, or prevention of the dehydration of the cell, via an increase of its osmotic pressure. In acclimated tissues, both the cell membrane and cell wall have become effective barriers for extracellular ice [ 20 , 22 ]. Non-acclimated tissues often show intracellular ice growth, originating from extracellular ice via secondary nucleation. Membranes with chilling injury allow passage of ice from the extracellular space to the intracellular space. Intracellular ice formation is often lethal for living tissue.

Cold acclimation as a pretreatment to freezing has been investigated for carrots [ 10 ] and spinach [ 106 ], showing a significant decrease of freezing damage. Carrots have been stored at 0 ∘ C to elicit cold acclimation. Cold acclimation of spinach leaves is elicited via changing the environmental temperature during growth. The experienced cold stress-induced cold acclimation.

However, if the acclimated vegetable is subject to blanching several protective effects provided by proteins or membrane composition will be eliminated, due to protein denaturation and thermal injury to the membrane [ 10 ]. Still, the amount of accumulated solutes in the vacuole will lead to a lowering of freezing point and decrease in amount and size of ice crystals.

Cold acclimation will be difficult for (sub)tropical produce. For these produces the cold storage temperature must also remain above a critical value, otherwise, chilling injury happens, leading to damage of the cell membrane, and loss of turgor [ 22 ].

Processed vegetables and fruits are often sliced or cut to reduce size for consumer convenience, or to enhance the heating rate of thermal processing like blanching, cooling or freezing. Slicing is often done if the product is still in the turgid state, because of its firmness. Hence, the plant is still metabolically active, and slicing will trigger biochemical defense mechanisms. Via slicing phenolic substrates, present in the vacuole, come into contact with enzymes leading to browning. Also the ripening biochemistry is accelerated, leading to faster softening of texture [ 108 , 109 , 110 ]. Also, slicing can enhance the loss of leaching of solute if further processing is performed in water (like blanching).

The negative effects of the activated biochemistry due to slicing can be stopped if the food is immediately blanched. For fruits blanching is often not done, and they can be frozen directly after slicing. As the freezing takes time, the sliced fruits can be dipped in water, to infuse it with anti-oxidants, Ca 2+ and PME enzyme [ 111 ]. Anti-oxidants can retard the browning, and the Ca 2+ and PME enzymes enhance the texture via cross-linking pectin. Furthermore, Ca 2+ also retards the accelerated softening.

Slicing plant-based products in the turgid state requires quite some energy, and consequently one sees the application of PEF (pulsed electric field) technology to puncture the cell membranes, leading to a reduction or loss of turgor pressure, and thus a reduction of the energy required for slicing [ 112 , 113 ]. It is applied in practice to frozen potato products. Also, it is claimed that PEF before slicing can already provide (partial) inactivation of enzymes [ 114 ]. However, this claim is controversial as it is debated that the enzyme inactivation can just be imparted by the thermal effects of PEF [ 115 ].

For consideration of the blanching of vegetables on freezing, it is important to know the actual functionality of this pretreatment. Often, it is primarily intended to inactivate enzymes. For green vegetables it is important to inactivate lipid oxidase [ 116 , 117 ]; otherwise, off-odours can develop due to lipid oxidation during freeze concentration. Also one desires to inactivate peroxidase, to reduce enzymatic browning [ 118 , 119 ]. However, for some vegetables like carrots it is questionable if blanching is required to inactive enzymes, for they have limited impact on texture and browning [ 8 ]. Other studies state that a very quick treatment of dipping carrots in boiling water for 10–20 s is sufficient for inactivation enzymes responsible for lipid oxidation [ 10 ].

The temperature treatment during blanching will give undesired effects on the texture. First of all, the permeability of cell membranes is imparted, leading to loss of turgidity [ 120 ]. Especially for fruits, this is highly undesired, and consequently fruits are seldom blanched prior to freezing [ 121 ]. Furthermore, blanching at high temperatures, T > 80 ∘ C, will solubilize the pectin of the cell walls via the β -elimination process, leading to cell separation and weakening of the tissue [ 7 ]. Water holding properties of cell wall materials are imparted by the solubilization of pectin [ 64 ].

Common blanching processes involve the immersion of the vegetables in hot water or steam [ 7 ]. Blanching via immersion in hot water renders a good heat transfer, but it can also lead to leaching of solutes. Sliced products are of course extra vulnerable to leaching of solutes. The loss of solutes implies an increase of the freezing point, and therefore an increase of amount and size of ice crystals—which are both negative for freezing quality. Solute leaching is reduced via steam blanching [ 122 ]. Steam blanching is also a fast process, due to the efficient transfer of heat via the condensation of steam [ 123 ]. The use of superheated steam allows for temperatures below 100 ∘ C [ 124 , 125 ].

The impact of the thermal treatment during blanching on texture can be reduced via two strategies: (a) LTLT (low temperature long time) blanching or (b) HTST (high temperature short time) blanching [ 126 , 127 , 128 ]. During LTLT blanching the low temperature enhances the activity of the PME enzyme, which enhances the strength of the cell wall via mediating cross-links between pectins with Ca 2+ [ 129 ]. Many vegetables already contain sufficient calcium in their cell wall material [ 126 , 130 ], but it can be added to the blanching water, or infused via vacuum impregnation [ 131 , 132 ]. The strengthened cell wall will have reduced water holding capacity but may resist better the compression induced by the growing ice crystals during freezing [ 8 ].

The HTST blanching strategy can be applied with technologies having a high heating rate. The texture degradation is reduced if the heating rate is significantly faster than the kinetics of the β -elimination of pectins [ 133 ]. A faster heating rate is often the target of novel blanching technologies like Ohmic heating [ 7 ], or radio-frequency or microwave heating [ 134 , 135 ]. These are volumetric heating methods, which can heat foods up to 80 ∘ C within one minute. The volumetric heating makes the heating rate quite independent of the size of the product. However, especially via microwave heating, there is often a large non-uniformity in temperature—making such a blanching technology hard to design and control. The HTST strategy can also be achieved by enhanced heat transfer coefficients as in jet impingement technology [ 124 , 136 , 137 ]. However, the enhanced heat transfer coefficient is only efficient if the size of the product is small enough that the heating rate is still determined by the external heat transfer coefficient, rather than the thermal conduction inside the product.

Good enzyme inactivation, while maintaining a firm texture, can be achieved with high-pressure blanching [ 138 , 139 ]. However, high-pressure treatment is a batch operation, requiring specialized equipment, which is difficult to incorporate in the continuous production chain of frozen vegetables and fruits.

Another measure against solute leaching is the addition of solutes to the blanching water [ 8 , 132 ]. This can even promote the diffusion of solutes into the vegetable if the solute concentration in the blanching water is higher than in the tissue. The addition of solutes will lower the food’s initial freezing point [ 8 ]. But, their effect might not be very large, due to diffusion barriers imposed by the cell wall material.

Some dry blanching techniques allow for combining the process with dehydration, i.e., drying, with the purpose of increasing solute concentration, and thus lowering of the initial freezing point, and the ultimate improvement of the quality of frozen product. Examples of these dry blanching technologies are infrared/hot air blanching [ 140 ], and blanching via hot air and radio-frequency heating [ 141 , 142 ]. Dehydration can only be performed at practical time scales for small (sliced) products. Also, one must be careful and prevent case hardening, the formation of a dense skin on the product, which leads to a significant lower appreciation of the texture by consumers, and limits possible rehydration after thawing.

Dehydration

Potato products and some vegetables are air-dried after blanching to remove surface water to avoid clumping during freezing operation in the tunnel [ 97 ] or oil splashing during par-frying [ 101 ]. Dehydration can also be performed to improve the quality of frozen fruits and vegetables. The dehydration lowers the amount of water and increases thus the solute concentrations, and consequently it lowers the initial freezing point of the food—leading to smaller ice crystals and a smaller amount of ice formed. These combined processes of dehydration and freezing is known as dehydrofreezing [ 7 , 52 ].

Although dehydrofreezing is already known for years, it has received little attention in the scientific literature. It has been argued that there is much room for improvement if the two processing steps are jointly optimized [ 52 ]. The dehydration step can be performed via conventional air drying [ 143 ] or via osmotic dehydration, but this process requires intact cell membranes [ 144 ]. This technology is then particularly applied to fruits, and to vegetables, which are nearly always blanched (destroying the integrity of the cell membrane). For vegetables, dehydration can conveniently be combined with blanching, as reviewed in the previous section.

To improve the quality of frozen vegetables or fruits the amount of water to be removed via dehydration should be in the order of 30–50% [ 52 ]. Next to improvement of texture, there is also a reduction of freezing time, which significantly reduces energy consumption [ 52 ].

Osmotic dehydration is performed via immersing food in an osmotic solution, containing a high concentration of solutes like sugars or salts. The process has some negative side effects like leaching of solutes from the food, and the increase of salt or sugar levels of the food [ 144 ], which can impair the sensorial and nutritional value of the food. The nutritional constraints of sugar or salt in osmotic solutions can be lifted via the use of sugar replacers with smaller molecular weight such as polyols [ 145 ]. Per mass of solute they also offer higher osmotic pressure, i.e., a higher driving force for osmotic dehydration.

Furthermore, osmotic dehydration is a slow process [ 146 ] as (1) water has to pass through the cell membrane, which is the dominant resistance to mass transfer and (2) the dehydrated crust of the food with increased concentration of the solute from the osmotic solution has a decreased permeability. The temperature and solute concentration of the dehydration solution are limited because at high temperature or high osmotic pressure cell lysis will occur (also known as osmotic plasmolysis) [ 147 , 148 ], leading to the loss of function of the membrane—becoming fully permeable. Generally, osmotic dehydration is performed at 30 ∘ C, avoiding the loss of cell membrane integrity [ 52 ]. Furthermore, the recycling of the osmotic solution is a concern from a technological and hygienic point of view [ 144 , 146 ].

Significant enhancement of osmotic dehydration is via the combination with vacuum impregnation [ 149 , 150 ]. During this pretreatment the air in the intercellular space is removed and replaced by the osmotic solution. This pretreatment is especially advantageous for highly porous foods like apples. Recently, it is also applied to carrot and strawberries, where sugar impregnation is combined with CaCl 2 and/or PME to promote cross-linking of pectin, which will increase the mechanical strength of the cell wall [ 151 , 152 ].

Because osmotic dehydration is inherently a slow process, there has been a variety of research studies of improving the mass transfer rate via combining the dehydration with techniques like high pressure or pulsed electric fields (PEF) [ 146 ], with the purpose to increase (temporarily) the permeability of the cell membrane. The treatment should be mild enough, such that the resistance to mass transfer is lowered, but the loss of membrane permeability is reversible, such that full turgor can be restored after osmotic dehydration [ 5 , 152 ].

Some studies concerning PEF-assisted osmotic dehydration have investigated whether the viability of the cells can still be retained after freezing. PEF is combined with vacuum impregnation of antifreeze agents and/or antifreeze proteins that are impregnated into the tissue, leading to enhanced cryoprotection of the tissue [ 153 ]. Retention of cell viability requires fast freezing rates as in cryogenic freezing, such that intracellular ice formation occurs. The small, round ice crystals will not puncture the cell membrane. However, this is achieved for few porous products, such as leafy vegetables (like spinach leaves) [ 154 , 155 ], and fruits as strawberries [ 150 ] and apples [ 156 ]. However, the texture and drip loss is still comparable with a vacuum impregnation treatment without PEF [ 150 ].

If one tries to achieve retained cell viability via air dehydration, the drying must be done at very mild conditions to prevent permeabilization of the cell membrane, which happens at temperatures above 40 degrees [ 143 ]. It is suggested to use vacuum-microwave drying performed at low temperatures [ 52 ].

Freezing Operation

Commonly, industrially processed fruits and vegetables are frozen using air blast freezing in conveyor belt tunnels [ 100 ]. Air is forced by big fans over a mechanical refrigerator unit, and then over the belt with produce. The belt allows vertical airflow to pass through it. The air temperature is often set between − 18 and − 40 ∘ C, and air velocities are in the order of 1 m/s. Heat transfer coefficients of 10–80 W/m 2 K are commonly reached [ 157 , 158 ], which are not as high as in fluidized bed freezing or impingement freezing [ 159 ]. Hence, there is definitely room for improvement of final product quality and texture. One can follow two strategies to improve freezing, aiming at either (1) the enhancement of the freezing rate or (2) the enhancement ice crystal nucleation [ 5 ].

Earlier I have discussed how the freezing rate impacts the ice crystal size, and thus the food texture [ 53 ]. The freezing rate is determined by several factors, namely the size of the food, the heat transfer coefficient of the coolant medium, and the freezer temperature, as indicated by Planck’s equation [ 53 ]. Heat transfer rate can be improved via enhancement of airflow velocity, as in jet impingement freezing [ 160 , 161 ] or in fluidized bed freezing [ 162 ], or by choosing another coolant medium like a liquid in immersion freezing, or a cryogenic liquid or gas from liquid nitrogen or CO 2 freezing [ 163 ]. Via jets of airflow impinging the surface of foods one can achieve a local heat transfer coefficient of 400 W/m 2 K [ 164 ]. It must be noted that the increase of air velocity also promotes mass transfer, i.e., the evaporation of water from the food, which might not be desired.

Cryogenic freezing is used in practice for freezing quite vulnerable fruits and vegetables, such as berries [ 151 , 165 , 166 ]. The high freezing rate of cryogenic freezing is mostly due to the low temperature of the coolant, cold nitrogen or CO 2 gas or sprays—which is generated from solid CO 2 or liquid nitrogen. The freezing temperature is due to the low boiling point of the cryogenic liquids, which are − 78 ∘ C or − 196 ∘ C for CO 2 and N 2 , respectively. Both gases are quite inert and do not penetrate the food. Cryogenic liquids can also be sprayed on the foods, and the freezing is then enhanced by the required heat for the evaporation of the cryogenic liquid droplet on the foods surface [ 100 ].

In immersion freezing the cooling medium should remain liquid below 0 ∘ C, which can be mixtures of water/glycerol, water/ethanol or similar liquids as used in osmotic dehydration (sugar or salt solutions) [ 167 , 168 ]. The heat transfer coefficients in liquids are considerably higher than those in airflow, which are in the range 150-1500 W/m 2 K [ 169 ]. During immersion freezing in such liquids there will be impregnation of food with solutes from the liquid if the food does not have any proper barrier. For large-sized foods like fish or meat, it is economical to use vacuum-sealed packagings as a barrier. However, if a similar liquid has already been used for osmotic dehydration, the use of immersion freezing in the same liquid can be a serious option for freezing fruits or vegetables, via combining freezing and osmotic dehydration [ 167 ]. Freezing rates can also be improved via the use of impingement technology.

There will be a limit to the freezing rate, as high freezing rates will impact the development of mechanical stress in the food, due to the volume expansion of ice [ 170 ]. At high freezing rates, the stress has insufficient time to relax. High stresses can lead to freeze cracking or damage during fast thawing [ 171 ].

At sufficient low freezing temperatures, the food attains the glassy state. The food is not in equilibrium but is supercooled. The amount of ice formed depends on the freezing rate [ 171 ]. However, if one thaws these supercooled foods too slowly, ice growth can occur at temperatures just below freezing point. All the protection offered by the glassy/supercooled state against freezing damage can be eliminated by the slow thawing [ 172 ].

For the stimulation of ice nucleation one has been tried several novel technologies, like ultrasound [ 173 ], pressure shift freezing [ 174 ], static or oscillation electric fields [ 175 ], and oscillating magnetic fields [ 176 , 177 ]. Whether (electro)magnetic fields have a significant influence on nucleation is doubtful [ 178 , 179 ]. Ultrasound and pressure shift freezing have significant effects on nucleation. However, ultrasound is only effective if the frozen food is submerged in liquid as in immersion freezing. For pressure shift freezing quite specialized equipment is required, which only works for batch operations. Hence, novel technology is difficult to combine with conventional air blast freezing tunnels. I view mainly potential for ultrasound technology if applied to immersion freezing of fruits [ 180 , 181 ]. Ultrasound will also enhance the heat transfer if the immersion fluid via the acoustic streaming effect.

In formulated foods like gels and ice creams, one can control nucleation also via additives, but that is difficult for fresh vegetables. Some trials are performed using pulsed electric fields, to permeate the cell membrane and to impregnate vegetables with antifreeze agents like trehalose and antifreeze proteins[ 155 ]. However, the additives can be costly.

Hence, for most foods control of ice nucleation must rather be sought in processing technologies, which can be applied with conventional freezing equipment. Examples of these technologies derive from the field of cryopreservation or freeze-drying. Examples of such methods as applied to are (1) shock freezing [ 19 ], (2) ice fog method [ 182 ], and (3) mechanical shock [ 49 ].

It is not likely that the ice fog method is applicable to vegetables and fruits because their epidermis does not permit direct contact of the liquid in the tissue and the deposited ice crystal seeds. In freeze-drying it is relatively easy to provide a mechanical shock to vials placed on the shelf of the freezer [ 49 ]. But, for belt freezing it is not evident how to apply mechanical shocks.

I view that for food processing the shock freezing method is the easiest to implement in practice. During shock freezing the product is first slowly cooled to a supercooling of about 5 ∘ C, and it is held in the supercooled state for some time. After the holding time, the freezing temperature is suddenly dropped to temperatures far below − 20 ∘ C. This temperature shock very often induces the nucleation, which is expected to give a more uniform and fine ice crystal distribution [ 19 ].

There are a few reports in food science, where the shock freezing method has shown to initiate indeed ice crystal nucleation [ 183 , 184 , 185 , 186 ]. The temperature shock leads to the nucleation of many small ice crystals. Due to the subsequent fast, deep freezing step the nucleated ice crystals have little time to grow and coalesce, as at low temperatures the water diffusion coefficient is too low for ice crystal growth [ 54 ]. However, a recent application of shock freezing to strawberries still shows freezing damage comparable with conventional slow freezing, despite the controlled nucleation of ice crystals [ 186 ]. The fast growth of ice crystals has caused much mechanical stress on the cell membranes and cell walls, which appears to be split in pieces similar to what happens during slicing. Apparently, the rigidity of the cell is an important factor to consider when applying shock freezing.

Frozen Storage

It is common practice that frozen foods are stored lower or equal to − 18 ∘ C, which is the limit set by legislation (such as the European directive for Quick Frozen Foods). This storage temperature is still above the glass transition for many fruits and vegetables [ 187 ]. Consequently, during frozen storage there can still be (bio)chemical activity and coarsening of ice crystals. Especially, the effects of temperature fluctuations due to the mechanical temperature control are expected to have a large impact on frozen food quality [ 188 ].

Often, it is advised for minimization of freezing damage to store foods at the intersection of freezing line and glass transition line, which lies in the range of − 20 to − 40 ∘ C for most fruits and vegetables [ 189 ]. However, one must mind that the chemical kinetics and ice coarsening are driven by diffusion, which decouples from the viscosity at temperatures around the glass transition [ 190 ]. Hence, the glass transition is not a good measure for diffusivity, but rather for viscosity. I have shown that near the glassy state diffusion is quite independent of the molecular weight of solutes [ 190 ]. Consequently, the effects of the chemical kinetics and ice coarsening are quite independent of the type of food, i.e., the type of solutes. But, ice coarsening can still be controlled by antifreeze proteins [ 191 , 192 ], as synthesized by cold acclimation or infused via PEF.

The large independence of the type of food concerning the physical and chemical changes during storage allows for a simple strategy for improving food storage: lowering of the storage temperature will have similar effects on all stored fruits and vegetables. If the storage temperatures is lower than − 25 ∘ C, the effect of temperature fluctuations is considerably lower due to reduced diffusion coefficients and lower (bio)chemical activitity. Of course, via packaging with good insulating properties the effects of these fluctuations can also be lowered [ 193 ].

During frozen storage, the thermal stresses accumulated during the freezing operation can relax. However, this is a very slow process with relaxation times of about half a year. The relaxation time also depends on how far the storage temperature deviates from the glass transition temperature of the frozen food [ 89 , 194 ].

Generally, thawing is performed more slowly than freezing. This is mainly inherently to the process for the following reasons [ 195 ]: (1) the thawed outer layer of food has a lower thermal conductivity than the frozen part, (2) the ambient temperature cannot be high, because of high temperatures promoting biochemical processes, as enzymatic activity, microbial decay or protein denaturation having adverse effects on product quality. One should mind that enzymes and microbes are often not (fully) inactivated, and proceed with their activity upon rewarming [ 95 ]. Furthermore, it is also because thawing is often left to the consumers, who either leave it in the open air in the kitchen, or in the refrigerator or immersed in tap water [ 195 ]. The latter is considerably faster, but allows solutes to leach out in the immersion liquid, but which can be minimized via having the food wrapped in plastic packaging foil [ 196 ].

On the other hand, slow thawing should give the food time to relax mechanical stresses induced by freezing and allow to reabsorb moisture, which would either be lost via drip loss [ 197 ]. The stress relaxation time is much faster in the thawed state as in frozen storage. But, for meat [ 198 ] and potato [ 33 ] it is found that thawing rate does not affect much the volume of drip loss. In general it is stated that thawing has little influence on texture, as much irreversible damage is already performed during freezing [ 196 ].

Thawing frozen food is applied in the industry if frozen food is mixed into a multi-component meal like a pizza, pie or salad [ 196 ]. Also some frozen foods are tempered, i.e., raised in temperature from − 18 to − 5 ∘ C to allow for easy slicing [ 199 ]. Conventional thawing methods are reviewed by James [ 199 ], which rely on heat transfer via (moving) air, water or condensing steam. Via forced convection and smart temperature/time strategies the thawing can be significantly faster than at the consumer site.

Novel processing technology has been investigated for industrial thawing [ 200 ]. Thawing can be achieved via pressure-shift freezing [ 201 ], where frozen food are subject to high pressures of about 200 MPa at − 15 ∘ C, by which the freezing point shifts. Thawing can happen at subzero temperatures, and subsequently rewarmed to room temperature. Application of this method is limited due to high costs of equipment, and their batch-like operation.

Fast thawing can also be achieved by electromagnetic means, via microwave, radio-frequency (RF) waves or ohmic heating [ 170 , 200 ]. Especially during microwave heating, there is strong non-uniform heating as the frozen part hardly absorbs microwaves. One can experience overheated edges of the food, while the inner core is still frozen.

Due to the longer wavelength of RF waves, the temperature distribution is much more uniform than with microwaves, but its non-uniformity is still limiting its application to thawing [ 170 ]. Thawing via Ohmic heating is quite promising, but it often requires immersion of the frozen food in a liquid, promoting leaching of solutes. However, the treatment times can be quite short, in the order of minutes. Thawing can also be done via ultrasonic acoustic waves, which are mainly absorbed by the frozen part. The efficient application of ultrasound requires immersion of frozen food in liquid, leading to leaching of solutes. The technique is still experimental, and still with some disadvantages like high power consumption and low penetration depth—leading to non-uniform heating [ 170 , 200 ].

Summary via the Causal Network

The above review of the effect of processing steps on the final quality of frozen vegetables and fruits is summarized by means of a causal network (also known as a concept map), cf. [ 202 , 203 , 204 , 205 ], as shown in Fig.  4 . The (aspects of) processing steps are indicated with rectangles, which are connected to physicochemical factors of influence to the final product quality. These connectors either indicate a positive correlation (blue arrows) or a negative correlation (red barred connectors). For example, the dehydration processing step promotes the amount of solute in the sample (via removing water). Most of the physicochemical factors do not have a direct effect on the final quality. Most times, their impact is indirect, which is indicated by the network of causal relations between these physicochemical factors. This network is based on the physics and chemistry behind freezing of vegetables and fruits, as summarized in the supplementary material. In the network one can distinguish several final quality aspects: drip loss, texture, water holding capacity (related to juiciness), and browning. In the causal network, I have included all processing steps from Fig.  3 , except for slicing. The latter is done because otherwise the causal network becomes too much cluttered up. The main effect of slicing is to reduce the size, which will affect the heating rate during blanching or thawing, and freezing rate. Hence, the effect of reducing size via slicing is absorbed in these processing factors.

Causal network of how processing steps in the production chain (rectangles) impact the physicochemical factors related to final product quality. Positive correlations are indicated by blue arrows, and negative correlations are indicated by barred red lines

As an example of the use of the causal network, I focus on a single quality aspect, drip loss. Via traversing the causal network from the processing factor to the drip loss factor, one obtains the final effect on the processing factor on the drip loss, via multiplication of the signs of the correlations.

The final results are captured in Table  1 . The left column lists all steps of the production chain of frozen vegetables and fruits. The next column indicates the input parameters one can adjust in these process steps. Each processing step can impact some physicochemical factors, which are stated in the third column. The fourth column indicates how the increase of the physicochemical factor in the third column impacts drip loss. This impact can either be negative (−) (decreasing drip loss), positive (+), or indifferent (0). If the impact is either negative or positive, we have placed (+/−) in the fourth column. In the table, the slicing operation is now incorporated.

Using the considerations from the above review and the results listed in Table  1 , I have come to the following recommendations for modification of the production chain of frozen vegetables and fruits for improved drip loss:

Perform cold storage at low temperature, just above the freezing point of the vegetable, to induce cold acclimation .

Match the size of the product during slicing to a shorter blanching time and/or faster freezing rate.

Match the blanching time to the size of the product and the enzyme inactivation kinetics and pectin degradation kinetics, i.e., perform blanching as short as possible by the use of the HTST treatment.

Before HTST perform LTLT blanching to promote pectin cross-linking, which might be combined with calcium immersion.

Combine dehydration and blanching via using superheated steam impingement or hot air/infrared heating.

Find an optimal freezing rate with minimal ice crystal size, and minimal stress accumulation

Low frozen storage temperature, T ≪− 18 ∘ C, to reduce Ostwald ripening and the product sensitivity to temperature fluctuations.

Improve packaging during frozen storage, with increased thermal insulation and moisture barrier, for minimization effects of temperature fluctuations.

Perform thawing at moderate temperature (0–10 degrees) and allow time for stress relaxation and resorption of drip loss.

From the list above, one can observe there are largely two main strategies to reduce drip loss: (1) improved freezing operations, with faster freezing rates and lower freezing temperatures, and (2) the increase of solute concentrations before freezing, which is achieved via other processing steps prior to freezing. However, one must also consider the adverse effects of increased solute concentration (i.e., reduced water content) on other quality factors like texture or water holding capacity/juiciness. It is clear that industry has to make a compromise between different quality traits.

Process Intensification and Production Chain Optimization

Above, I have discussed the individual processing steps in the production chain. Here, I discuss potential improvements if larger parts or the complete product chain is taken into consideration. First, I discuss the possibilities of process intensification, where one tries to integrate several unit operations into a new one. In the production chain of frozen foods, there are opportunities for process intensification, if there are subsequent processing steps which use the same medium for heat transfer. Via combining such processing steps one shortens the residence time, often leading to improving final product quality. From this perspective, I see two possibilities for process intensification: (1) combination of blanching and dehydration of vegetables if performed in hot air or (superheated) steam and (2) combination of osmotic dehydration and freezing for fruits if immersed in a liquid.

If the blanching is performed in air or steam, there is little leaching of solutes and nutrients. This dry blanching is easily combined with dehydration, which can be viewed as the first step in dehydrofreezing. However, dehydration can lead to loss of texture, if there is structural collapse of the tissue [ 27 ]. Volumetric heating can help in preventing collapse during dehydration due to the generation of pressure inside the tissue due to internal evaporation [ 27 ]. Another suggested measure to reduce texture loss is to strengthen the cell wall via PME-enhanced cross-linking of pectin with Ca 2+ , as in the LTLT blanching method. The dehydration/blanching step should be designed such that the reduction in texture breakdown in the following freezing step significantly outweighs the texture loss imposed by the dehydration. Recall, that dehydrated vegetables will have less freezing damage due to a reduced ice crystal size, and a reduced amount of ice formation.

The other possibility for process intensification is foreseen for fruits, that are immersed in osmotic solutions, to perform both osmotic dehydration and immersion freezing [ 167 ]. There is good potential to combine it with novel technology. Mild PEF treatments can make the cell membrane more permeable but can be repaired if the cells remain viable [ 150 ]. Furthermore, thanks to the immersion liquid ultrasound can be applied efficiently to promote ice nucleation [ 173 ]. This possibility for process intensification offers opportunity for fruits, where the conventional freezing process leads to too much freezing damage due to their fragile tissue, such as strawberries. Still, the design of the intensified process should be done with care to overcome the two drawbacks of immersion freezing [ 167 ]: (a) to keep the immersion liquid free from microbial contamination and (b) the influx of solutes from the immersion liquid into the fruit. The latter can affect the health impact of the fruit, due to the increase in sugar levels. However, this might be solved by the use of polyols as the osmolyte [ 145 ].

Next to process intensification, whose implementation is still quite involving for the food industry, I view there are still ample opportunities for simultaneous optimization of the complete production chain. Taking into account the interaction between various processing steps, one could optimize the settings of each processing step for the optimal value of the final product quality.

This optimization requires good quantitative knowledge on how the processing impacts the product quality. Over the years a large body of knowledge on this matter is generated, but still, some knowledge is lacking especially the impact of processing on mechanical damage and water holding properties. Below, I will summarize some of the findings, that can be found in literature. Most of this knowledge is on the kinetics on biochemical processes impacting product quality, which links to the functionality of several processing steps like blanching and freezing, i.e., to lower the activity of these biochemical processes. For optimizing the production chain for the final quality one has to minimize the thermal impact of the processing steps. This can be achieved via matching the time scale and/or residence time of the thermal processing with the time scale of biochemical kinetics. Below, we discuss the time scales of relevant biochemical processes in several processing steps.

Cold acclimation happens at relatively long time scales of several weeks [ 104 ]. Hence, to invoke cold acclimation, one has to match cold storage times of freshly harvested vegetables and fruits to this long time scale of cold acclimation. Diurnal fluctuating storage temperatures can help to shorten this time scale, as the metabolism is higher at elevated temperatures.

Blanching affects the inactivation of enzymes, the degradation of color and simultaneously modifies the cell wall structure. These kinetics have been investigated for several vegetables [ 212 , 213 , 214 , 215 , 216 ]. The cell wall structure is modified enzymatically via the PME enzyme, and via β -elimination. PME removes methoxy side groups from pectin, which makes them less vulnerable to β -elimination. The interaction between these processes is captured in the model by Verlinden [ 214 ]. The time scale of the heat transfer process can be estimated with simple models, as proposed earlier [ 217 , 218 ]. The time scale of the heat transfer depends on (1) the size of the product, (2) the external heat resistance as quantified by the heat transfer coefficient h , and (3) the internal heat resistance. For determining the endpoint of blanching the limiting time scale of the most important biochemical processes has to match the time scale of heat transfer [ 219 ].

Blanching modifies the mass transfer during a subsequent dehydration step, due to the permeation of the cell membranes, allowing faster diffusion of moisture during drying.[ 220 ]. Even blanching enhances the mass transfer during osmotic dehydration, but induces loss of cell membrane integrity. But, this makes the purpose of osmotic dehydration obsolete, which is to remove moisture, while maintaining turgor and cell membrane integrity [ 221 ]. For conventional dehydration methods like air drying or via superheated steam it is very beneficial to apply blanching beforehand due to the enhancement of the mass transfer. During dehydration the time scale for mass transfer is usually much longer than the time scale for heating. However, the time scale for mass transfer can still be computed using a similar approach as used for heat transfer [ 217 , 218 ]. But, it requires knowledge of moisture diffusion coefficients, for which predictive theories are at hand [ 190 , 222 , 223 ].

The time scale for the freezing operation can be estimated using the Planck method, as detailed in [ 53 ]. With the help of the freezing time, and the difference between initial freezing point and coolant temperature (or desired final product temperature), and thermal conductivity [ 224 ] one can compute the freezing rate. Knowing the freezing rate one can estimate the ice crystal size via a power-law [ 53 ]. The ice crystal size can be taken as a measure for freezing damage [ 62 ].

During frozen storage, there can be further recrystallization, i.e., ice coarsening via Ostwald ripening. For sugar solutions, which can be regarded as a model system for fruits, a study has been performed concerning the kinetics [ 56 ]. The growth rate of the ice crystal volume is linear with the diffusion coefficient of water [ 225 ], which is known for carbohydrate solutions [ 190 ]. The model shows that the diffusion coefficient depends on temperature and mass fraction of water. Hence, recrystallization is minimized at lower storage temperatures, which lowers both the diffusion coefficient and the mass fraction of water (due to progressed ice fraction). For computing the ice fraction and the remaining water fraction in the unfrozen phase a simple relation with the storage temperature and initial freezing point can be used [ 224 , 226 ].

Conclusions

In this paper, I have reviewed the production of frozen vegetables and fruits from a chain perspective, which is instigated by our experience with industrial projects that final product quality can significantly be improved via optimization of the complete production chain [ 205 ].

This optimization requires good knowledge of the fundamental physicochemical and biochemical processes underlying the changes in product quality during their processing in the production chain. This knowledge is summarized in a network diagram, showing the causal relations between processing steps and physicochemical factors impacting various quality aspects. The use of the causal network is shown for the example of drip loss, an important final product quality.

In general, it is found that it is difficult to implement novel processing technology in current production chains. But, I do see opportunities for the use of novel processing technology if combined with process intensification, where several unit operations can be combined into a new single operation. In particular, I see the potential for combining blanching and dehydration for vegetables or combining osmotic dehydration and immersion freezing for fruits. But, the implementation of process intensification can still be quite involving modification of existing production chains.

Still, I view there is an opportunity to improve final quality of frozen food via optimization of the existing production chain, via careful matching the processing times to the time scales of the fundamental physicochemical and biochemical processes. For this optimization the causal network can be a good guiding tool.

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The author has received funding for this review as part of the PPS Tasty Sustainable Frozen Foods, which was co-financed by the Dutch Ministry of Economic Affairs; the project is registered under contract number TKI-AG-15235.

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van der Sman, R.G.M. Impact of Processing Factors on Quality of Frozen Vegetables and Fruits. Food Eng Rev 12 , 399–420 (2020). https://doi.org/10.1007/s12393-020-09216-1

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Fermented Fruits and Vegetables of Asia: A Potential Source of Probiotics

Manas ranjan swain.

1 Department of Biotechnology, Indian Institute of Technology Madras, Chennai, Tamil Nadu 600036, India

Marimuthu Anandharaj

Ramesh chandra ray.

2 Centre for Tuber Research Institute, Bhubaneshwar, Orissa 751019, India

Rizwana Parveen Rani

3 Gandhigram Rural Institute-Deemed University, Gandhigram, Tamil Nadu 624302, India

As world population increases, lactic acid fermentation is expected to become an important role in preserving fresh vegetables, fruits, and other food items for feeding humanity in developing countries. However, several fermented fruits and vegetables products (Sauerkraut, Kimchi, Gundruk, Khalpi, Sinki, etc.) have a long history in human nutrition from ancient ages and are associated with the several social aspects of different communities. Among the food items, fruits and vegetables are easily perishable commodities due to their high water activity and nutritive values. These conditions are more critical in tropical and subtropical countries which favour the growth of spoilage causing microorganisms. Lactic acid fermentation increases shelf life of fruits and vegetables and also enhances several beneficial properties, including nutritive value and flavours, and reduces toxicity. Fermented fruits and vegetables can be used as a potential source of probiotics as they harbour several lactic acid bacteria such as Lactobacillus plantarum , L. pentosus , L. brevis , L. acidophilus , L. fermentum , Leuconostoc fallax , and L. mesenteroides . As a whole, the traditionally fermented fruits and vegetables not only serve as food supplements but also attribute towards health benefits. This review aims to describe some important Asian fermented fruits and vegetables and their significance as a potential source of probiotics.

1. Introduction

Fermented foods and beverages have heterogeneity of traditions and cultural preferences found in the different geographical areas, where they are produced. Fermentation has enabled our ancestors in temperate and cooler regions to survive during the winter season and those in the tropics to survive drought periods. Fermentation is a slow decomposition process of organic substances induced by microorganisms or enzymes that essentially convert carbohydrates to alcohols or organic acids [ 1 ]. In many instances, production methods of different traditional fermented foods were unknown and passed down to subsequent generations as family traditions. Drying and salting are common fermentation practices in the oldest methods of food preservation. Fermentation processes are believed to have been developed in order to preserve fruits and vegetables for times of scarcity by preserving the food by organic acid and alcohols, impart desirable flavour, texture to foods, reduce toxicity, and decrease cooking time [ 2 ].

World Health Organization (WHO) and Food and Agriculture Organization (FAO) recommended intake of a specific dose of vegetable and fruits in daily food to prevent chronic pathologies such as hypertension, coronary heart problems, and risk of strokes. The consumers tend to prefer the foods and beverages which is fresh, highly nutritional, health promoting and ready to eat or ready to drink [ 3 ]. Lactic acid (LA) fermentation of vegetables and fruits is a common practice to maintain and improve the nutritional and sensory features of food commodities [ 4 – 6 ]. A great number of potential lactic acid bacteria (LAB) were isolated from various traditional naturally fermented foods [ 7 ]. Asian traditional fermented foods are generally fermented by LAB such as Lactobacillus plantarum , L . pentosus , L . brevis , L . fermentum , L. casei , Leuconostoc mesenteroides , L. kimchi , L . fallax , Weissella confusa , W. koreenis , W. cibaria, and Pediococcus pentosaceus , which are considered as the probiotic source of the food practice. Availability of certain specific nutrients such as vitamins, minerals, and acidic nature of fruits and vegetables provides conducible medium for fermentation by LAB.

Probiotic is a relatively new word meaning “for life” and it is generally used to name the bacteria associated with beneficial effects for humans [ 8 , 9 ]. Probiotics are defined as live microbial feed such as Lactobacillus plantarum , L. casei , L. acidophilus , and Streptococcus lactis which are supplemented by food that beneficially affect the host by improving its intestinal balance [ 10 ]. Several studies have shown that supplementation of probiotics to food provides several health benefits such as reduction of serum cholesterol, improved gastrointestinal function, enhanced immune system, and lower risk of colon cancer [ 11 – 15 ]. This review provides an overview on the current research prospects of LA fermentation of fruits and vegetables with regard to human nutrition and health.

2. Fermentation of Fruits and Vegetables by LAB

Shelf life of the perishable food can be improved by fermentation which is considered as the oldest technology compared to the refrigeration. Fermentation is one of the oldest processing techniques to extend the shelf life of perishable food and was particularly important before refrigeration. LA fermentation of cabbage to produce sauerkraut has been widely studied for many years [ 16 , 17 ]. Basic outline of the fruit and vegetable fermentation is given in Figure 1 . With the popularity and success of sauerkraut, fermentation of many other vegetables has emerged, such as cucumbers, beets, turnips, cauliflower, celery, radishes, and carrots [ 18 ] ( Table 1 ).

Overall fermentation process of fruits and vegetables.

Examples of traditional fermented fruits and vegetables, which are used in various parts of Asian subcontinent.

Depending on the type of raw materials in final fermented products, vegetable fermentation is characterized accordingly. Sauerkraut, fermented cucumbers, and kimchi are the most studied lactic acid fermented vegetables mainly due to their commercial importance. Canning or freezing is often too expensive method in food preservation which cannot be affordable by millions of world's economically deprived people and lactic acid fermentation [ 19 ].

Fermented fruits and vegetables ( Table 2 ) have an important role in feeding the world's population on every continent today [ 20 , 21 ]. They play an important role in preservation, production of wholesome nutritious foods in a wide variety of flavours, aromas, and textures which enrich the human diet and remove antinutritional factors to make the food safe to eat [ 4 ]. Fermentation serves many benefits, which include food security, improved nutrition, and better social well-being of the people living in marginalized and vulnerable society [ 22 ]. Fermentation-based industries are an important source of income and employment in Asia, Africa, and Latin America [ 23 ]. Fermentation of fruits and vegetables can occur “spontaneously” by the natural lactic bacterial surface microflora, such as Lactobacillus spp., Leuconostoc spp., and Pediococcus spp.; however, the use of starter culture such as L. plantarum , L . rhamnosus , L . gasseri , and L. acidophilus provides consistency and reliability of performance [ 24 ].

Nutritive values and scientific names of fruits and vegetables mostly used for lactic acid fermentation.

Fruits and vegetables are exclusive sources of water-soluble vitamins C and B-complex, provitamin A, phytosterols, dietary fibres, minerals, and phytochemicals for the human diet [ 25 ]. Vegetables have low sugar content but are rich in minerals and vitamins and have neutral pH and thus provide a natural medium for LA fermentation [ 26 ]. LA fermentation enhances the organoleptic and nutritional quality of the fermented fruits and vegetables and retains the nutrients and coloured pigments [ 27 ]. LA fermentation of vegetable products applied as a preservation method for the production of finished and half-finished products is considered as an important technology and is further investigated because of the growing amount of raw materials processed in the food industry [ 22 ], and these foods are well suited to promoting the positive health image of probiotics [ 28 ]. The consumption of LA fermented fruits and vegetables helps to enhance human nutrition in several ways such as the attainment of balanced nutrition, providing vitamins, minerals, and carbohydrates, and preventing several diseases such as diarrhoea and cirrhosis of liver because of probiotic properties [ 29 ]. Some of the fermented fruits and vegetables contain coloured pigments such as flavonoids, lycopene, anthocyanin, β -carotene, and glucosinolates, which act as antioxidants in the body by scavenging harmful free radicals implicated in degenerative diseases like cancer, arthritis, and ageing [ 30 ]. Lactic acid fermentation of vegetables has an industrial significance only for cucumbers, cabbages, and olives [ 22 ]. In Italy, the industrial production of fermented vegetables is limited to sauerkrauts and table olives [ 31 ].

According to Kim et al. the Chinese cabbage, cabbage, tomato, carrot, and spinach provide relatively higher fermentability than other vegetables (okra and gourds) because they have more fermentable saccharides [ 32 ]. The most reported fermented fruits and vegetables are categorized as follows.

• Root vegetables: carrots, turnips, beetroot, radishes, celeriac, and sweet potato [ 72 ].
• Vegetable fruits: cucumbers, olives, tomatoes, peppers, okra, and green peas [ 27 ].
• Vegetables juices: carrot, turnips, tomato pulp, onion, sweet potato, beet, and horseradish [ 75 ].
• Fruits: apples, pears, immature mangoes, immature palms, lemons, and fruit pulps such as banana [ 22 ].

3. Traditional Fermented Fruits and Vegetables in India

In eastern Himalayan regions of India a wide range of fermented vegetable products are prepared for bioprocessing the perishable vegetable for storage and further consumption [ 33 ]. Lactic acid fermentation vegetables such as gundruk, sinki, and khalpi are fermented vegetable product of Nepal, Sikkim, and Bhutan. Lactobacillus brevis , L. plantarum , Pediococcus pentosaceus , P. acidilactici , and Leuconostoc fallax are the predominant LAB involved in ethnic fermented vegetables. Predominant functional LAB strains associated with the ethnic fermented tender bamboo shoot products, mesu, soidon, soibum, and soijim of the Himalayas, were identified as L. brevis , L. plantarum , L. curvatus , P. pentosaceus , L. mesenteroides subsp. mesenteroides , L. fallax , L. lactis , L. citreum , and Enterococcus durans [ 33 ]. Some of the LAB strains may also possess protective and functional properties that render them as interesting candidates for use as starter culture(s) for controlled and optimized production of fermented vegetable products [ 34 ].

3.1. Gundruk

Gundruk is a nonsalted, fermented, and acidic vegetable product indigenous to the Himalayas. During fermentation of gundruk, fresh leaves of local vegetables known as rayosag ( Brassica rapa subsp. campestris var. cuneifolia), mustard leaves ( Brassica juncea (L.) Czern), cauliflower leaves ( Brassica oleracea L. var. botrytis L.), and cabbages ( Brassica sp.) are wilted for 1-2 days. Wilted leaves are crushed mildly and pressed into a container or earthen pot, made airtight and fermented naturally for about 15–22 days. After desirable fermentation, products are removed and sun-dried for 2–4 days. Gundruk is consumed as pickle or soup and has some resemblance with other fermented acidic vegetable products such as kimchi of Korea, sauerkraut of Germany, and sunki of Japan [ 36 ]. The predominant microflora of Gundruk includes various LAB such as L. fermentum , L. plantarum , L. casei , L. casei subsp. pseudoplantarum , and Pediococcus pentosaceus [ 33 , 35 ].

Sinki, an indigenous fermented radish tap root food, is traditionally prepared by pit fermentation, which is a unique type of biopreservation of foods by LA fermentation in the Sikkim Himalayas. For sinki production, a pit was dug with 2-3 ft diameter in a dry place. The pit is cleaned, plastered with mud, and warmed by burning. After removing the ashes, the pit is lined with bamboo sheaths and paddy straw. Radish tap roots are wilted for 2-3 days, crushed, dipped in lukewarm water, squeezed, and pressed tightly into the pit, covered with dry leaves and weighted down by heavy planks or stones. The top of the pit is plastered with mud and left to ferment for 22–30 days. After fermentation, fresh sinki is removed, cut into small pieces, sun-dried for 2-3 days, and stored at room temperature for future consumption [ 36 ]. Pit fermentation has been practiced in the South Pacific and Ethiopia for preservation of breadfruit, taro, banana, and cassava [ 37 ]. Sinki fermentation is carried out by various LAB including L. plantarum , L. brevis , L. casei , and Leuconostoc fallax [ 33 , 38 ].

3.3. Khalpi

Khalpi or khalpi is a fermented cucumber ( Cucumis sativus L.) product, commonly consumed by the Brahmin Nepalis in Sikkim. It is the only reported fermented cucumber product in the entire Himalayan region [ 36 ]. Ripened cucumber is cut into suitable pieces and sun-dried for 2 days, and then put into a bamboo vessel and made airtight by covering with dried leaves. It is fermented naturally at room temperature for 3–5 days. Fermentation after 5 days makes the product sour in taste. Khalpi is consumed as pickle by adding mustard oil, salt, and powdered chilies. Khalpi is prepared in the months of September and October. Microorganisms isolated from Khalpi include L. plantarum , L. brevis , and Leuconostoc fallax [ 10 , 33 ].

3.4. Inziangsang

In Northeast India, especially the people of Nagaland and Manipur consume Inziangsang, traditional fermented leafy vegetable product prepared from mustard leaves and similar to gundruk [ 36 ]. Preparation process of inziangsang is like of gundruk. Mustard leaves, locally called hangam ( Brassica juncea L. Czern), are collected, crushed, and soaked in warm water. Leaves are squeezed to remove excess water and pressed into the container and made airtight to maintain the anaerobic condition. The container is kept at ambient temperature (20°C–30°C) and allowed to ferment for 7–10 days. Like gundruk, freshly prepared inziangsang is sun-dried for 4-5 days and stored in a closed container for a year or more at room temperature for future consumption. Nagaland people consume inziangsang as a soup time with steamed rice. In resident meal, the fermented extract of ziang dui is used as a condiment. This fermentation is also supported by set of LAB which includes L. plantarum , L. brevis , and Pediococcus [ 10 , 33 ].

3.5. Soidon

Soidonis a widespread fermented product of Manipur prepared from the tip of mature bamboo shoots. Main source of fermentation is the tips or apical meristems of mature bamboo shoots ( Bambusa tulda , Dendrocalamus giganteus , and Melocanna bambusoides ). Outer casings and lower portions of the bamboo shoots were removed and whole tips are submerged in water in an earthen pot. The sour liquid (soijim) of a previous batch is added as starter in 1 : 1 dilution, and the preparation is covered. Fermentation was carried out for 3–7 days at room temperature. Leaves of Garcinia pedunculata Roxb. (family: Guttiferae), locally called heibungin in Manipuri language, may be added in the fermenting vessel during fermentation to enhance the flavor of soidon. After 3–7 days, soidon is removed from the pot and stored in a closed container at room temperature for a year. L. brevis , Leuconostoc fallax , and Lactococcus lactis take part in fermentation [ 10 , 39 ].

3.6. Goyang

Goyang, a prominent traditional fermented vegetable foodstuff of the Sikkim and Nepal, leafs of magane-saag ( Cardamine macrophylla Willd.), belonging to the family Brassicaceae, are collected, washed, cut into pieces, and then squeezed to drain off excess water and are tightly pressed into bamboo baskets lined with two to three layers of leaves of fig plants. The tops of the baskets are then covered with fig plant leaves and fermented naturally at room temperature (15°C–25°C) for 25–30 days. L. plantarum , L. brevis , Lactococcus lactis , Enterococcus faecium, and Pediococcus pentosaceus , yeasts Candida spp., were LAB isolated from goyang [ 40 ].

4. Traditional Fermented Fruits and Vegetables in Other Asian Countries

4.1. kimchi.

Kimchi is a Korean traditional fermented vegetable made from Chinese cabbage (beachu), radish, green onion, red pepper powder, garlic, ginger, and fermented seafood (jeotgal), which is traditionally made at home and served as a side dish at meals [ 41 ]. Kimchi is a generic term indicating a group of traditional LA fermented vegetables in Korea [ 42 ]. The major raw materials (oriental cabbage or radish) are salted after prebrining, blended with various spices (red pepper, garlic, green onion, ginger, etc.) and other minor ingredients (seasonings, salted sea foods, fruits and vegetables, cereals, fish, and meats, etc.), and then fermented at low temperature (2–5°C). Kimchi fermentation is temperature-dependent process. It ripens in one week at 15°C and took three days at 25°C. But low temperature is preferred in kimchifermentation to prevent production of strong acid, overripening, and extended period of optimum taste [ 43 ]. Kimchi is characterised particularly by its sour, sweet, and carbonated taste and differs in flavour from sauerkrautand pickles that are popular fermented vegetables [ 44 ]. The classical identification of bacterial isolates from kimchi revealed that Leuconostoc mesenteroides and Lactobacillus plantarum were the predominant species [ 41 ]. Several results suggested that LAB contributing to kimchi fermentation include L. mesenteroides , L. citreum , L. gasicomitatum , Lactobacillus brevis , L. curvatus , L. plantarum , L. sakei , L. lactis , P. pentosaceus , W. confusa , and W. koreensis [ 45 ]. Some important species thought to be responsible for kimchi fermentation are Leuconostoc mesenteroides , L. pseudomesenteroides , and L. lactis , as the pH gradually falls to 4.0 [ 41 , 42 ].

Kimchi contains various health-promoting components, including β -carotene, chlorophyll, vitamin C, and dietary fibre [ 43 ]. In addition, antimutagen [ 46 ], antioxidation, and angiotensin-converting enzyme inhibition activities of kimchi are thought to protect against disease [ 47 ]. Bacteria isolated from kimchi produce beneficial enzymes, such as dextransucrase and alcohol/acetaldehyde dehydrogenase [ 48 ]. Because of these beneficial properties, kimchi was nominated as one of the world's healthiest foods in a 2006 issue of Health Magazine [ 43 ]. Optimum taste of kimchiis attained when the pH and acidity reach approximately 4.0–4.5 and 0.5-0.6, respectively. Vitamin C content is maximal at this point.

4.2. Sauerkraut

Sauerkraut means sour cabbage. In sauerkraut fermentation, fresh cabbage is shredded and mixed with 2.3–3.0% salt before allowing for natural fermentation. Sauerkrautproduction typically relies on a sequential microbial process that involves heterofermentative and homofermentative LAB, generally involving Leuconostoc spp. in the initial phase and Lactobacillus spp. and Pediococcus spp. in the subsequent phases [ 42 ]. The pH of final product varies from 3.5 to 3.8 [ 49 ]. At this pH, the cabbage or other vegetables will be preserved for a long period of time [ 37 ]. Sauerkraut brine is an important byproduct of the cabbage fermentation industry and can be used as a substance for the production of carotenoids by Rhodotorula rubra or for β -glucosidase production by Candida wickerhamii for commercial applications [ 50 ].

4.3. Paocai

The most favored customary tableware of Chinese is Paocai, a lactic acid fermented vegetable with saltish palate. In certain places of China, the surplus vegetables such as cabbage, celery, cucumber, and radish were retained during superfluous season. Usually Paocai is served as an accompaniment with the chief meal and occasionally used as a Nipple. Paocai is a type of pickle, varies in terms of taste and method of preparation in different areas. Taiwanese paocai has crunchy texture and tangy taste, which is made with many kinds of vegetables, spices, and other ingredients by anaerobic fermentation in a special container. Paocai fermentation is initiated by various microorganisms presented in the raw materials, and LAB become the dominate bacterial finally. Lactobacillus pentosus , L. plantarum , L. brevis , L. lactis , L. fermentum , and Leuconostoc mesenteroides are the LAB isolated from paocai [ 36 , 51 ].

4.4. Yan-Dong-Gua

In Taiwan, the extensively used customary fermented nutriment is Yan-dong-gua, prepared using wax gourd. Harvested wax gourd is washed and sliced into little pieces, dried in sunlight, combined with salt, sugar, and fermented soybeans, and layered in a bucket. Usually, minor mass of Mijiu (Taiwanese rice wine) is mixed in the earlier stage of fermentation and the bucket was sealed. The time of fermentation process is for one month, but it may be elongated even more than two months. Yan-dong-gua is usually used as a seasoning for fish, pork, meatballs, and various other foods. Weissella cibaria and W. Paramesenteroides are the bacteria responsible for fermentation [ 52 ].

4.5. Tempoyak

Tempoyak is a traditional Malaysian fermented condiment made from the pulp of the durian fruit ( Durio zibethinus ). Salt is sometimes added to proceed fermentation at ambient temperature. Seeded durian is mixed with small amount of salt and left to ferment at ambient temperature in a tightly closed container for 4–7 days. The acidity of tempoyak was reported as approximately 2.8 to 3.6%. The sour taste of tempoyak is attributed to the acid produced by lactic acid bacteria (LAB) during fermentation. LAB were the predominant microorganisms including Lactobacillus brevis , L. mali , L. fermentum , L. durianis , Leuconostoc mesenteroides , and an unidentified Lactobacillus sp. [ 53 ].

4.6. Sayur Asin

Sayur asin is a fermented mustard cabbage leaf food product of Indonesia. A similar product, hum choy, is produced in China and other South East Asian countries. Mustard cabbage leaves ( Brassica juncea var. rugosa) are wilted, rubbed, or squeezed with 2.5%–5% salt. Liquid from boiled rice is added to provide fermentable carbohydrates to ensure that sufficient acid is produced during the fermentation. Fermentation was characterized by a sequential growth of the lactic acid bacteria, Leuconostoc mesenteroides , Lactobacillus confusus , Lactobacillus curvatus , Pediococcus pentosaceus , and Lactobacillus plantarum . Starch degrading species of Bacillus , Staphylococcus , and Corynebacterium exhibited limited growth during the first day of fermentation. The yeasts, Candida sake and Candida guilliermondii , contributed to the fermentation [ 54 ].

4.7. Salam Juice

Shalgam juice is prepared from the mixture of turnips, black carrot bulgur (broken wheat) flour, salt, and water by lactic acid fermentation. Shalgam is widely used in Turkey [ 55 ]. Shalgam juices were prepared by two methods for commercial production, which are the traditional and direct methods. Traditional method has two stages of fermentation that includes sour-dough fermentation (first fermentation) and carrot fermentation (second fermentation). The direct method has only second fermentation [ 56 , 57 ]. The shalgam juice fermentation was mainly carried out by LAB that belong to the genera Lactobacillus , Leuconostoc , and Pediococcus [ 58 , 59 ]. The LAB species predominantly include Lactobacillus plantarum , L. brevis , L. paracasei , L. buchneri , and Pediococcus pentosaceus [ 56 , 57 , 60 , 61 ].

4.8. Yan-Taozih

Yan-taozih (pickled peaches) is a popular pickled fruit in China and Taiwan. Fresh peaches ( Prunus persica ) are mixed with 5%–10% salt and then shaken gently until water exudes from the peaches. The peaches are then washed and mixed with 5%–10% sugar and 1%-2% pickled plums. All of the ingredients are mixed well and then allowed to ferment at low temperature (6–10°C) for 1 day. Chen et al. isolated Leuconostoc mesenteroides , L. lactis, Weissella cibaria , W. paramesenteroides , W. minor , Enterococcus faecalis, and Lactobacillus brevis from Yan-taozih [ 62 ].

4.9. Pobuzihi

Pobuzihi is a widely used traditional fermented food prepared with cummingcordia in Taiwan. Two types of Pobuzihi are mainly available that can be easily differentiated from the appearance of the final products. Caked or granular pobuzihi is prepared by boiling cummingcordia ( Cordia dichotoma Forst. f.) for several minutes and mixing it with salt. The caked pobuzihi is prepared by filling up the boiled cummingcordia into containers and after cooling removed from the containers. Chen et al. isolated novel Lactobacillus pobuzihii , L. plantarum , Weissella cibaria , W. paramesenteroides , and Pediococcus pentosaceus from fermented pobuzihi [ 63 , 64 ].

4.10. Nozawana-Zuke

Nozawana-zuke is a low-salt pickle prepared by using field mustard, locally called Nozawana ( Brassica campestris var. rapa), a leafy turnip plant. It is majorly consumed by Japanese people. The pickle is manufactured by lactic acid fermentation after adding various inorganic salts and red pepper powder containing spicy components to nozawana. The fermentation is achieved by various plant-derived genera of lactic acid bacteria (LAB), including Lactobacillus and Leuconostoc. These LAB contribute to generating the sensory properties of Nozawana zuke and preventing its contamination from disadvantageous bacteria by producing organic acids. The fermentation was carried out by Lactobacillus curvatus [ 65 ].

4.11. Yan-Jiang

Yan-jiangis a traditional fermented ginger widely used in Taiwan. It is prepared by two methods, such as with addition of plums and without addition of plums. The ginger ( Zingiber officinale Roscoe) was washed, shredded, mixed with salt (NaCl), and layered in a bucket for 2–6 h. After the exuded water is removed, the ginger is mixed with sugar, and pickled plums are added only in method P. Salt and sugar are added to a final concentration of approximately 30–60 g kg −1 . Fermentation usually continues for 3–5 days at low temperature (6–10°C), but some producers maintain a fermentation time of 1 week or even longer. Initial fermentation was carried out by Lactobacillus sakei and Lactococcus lactis subsp. Lactis and this species are replaced by Weissella cibaria and L. plantarum at the final stages of fermentation [ 66 ].

4.12. Yan-Tsai-Shin

Yan-tsai-shin is a fermented Broccoli ( Brassica oleracea ) stem, which is belonging to cabbage family. It is widely used in Taiwan. Harvested broccoli is washed, peeled, cut, mixed with salt (NaCl), and filled in a bucket for approximately 6 h. After the exuded water is removed, fermented broccoli is mixed with various ingredients, including sugar, soy sauce, and sesame oil. Some producers also add rice wine or sliced hot pepper to obtain a unique flavour. The ingredients were mixed well and then fermented at low temperature (6–10°C) for 1 day. The most common bacterial species include Weissella paramesenteroides , W. cibaria , W. minor , Leuconostoc Mesenteroides , Lactobacillus Plantarum , and Enterococcus sulphurous [ 67 ].

4.13. Jiang-Gua

Jiang-guais a popular traditional fermented cucumber in Taiwan that can be served as a side dish or a seasoning. Harvested cucumbers ( Cucumis sativus L.) are washed, cut, mixed with salt (NaCl), layered in a bucket, and then sealed with heavy stones on the cover. This process usually continues for 4-5 h, but some producers maintain a longer processing time. After the exuded water has been drained off, the cucumbers are mixed with sugar and vinegar. In addition, soy sauce is added optionally depending on the recipe. Fermentation usually continues for at least 1 day at low temperature (6–10°C). Fermentation depends upon Weissella cibaria , W. hellenica , L. Plantarum , Leuconostoc lactis , and Enterococcus casseliflavus [ 68 ].

5. Other Fermented Vegetables and Fruits

Pickles from various vegetables and fruits such as mango ( Mangifera indica L.) and amla ( Emblica officinalis L.) are dietary supplements and used for culinary purposes in several parts of the world. Pickling of cucumber is made in Africa, Asia, Europe, and Latin America [ 69 ]. Khalpi is a cucumber pickle popular during summer months in Nepal [ 27 ]. Although, a variety of methods are used, placing the cucumbers in 5% salt brine is a satisfactory method. The cucumbers absorb salt until there is equilibrium between the salt in the cucumbers and the brine (about 3% salt in the brine) [ 70 ]. When the pH attains at about 4.7–5.7, the brine is inoculated with either L. plantarum or Pediococcus pentosaceus or a combination of these organisms for a total cell count of 1–4 billion cells/gallon of brined cucumbers. The final product has an acidity of 0.6–1.0% (as LA) and a pH of 3.4–3.6 in about two weeks, depending upon the temperature [ 71 ]. Similarly, sweet potato lacto-pickles may serve as an additional source of pickle with usual beneficial probiotic properties [ 72 ].

Different varieties of onions ( Allium cepa ) such as sweet, white and yellow storage were used for LA fermentation. White and yellow storage onions are typically used for processing due to their high solid content, so they were chosen for fermentation. Sweet onions are a spring/summer variety with low solids and mild flavour and are often consumed fresh.

Sweet cherry ( Prunus avium L.) is one of the most popular of temperate fruits. Italy, together with United States, Iran, and Turkey, is one of the main world producers of sweet cherries [ 73 ].

The fermentation of beetroot and carrot juices, with addition of brewer's yeast autolysate, was also carried out by various workers like Rankin et al. A mixture of beetroot and carrot juices with brewer's yeast autolysate (fermented bio product) has optimum proportions of pigments, vitamins, and minerals. This balanced material represents a valuable product as far as nutrition and health are concerned [ 74 ]. Red beets were evaluated as a potential substrate for the production of probiotic beet juice by four species of lactic acid bacteria ( Lactobacillus acidophilus , L. casei , L. delbrueckii , and L. plantarum ).

Spontaneous cauliflower fermentation is commonly encountered in many countries with local variations depending mainly upon tradition and availability of raw materials. L. plantarum and Leuconostoc mesenteroides were isolated from the cauliflower fermentation [ 19 ].

The consumption of LA fermented vegetable juices (lacto-juice) has increased in many countries. Lacto-juices are produced mainly from cabbage, red beet, carrot, celery, and tomato [ 4 ]. They can be produced by either of the following procedures:

• usual way of vegetable fermentation and then processed by pressing the juice (manufacture from sauerkraut);
• fermentation of vegetable mash or juice.

There are three types of lactic fermentation of vegetable juices:

• spontaneous fermentation by natural microflora;
• fermentation by starter cultures that are added into raw materials;
• fermentation of heat-treated materials by starter cultures.

During the manufacture of lacto-juices, the pressed juice can be pasteurized at first and consecutively it is inoculated by a culture of selected LAB at a concentration varying from 2 × 10 5 to 5 × 10 6  CFU/mL [ 4 , 75 ]. For fermentation of juices of highest quality, it is imperative to use commercially supplied starter cultures such as L. plantarum , L. bavaricus , L. xylosus , L. bifidus , and L. brevis . The criteria used for finding out suitability of a strain are as follows [ 76 ]:

• the rate and total production of LA, change in pH, loss of nutritionally important substances;
• decrease in nitrate concentration and production of biogenic amines (BAs);
• ability of substrate to accept a starter culture;
• type of metabolism and ability of culture to create desirable sensory properties of fermented products.

6. Probiotic Microorganisms

6.1. lactic acid bacteria.

The genus Lactobacillus is a heterogeneous group of LAB with important application in food and feed fermentation. Lactobacilli are used as probiotics inoculants and as starters in fermented food [ 77 ]. The genus Lactobacillus is Gram-positive organisms which produce lactic acid by fermentation which belongs to the large group of LAB. Other genera such as Lactococcus , Enterococcus , Oenococcus , Pediococcus , Streptococcus , Leuconostoc , and Lactobacillus are also considered in LAB group due to lactic acid production ability [ 78 ].

The genus Lactobacillus is a heterogeneous group of LAB with important implications in food and feed fermentation. Lactobacilli are currently used as probiotics, silage inoculants, and as starters in fermented food [ 77 ]. The genus Lactobacillus belongs to the large group of LAB, which are all Gram-positive organisms which produce lactic acid by fermentation. Genera of LAB include, among others, Lactococcus , Enterococcus , Oenococcus , Pediococcus , Streptococcus , Leuconostoc , and Lactobacillus [ 78 ]. Lactobacillus is rod shaped, often organized in chain belonging to a large group within a family Lactobacillaceae. They grow well in anaerobic condition and strictly fermentative in nature. Lactobacillus is generally divided into two groups depending on the ability of the sugar fermentation: homofermentative species, converting sugars mostly into lactic acid and heterofermentative species, converting sugars into lactic acid, acetic acid and CO 2 . LAB can influence the flavour of fermented foods in a variety of ways. During fermentation, lactic acid is produced due to the metabolism of sugars. As a result, the sweetness tastes will likely decrease as sourness increases [ 76 ].

Lactobacilli prefer relatively acidic conditions ranges from pH 5.5 to 6.5 due to the main catabolite as lactic acid. It can be found in a wide ranges of ecological niches such as plant, animal, raw milks, and in insects [ 79 ]. Due to the wide verity in habitat Lactobacillus possess a wide range of metabolites versatility in the LAB group. It has been used for food preservation, starter for dairy products, fermented vegetables, fish, and sausages as well as silage inoculants for decades. Lactobacillus is proposed as potential probiotics due to its potential therapeutic and prophylactic attributes. L. paracasei , L. rhamnosus , and L. casei belong to the group of lactobacillus which are commonly found in food and feed as well as common inhabitants of the animal/human gastrointestinal tract (GIT) [ 80 ]. L. plantarum is considered a food-grade microorganism because of its long and documented history of safe use in fermented foods [ 81 ]. L. fermentum , one of the best-known species of this group, has been isolated from vegetable and dairy fermentation [ 77 , 80 , 82 ].

The Weissella species are Gram-positive, catalase negative, non-spore-forming, heterofermentative, nonmotile, irregular, or coccoid rod-shaped organisms [ 83 ]. Members of the genus Weissella have been isolated from a variety of sources, such as fresh vegetables and fermented silage [ 84 – 86 ]. The genus Weissella encompasses a phylogenetically coherent group of lactic acid bacteria and includes eight Leuconostoc -like species, including Weissella confuse (formerly Lactobacillus confuses ), W. minor (formerly Lactobacillus minor ), W. kandleri (formerly Lactobacillus kandleri ), W. halotolerans (formerly Lactobacillus halotolerans ), W. viridescens (formerly Lactobacillus viridescens ), W. paramesenteroides (formerly Leuconostoc paramesenteroides ), and W. hellenica [ 83 ].

6.2. Definition and Mechanism of Action of Probiotics

According to the Food and Agriculture Organization (FAO) Probiotics are defined as “living microorganisms which, when administrated in adequate amounts, confer health benefit on the host”. Many studies supported that maintenance of health gut microflora provides protection against gastrointestinal disorder including gastrointestinal infections and inflammatory bowel diseases. On the other hand, probiotics can be used as an alternative to the use of antibiotics in the treatment of enteric infection or to reduce the symptoms of antibiotic associated diarrhea [ 87 ]. Probiotic bacterial cultures support the growth of intestinal microbiota, by suppressing potentially harmful bacteria and reinforce the body's natural defence mechanisms. Currently, much evidence exists on the positive effects of probiotics on human health [ 77 , 88 – 91 ].

6.3. Selection and Application of Probiotics

Lactobacilli are the most extensively studied and widely used probiotics within the LAB. Most Lactobacillus strains belong to the L. acidophilus group. L. paracasei , L. plantarum , L. reuteri , and L. salivarius , which represent the respective phylogenetic groups, are known to contain probiotic strains. In order for a probiotic to be of benefit to human health, it must fulfil several criteria ( Figure 2 ). It must survive passage through the upper GIT and reach its site of action alive, and it must be able to function in the gut environment. The functional requirements of probiotics include tolerance to human gastric juice and bile, adherence to epithelial surfaces, persistence in the human GIT, immune stimulation, antagonistic activity toward intestinal pathogens (such as Helicobacter pylori , Salmonella spp., Listeria monocytogenes , and Clostridium difficile ), and the capacity to stabilize and modulate the intestinal microbiota [ 88 – 92 ].

Basic characteristics of selection of a probiotic strains.

7. Raw Materials Pretreatments

Pretreatments can promote growth of lactic flora that can be used depending on the fruit or vegetable to be fermented. Washing fruits and vegetables prior to fermentation reduces the initial microbial count, thus favouring the development of lactic flora [ 93 ]. Vegetables are also macerated with pectinolytic enzymes [ 75 ] to allow for their homogenization prior to lactic fermentation, mainly for the production of cocktails and juices [ 4 ]. Many vegetables contain glycosides that hamper efficient fermentation [ 94 ]. For LA fermentation of tomatoes, choosing very ripe fruit is recommended, since the high solanin content of unripe fruit might inhibit the growth of LAB.

8. Role of Ingredients Used in Fermentations of Fruits and Vegetables

8.1. addition of salt.

LA fermentation of fruits and vegetables is mostly carried out in a salted medium [ 95 ]. Salting is done by adding common dry salt (NaCl) with high water content or by soaking in brine solution. The optimum salt concentration depends on the type of vegetables or fruits [ 96 ]. Substituting NaCl by KCl up to 50% in the preparation of kimchi from cabbage did not affect the sensory qualities (saltiness, bitterness, sourness, hotness, and texture). The main role of salt is to promote the growth of LAB over spoilage bacteria and to inhibit potential pectinolytic and proteolytic enzymes that can cause vegetable softening and further putrefaction. Salt induces plasmolysis in the plant cells and the appearance of a liquid phase, which creates anaerobic conditions around the submerged product. Anaerobic conditions are more effective in the finely cut and shredded cut material.

8.2. Ingredients Favouring Bacterial Growth

Some ingredients when added to LA fermented vegetables or fruits seem to enhance the development of lactic flora. They have three major roles:

• they are a source of nutrients such as sugars, vitamins, and minerals which initiate fermentation;
• they add desirable aroma, flavour, and taste to the fermented product;
• they help in combating the spoilage bacteria by lowering the pH.

For some vegetables with low nutrient contents, such as turnip and cucumber, the addition of sugar promotes bacterial growth, thereby accelerating fermentation. In Spanish-style olive fermentation, olives have undergone alkaline treatment to eliminate their bitterness, followed by repeated washings. They are then replaced with glucose on sucrose to improve LA fermentation [ 71 ]. Whey is often recommended for use in traditional LA vegetable fermentation processes as it has high lactose content, which is a potential energy substrate for LAB. It also supplies minerals salts and vitamins necessary for the lactic flora metabolism.

8.3. Ingredients with Antiseptic Properties

Spices or aromatic herbs are added to most of the lactic fruits and vegetable fermentation to improve the flavour of the end products [ 21 ]. Certain spices, mainly garlic, cloves, juniper berries, and red chillies help to inhibit the growth of spoilage bacteria [ 22 ]. There are many sulphur compounds with antibacterial properties in garlic which must be combined with other vegetables at ratios not higher than 150 g/kg of vegetables. Chemical preservatives such ascorbic on benzoic acid salts are sometimes used in industrial production of LA fermented sauerkraut , olives, or cucumbers [ 69 ]. The role of essential spice oils such as thyme, sage, lemon, and dill is to inhibit the growth during fermentations of olives [ 70 ]. Mustard seed contains allyl isothiocyanate, a volatile aromatic compound with antibacterial and antifungal properties, which inhibits the growth of yeast ( Saccharomyces cerevisiae ) and promotes growth of LAB [ 69 , 70 ].

8.4. Ingredients Modifying the pH and Buffers Effect of Brines

To promote the growth of LAB over yeasts, moulds and other pathogenic or unwanted bacterial strains, acids, or buffer systems (acid + acid salts) are often added to the fermentation medium. During the fermentation of fruits and vegetables with high fermentable sugar contents, the fermentation medium has to be buffered to slow down acidification, thus allowing the LAB to consume all the sugars. An acetic acid + calcium acetate buffer system has been reported to improve the LA cucumber fermentation process.

9. Beneficial Effect of Fermented Fruits and Vegetables

9.1. enhancing food quality and safety.

Nutritional quality of food can be enhanced by fermentation, which may improve the digestibility and beneficial components of fermented food. The raw materials have increased the level of vitamin and mineral content compared to its initial content. Several antimicrobial compounds such as organic acids, hydrogen peroxide, diacetyls, and bacteriocins are produced during the fermentation process, which impacts unrequited bacterial growth and on the other hand increases the shelf life of the food.

Lactic acid content of fermented food product may enhance the utilization of calcium, phosphorus, and iron and also increase adsorption of iron and vitamin D. Fermented foods have a variety of enzymes and each enzyme can play a different role in increasing food quality. Lactase in fermented food product degrades the lactose into galactose. Galactose is an important constituent of cerebroside that can promote brain development in infants. Similarly proteinases produced by LAB can break down the casein into small digestible molecules. Fermented foods are rich in globular fats which can be easily digested.

9.2. Removal of Antinutrient Compounds

Most of the fruits and vegetables contain toxins and antinutritional compounds. These can be removed or detoxified by the action of microorganisms during fermentation process. Plant foods contain a series of compounds, collectively referred to as antinutrients, which generally interfere with the assimilation of some nutrients and in some cases may even confer toxic or undesirable physiological effects. Such antinutrients include oxalate, protease, and α -amylase inhibitors, lectins, condensed tannins, and phytic acid. Numerous processing and cooking methods have been shown to possibly reduce the amount of these antinutrients and hence their adverse effects. It has been concluded that the way food is prepared and cooked is equally important as the identity of the food itself. Research is currently focused on identifying the antinutrient effect of several constituents rather than studying their fate during lactic acid fermentation.

9.3. Improving the Health Benefits of Humans

Several researchers have described the beneficial effects of LAB. This can modify the intestinal microbiota positively and prevent the colonization of other enteric pathogens. LAB strains also improve the digestive functions, enhance the immune system, reduce the risk of colorectal cancer, control the serum cholesterol levels, and eliminate the unrequired antinutritional compounds present in food materials. The overall health benefits of LAB are elucidated in Figure 3 .

Beneficial effects of probiotics.

9.4. Biopreservation

Nowadays, consumers are particularly aware of the health concerns regarding food additives; the health benefits of “natural” and “traditional” foods, processed with no added chemical preservatives, are becoming more and more attractive. Chemical additives have generally been used to combat-specific microorganisms. In the case of fermented foods, lactic acid bacteria (LAB) have been essential for these millennia. LAB play a defining role in the preservation and microbial safety of fermented foods, thus promoting the microbial stability of the final products of fermentation. Protection of foods is due to the production of organic acids, carbon dioxide, ethanol, hydrogen peroxide, and diacetyl antifungal compounds such as fatty acids or phenyllactic acid, bacteriocins, and antibiotics such as reutericyclin [ 97 ].

The term “bacteriocin” was coined in 1953 to define colicin produced by Escherichia coli . Like LAB, also bacteriocins have been consumed for millennia by mankind as products of LAB and, for this reason, they may be considered as natural food ingredients. As reported by Cotter et al. “bacteriocins can be used to confer a rudimentary form of innate immunity to foodstuffs.” Bacteriocins are ribosomally synthesised, extracellularly released low molecular-mass peptides or proteins (usually 30–60 amino acids), which have a bactericidal or bacteriostatic effect on other bacteria, either in the same species (narrow spectrum) or across genera (broad spectrum) [ 97 – 99 ]. Bacteriocin production has been found in numerous species of bacteria, among which, due to their “generally recognized as safe” (GRAS) status, LAB have attracted great interest in terms of food safety. In fact, LAB bacteriocins enjoy a food grade and this offers food scientists the possibility of allowing the development of desirable flora in fermented foods or preventing the development of specific unwanted (spoilage and pathogenic) bacteria in both fermented and nonfermented foods by using a broad- and narrow-host-range bacteriocins, respectively.

Regarding the application of bacteriocin-producing starter strains in food fermentation, the major problem is related to the in situ antimicrobial efficacy that can be negatively influenced by various factors, such as the binding of bacteriocins to food components (fat or protein particles) and food additives (e.g., triglyceride oils), inactivation by proteases or other inhibitors, changes in solubility and charge, and changes in the cell envelope of the target bacteria [ 97 , 100 ]. The most recent food application of bacteriocins encompasses their binding to polymeric packaging, a technology referred to as active packaging. Bacteriocins have generally a cationic character and easily interact with Gram-positive bacteria that have a high content of anionic lipids in the membrane determining the formation of pores [ 97 ].

10. Modern Techniques Used for Analyzing Microflora of Fermented Fruits and Vegetables

In addition to traditional methods (microscopy, plate count, etc.), several modern techniques like RAPD- (Random Amplified Polymorphic DNA-) PCR (Polymerase Chain Reaction), species-specific PCR, multiplex PCR, 16s rDNA sequencing, gradient gel electrophoresis, RFLPs (Restriction Fragment Length Polymorphism), and cluster analysis of TTGE (Temporal Temperature Gradient Electrophoresis) are employed to isolate and characterize different type of LAB strains of fermented fruits and vegetables [ 101 ]. RFLPs and 16s rDNA were employed to isolate and characterize lactic acid bacteria from dochi (fermented black beans) and suan-tsai (fermented mustard), a traditional fermented food in Taiwan [ 102 ]. The isolated strains are L. plantarum , Salmonella enterica , E. coli , P. pentosaceus , Tetragenococcus halophilus , Bacillus licheniformis , and so on. Tamang [ 10 ] isolated 269 strains of LAB from gundruk, sinki, inziangsang (a fermented leafy vegetable), and Khalpi samples and studied the phenotypic characteristics of these strains by genotyping using RAPD-PCR, repetitive element PCR, and species-specific PCR techniques. The major representatives of LAB involved in these fermentations were L. plantarum , L. brevis , P. acidilactici, and L. fallax . RAPD-PCR and gradient gel electrophoresis were used to isolate L. plantarum strains from ben saalga, a traditional fermented gruel from Burkina Faso . MALDI-TOF mass spectrometry and DGGE analysis were also used to analyze the fermented vegetable samples [ 103 ]. Characterization of LAB isolates by using MALDI-TOF MS fingerprinting revealed genetic variability within highly heterogeneous species. Previous research investigated the genetic diversity of LAB isolates associated with the production of fermented Almagro eggplants using a combination of randomly amplified polymorphic DNA (RAPD) and pulsed-field gel electrophoresis (PFGE) [ 104 ].

11. Research Prospects and Future Applications

Even though it has been broadly verified that dairy fermented products are the best matrices for delivering probiotics, there is growing evidence of the possibility of obtaining probiotic foods from nondairy matrices. Several raw materials (such as cereals, fruits, and vegetables) have recently been investigated to determine their suitability for designing new, nondairy probiotic foods [ 115 ]. Generally existing probiotics belong to the genus Lactobacillus . However, few strains are commercially obtainable for probiotic function ( Table 1 ). Gene technology and relative genomics will play a role in rapid searching and developing new strains, with gene sequencing allowing for an increased thoughtful of mechanisms and the functionality of probiotics [ 77 , 116 ].

12. Conclusion

In Asian continent, fermented fruits and vegetables are associated with several social and cultural aspects of different races. Studies showed that fruits and vegetables may serve as a suitable carrier for probiotics. Fermented fruits and vegetables contain a diverse group of prebiotic compounds which attract and stimulate the growth of probiotics. Basic understanding about the relationship between food, beneficial microorganism, and health of the human being is important to improve the quality of food and also prevention of several diseases. The amount of food ingredients and additives in fermented foods, such as sugar, salt, and monosodium glutamate, should conform to the accepted standards established by the regulations of target markets. Mixed fermentation with high variability should be replaced by pure cultivation to achieve large-scale production. Although challenges remain, it is possible that fermented foods, handed down for many generations, will play a major role in the global food industry. Detailed studies on the microbial composition and characteristics of fermented fruits and vegetables lead to the further application.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.