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FERMENTED FOODS | Fermented Vegetable Products
Fermented Vegetable Products R Di Cagno and R Coda, University of Bari, Bari, Italy Ó 2014 Elsevier Ltd. All rights reserved. This article is a revision of the previous edition article by Guillermo Oliver, Martha Nuñez, Silvia Nelina Gonzalez, volume 2, pp. 739–744, Ó 1999, Elsevier Ltd.
Introduction Minimally processed fresh vegetables and fruits have a short shelf life because they are subjected to rapid microbial spoilage, and, in some cases, to pathogen microorganisms as a result of their contact with soil during cultivation and harvesting. Cooking, pasteurization, and other similar processes, as well as the addition of preservatives, are traditional technology options that may guarantee safe vegetables; however, these methods would bring about a number of not always desirable changes in the physical characteristics and chemical composition of vegetables. To decrease such drawbacks, some novel technologies are considered, including high-hydrostatic pressure processing, ionization radiation, pulsed-electric fields, new packaging systems, and the use of natural antimicrobial compounds. Among the various technological options, lactic acid fermentation, as the traditional biopreservation method for the manufacture of finished and half-finished foods, may be considered to be a simple and valuable biotechnology for maintaining or improving the safety, nutritional, sensory, and shelf-life properties of vegetables and fruits. Lactic acid fermentation has an industrial significance that goes beyond simple preservation, and it is used to improve and develop characteristic sensory and nutritional properties, to enhance digestibility, to destroy undesirable components, to convey probiotics, and to develop new products. However, most of the fermented vegetables undergo a spontaneous fermentation, which may be responsible for undesirable variations of the sensory properties or may occur too slowly to inhibit spoilage and pathogen microorganisms. In addition, selection of starter cultures within the autochthonous microbiota of vegetables and fruits should be recommended because autochthonous cultures may ensure better performance compared with allochthonous strains. On the basis of these considerations, during the past decade, lactic acid fermentation of vegetables and fruits generated an increasing interest.
Microbiota of Raw Vegetables and Fruits Each particular type of vegetable provides a unique environment in terms of type, availability, and concentration of substrate, buffering capacity, competing microorganisms, and perhaps natural plant antagonisms. Nevertheless, analyses by molecular methods showed that each species of vegetables and fruits harbors a dominant and constant microbiota. The cell density of the microbial groups may vary depending on the plant species, temperature, and harvesting conditions. The microbiota of fruits is essentially represented by yeasts and fungi, which can cause discoloration and generate unpleasant odors and flavors and, in extreme cases, synthesize compounds that are toxic to the consumer. Inhibition of growth of yeasts is one of the main objectives for minimally processed products
Encyclopedia of Food Microbiology, Volume 1
based on fruits. Because of their faster growth, yeasts generally anticipate the colonization by fungi. Overall, the microbial population of vegetables and fruits is estimated to fluctuate between 5 and 7 log cfu g1. The number of yeasts may range between 2 and 6 log cfu g1. Dominant yeasts, such as Cryptococcus spp., Candida spp., Saccharomyces spp., and Rhodotorula spp. were found in banana; Hansenula, Kloeckera, Candida, Pichia, and Saccharomyces spp. were found in cocoa; Saccharomyces cerevisiae, Candida krusei, and Debaryomyces hansenii were found in melon pod; Pichia guilliermondii and Hanseniaspora uvarum were found in pineapple, and Candida, Cryptococcus, Kloeckera, Rhodotorula, and Kluyveromyces spp. were found in other tropical fruits (e.g., pitanga, umbu, and acerola).
Taxonomic Structure of Lactic Acid Bacteria Microbiota of Vegetables and Fruits Overall, lactic acid bacteria are a small part (2–4 log cfu g1) of the autochthonous microbiota of raw vegetables and fruits, and their cell density is mainly influenced by the species of vegetables, temperature, and harvesting conditions. Recently, the lactic acid bacteria microbiota of raw carrots, marrows, and French beans was characterized. The raw vegetables used in this study harbored autochthonous lactic acid bacteria at cell densities of w2.7–3.0 log cfu g1. The following species were identified for each vegetable: carrots, Leuconostoc mesenteroides, Lactobacillus plantarum, and Weissella soli/W. koreensis; French beans, Enterococcus faecalis, Pediococcus pentosaceus, and Lactobacillus fermentum; and marrows, L. plantarum. Carrots and French beans had the most heterogeneous composition of autochthonous lactic acid bacteria, and L. plantarum was the only species found in marrows. In fact, L. plantarum is considered as an ubiquitous and metabolic versatile bacterium largely found in fruits and vegetables. Leuconostoc spp., including Leuc. mesenteroides subsp. mesenteroides, mainly dominated the early spontaneous fermentation of carrots. Pediococci and lactobacilli, mainly L. plantarum, were identified in many raw vegetables and, especially, in fermented vegetable juices. L. plantarum, Weissella cibaria/confusa, Lactobacillus brevis, P. pentosaceus, Lactobacillus spp., and Enterococcus faecium/faecalis were identified in raw tomatoes, and Lactobacillus curvatus, Leuc. mesenteroides, L. plantarum, and W. confusa were identified in red and yellow peppers. Several species of lactic acid bacteria, such as L. fermentum, L. brevis, and E. faecalis were identified in melon pod. Pineapple fruits harbored autochthonous lactic acid bacteria belonging to L. plantarum and Lactobacillus rossiae at cell densities of w5.5 log cfu g1. Several cultivars of sweet cherry fruits harbored a low population of lactic acid bacteria at a cell density lower than that usually found in other fruits and vegetables (w2.5 log cfu g1). L. plantarum, Pediococcus acidilactici, P. pentosaceus, and Leuc. mesenteroides subsp. mesenteroides were the only species identified in the eighth cultivars of sweet cherry. A similar trend was also found for other fruits,
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Table 1 Lactic acid bacteria isolated from raw or spontaneously fermented vegetables and fruits Lactic acid bacteria species
Source
Lactobacillus plantarum
Tomatoes, marrows, carrots, cucumbers, eggplants, sauerkraut, red beets, capers, kimchi, pineapple, plums, kiwi, papaya, fennels, cherries, cabbages, grape must Capers, papaya, eggplants, cucumbers Pineapple French beans, red beets, capers, eggplants Peppers, sauerkraut, kimchi Kimchi Tomatoes, sauerkraut, capers, eggplants, cabbages, cucumbers, grape must Cider, cabbages, capers Cider Cider White cabbages, carrots, peppers, cucumbers, eggplants, lettuce, sauerkraut, kimchi, cherries, grape must Carrots Peppers, tomatoes, blackberries, papaya
Lactobacillus pentosus Lactobacillus rossiae Lactobacillus fermentum Lactobacillus curvatus Lactobacillus sakei Lactobacillus brevis Lactobacillus paraplantarum Lactobacillus collinoides Lactobacillus casei Leuconostoc mesenteroides subsp. mesenteroides
Weissella soli Weissella confusa, Weissella cibaria Enterococcus faecalis, Enterococcus faecium Oenococcus oeni Pediococcus pentosaceus Pediococcus parvulus
French beans, tomatoes, capers Cider, grape must French beans, tomatoes, cucumbers, sauerkraut, capers, cherries, cabbages Cider
Adapted from Ciafardini, Di Cagno, 2012. Olive da mensa ed altri prodotti vegetali. In: Farris, A., Gobbetti, M., Neviani, E., and Vincenzini, M. (Eds.), Microbiologia dei prodotti alimentari, CEA – Casa Editrice Ambrosiana, Milan, Italy, pp. 365–382, ISBN: 978-8808-18246-3.
such as blackberries, prunes, kiwifruits, and papaya, where L. plantarum, Lactobacillus pentosus, and W. cibaria were the only species identified. The same species were commonly identified within the microbiota of cucumber, olives, pumpkins, peppers, carrots, persimmons, and eggplants when spontaneously fermented. Examples of lactic acid bacteria isolated from raw or spontaneously fermented vegetables and fruits are shown in Table 1.
Fermentation of Vegetable Products Spontaneous Fermentation In raw vegetables and fruits, lactic acid fermentation may take place spontaneously, when anaerobic conditions, water activity, moisture, salt concentration, and temperature are favorable to the growth of the autochthonous lactic acid bacteria. Spontaneous fermentation may be optimized through back slopping, commonly used for sauerkraut production (i.e., inoculation of the raw material with a small quantity of a previously performed successful fermentation). Hence, back slopping results in dominance of the best adapted strains and represents a way of using a selected starter culture to shorten the fermentation and to reduce the risk of fermentation failure. When occurring spontaneously, the fermentation of vegetables
and fruits is frequently characterized by a succession of heteroand homo-fermentative lactic acid bacteria, together with or without yeasts, which are responsible for multistep fermentation processes. The spontaneous lactic acid fermentation of raw vegetables may also require some days (4–6 days) before the pH value undergoes a significant decrease. Consequently, in some cases, spontaneous fermentation may be responsible for undesirable variations of the sensory and rheological properties of fresh vegetables and fruits or it may occur too slowly to inhibit spoilage and pathogen microorganisms. Both from hygiene and safety point of views, the use of starter cultures is recommended, as it would lead to a rapid inhibition of spoilage and pathogenic bacteria, and to a processed product with consistent sensory and nutritional properties. The use of starter cultures is increasing in vegetable fermentation. Contrarily, to other fermented foods (e.g., dairy, meat, and baked goods), only few cultures are used for fruit and vegetable fermentations, with L. plantarum being the most frequently used.
Commercial or Allochthonous Lactic Acid Bacteria Starter Cultures The use of commercial starter cultures was considered a breakthrough in the processing of fermented vegetables and fruits, resulting in a high degree of control over the fermentation process and standardization of the product. Some examples of the use of commercial lactic acid bacteria starters may be found in the literature. The controlled fermentation of peeled and blanched garlic using a starter culture of L. plantarum LP91 in comparison with unblanched started garlic was studied. The effect of the starter on the sensory properties of fermented garlic was also evaluated. Starter grew well in blanched garlic (w9 log cfu g1 after 2 days of fermentation) and lactic acid was the main synthesized metabolite. On the contrary, its growth was inhibited in unblanched garlic, and ethanol, fructose, and the appearance of an undesired green pigment were found. From a sensory point of view, started and unstarted garlic did not show significant differences. However, in most of the cases commercial starter cultures do not correspond to autochthonous strains, whereas, the selection of starter cultures within the autochthonous microbiota of vegetables and fruits should be recommended because autochthonous cultures may ensure better performance compared with allochthonous strains. Commercial or allochthonous starter cultures may exhibit different limitations, such as low flexibility with regard to the desired properties and functionality of the product and low diversity of metabolic activities.
Autochthonous Starter Although lactic acid bacteria represent a small part of the microbiota of vegetables and fruits, they may have various functions: (1) to exert intrinsic antagonistic activity toward spoilage and pathogen microorganisms, (2) to deliver health relevant microorganisms to the gastrointestinal tract, and (3) to supply autochthonous lactic acid bacteria suitable to be reused as starters. Recently, it was shown that the use of selected autochthonous lactic acid bacteria starters compared with allochthonous or spontaneous fermentation guaranteed the
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4.5
4.4
10
4.3
4.2
pH
log cfu g –1
8
6
4.1 4
4.0
3.9
2 0
2
4
6
8
10
12
14
16
18
Time (h)
Tomato juice without starter inoculum (kinetic of acidification) Tomato juice without starter inoculum (kinetic of growth) Tomato juice fermented with the autochthonous starter Lactobacillus plantarum POM35 (kinetic of acidification) Tomato juice fermented with the autochthonous starter Lactobacillus plantarum POM35 (kinetic of growth) Tomato juice fermented with the allochthonous starter Lactobacillus plantarum LP54 (kinetic of acidification) Tomato juice fermented with the allochthonous starter Lactobacillus plantarum LP54 (kinetic of growth)
Figure 1 Kinetics of growth and acidification of tomato juice fermented with autochthonous and allochthonous starters. Adapted from Ciafardini and Di Cagno. Olive da mensa ed altri prodotti vegetali. In: Farris, A., Gobbetti, M., Neviani, E., and Vincenzini, M. (Eds.), Microbiologia dei prodotti alimentari, CEA – Casa Editrice Ambrosiana, Milan, Italy, pp. 365–382, ISBN: 978-8808-18246-3.
prolonged shelf life of fermented vegetables and fruits and maintained agreeable nutritional, rheological, and sensory properties. Autochthonous strains always had better performances than allochthonous strains. Allochthonous strains showed, in particular, longer latency phases of growth and acidification with respect to selected autochthonous strains. Figure 1 shows the representative kinetics of growth and acidification of unstarted and fermented tomato juice with autochthonous and allochthonous strains of L. plantarum. When fermented with selected autochthonous strains of L. plantarum, Leuc. mesenteroides, and P. pentosaceus, carrots, French beans, or marrows were characterized by rapid decrease of pH, marked consumption of fermentable carbohydrates, and inhibition of Enterobacteriaceae and yeasts. Autochthonous strains of L. plantarum, L. curvatus, and W. confusa were used as mixed starters to ferment red and yellow peppers. Compared with unstarted vegetables, the rapid decrease of pH and the marked consumption of fermentable carbohydrates inhibited the growth of enterobacteria and yeasts. After 30 days of storage at room temperature, started vegetables positively differentiated also for higher firmness and color indexes with respect to unstarted red and yellow peppers. Recently, a protocol was set up for the minimal processing of pineapple to increase its shelf life and to maintain agreeable sensory and
nutritional features. After 30 days of storage at 4 C, pineapple fruits started with selected autochthonous strains of L. plantarum and L. rossiae had a number of lactic acid bacteria up to 1 000 000 times higher than the other processed pineapples as well as the lowest number of yeasts. The highest antioxidant activity and firmness, better preservation of the natural colors, and preference for odor and overall acceptability were also found. Fermentation of sweet cherry (Prunus avium L.) puree added by stem infusion by selected autochthonous lactic acid bacteria starters showed that, despite the hostile environment (e.g., low pH, presence of phenolic compounds), the selected autochthonous strains of P. pentosaceus and L. plantarum used as starters grew well, showing peculiar metabolic traits. Consumption of carbohydrates and the lactic acid fermentation was limited, whereas consumption of organic acids (e.g., malic acid) and free amino acids was evident, especially, throughout storage. Both lactic acid bacteria remained viable during 60 days of storage at cell numbers that exceeded those of potential probiotic beverages, presumably on the basis of these metabolic activities. Overall, the use of autochthonous lactic acid bacteria starters to ferment vegetables and fruits ensured better preservation of different properties: natural colors, firmness, antioxidant activities, and other health-promoting
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Raw vegetables and fruits
Table 2 Main metabolic traits to consider for the selection of starters for vegetable fermentation Criteria
Isolation, typing, and identification of autochthonous lactic acid bacteria
Pro-technological
Selection of single or mixed starters
Processing into puree, juices, or pieces eventually followed by a thermal treatment (e.g., 70 °C for 5 min)
Inoculum of the starter at a final cell density of ca. 7 log cfu g–1
Lactic acid fermentation (25–30 °C for 15–24 h)
Storage under refrigeration up to 40 days
Figure 2 Example of biotechnology protocol to ferment raw vegetables and fruits. Adapted from Ciafardini and Di Cagno. Olive da mensa ed altri prodotti vegetali. In: Farris, A., Gobbetti, M., Neviani, E., and Vincenzini, M. (Eds.), Microbiologia dei prodotti alimentari, CEA – Casa Editrice Ambrosiana, Milan, Italy, pp. 365–382, ISBN: 978-8808-18246-3.
compounds. This effect may be the consequence of modification of the profile of organic acids and of the synthesis of lactic and acetic acids and free amino acids. These modifications might have had direct (pH) or indirect (redox potential) repercussions on the activity of endogenous browning enzymes, oxidation, and sensory properties (color, flavor, and aroma) of vegetables and fruits. An example of biotechnology protocol to ferment raw vegetables and fruits is reported in Figure 2. The main criteria for selecting starters to be used for vegetable fermentation are the (1) rate of growth, (2) rate and total production of acids which, in turn, affect the changes of pH, and (3) environmental adaptation and tolerance. Major metabolic traits to be considered for the selection of autochthonous lactic acid bacteria starters for vegetables and fruits fermentation are shown in Table 2. The criteria for selection of starters may be divided into three main categories: (1) technological, (2) nutritional, and (3) sensory. Predominance of
Sensory Nutritional
Metabolic traits Velocity of growth Velocity of acidification Salt tolerance Low pH values tolerance Capacity of growth at low pH values Completeness of fermentation Developing of malolactic fermentation Mild acid producing or fermentation Tolerance to high concentration of polyphenolic compounds Synthesis of antimicrobial compounds Synthesis of exopolysaccharides Pectinolytic activity Heterofermentative metabolism Synthesis of aroma precursor compounds Synthesis of biogenic compounds Synthesis of exopolysaccharides
Adapted from Ciafardini and Di Cagno. Olive da mensa ed altri prodotti vegetali. In: Farris, A., Gobbetti, M., Neviani, E., and Vincenzini, M. (Eds.), Microbiologia dei prodotti alimentari, CEA – Casa Editrice Ambrosiana, Milan, Italy, pp. 365–382, ISBN: 978-8808-18246-3.
growth by a species of lactic acid bacteria is influenced by the chemical and physical environment in which it has to compete. L. plantarum, which predominates the later stage of vegetable fermentation because of its high acid tolerance and metabolic versatility, seems a likely choice when homolactic fermentation is desired. Moreover, robustness of autochthonous starters throughout fermentation and storage processes, able to achieve high cell numbers (w8.0–9.0 log cfu g1), is an indispensable prerequisite to ensure hygiene, safety, and potential probiotic properties of the product. The synthesis of exopolysaccharides is another metabolic tract to be considered for selection, especially for juices and puree. In addition, the capacity of lactic acid bacteria to synthesize protopectinases, which may enhance the viscosity of fruit matrices, may be an important characteristic for starters. Growth and viability of lactic acid bacteria, in particular L. plantarum and pediococci, were frequently shown on plant materials when polyphenolic compounds were abundant. L. plantarum had the metabolic capacity to degrade some phenolic compounds and other chemical compounds strictly related. Starter cultures with probiotic properties were largely considered in these last years, especially for the production of nondairy probiotic foods. More than other food ecosystems, raw fruits and vegetables possess intrinsic chemical and physical parameters that, for some traits, mimic those of the human gastrointestinal tract. The extremely acidic environment, buffering capacity, high concentration of indigestible nutrients (e.g., fiber, inulin, and fructooligosaccharides (FOS)) and antinutritional factors (e.g., tannins and phenols) are the main characteristics of raw fruits and some vegetables. In most of the cases, the autochthonous microbiota of fruits and vegetables has to colonize and adhere to surfaces, and exerts antagonistic activity toward spoilage and pathogenic microorganisms. A large number of autochthonous lactic acid bacteria isolated from carrots, French beans, cauliflower, celery, tomatoes,
FERMENTED FOODS j Fermented Vegetable Products pineapples, kimchi, several ethnic fermented vegetables of the Himalayas, and Japanese pickles belonging to the genera Lactobacillus, Weissella, Pediococcus, Enterococcus, and Leuconostoc were characterized for their probiotic potential, showing their adaptability to gastrointestinal environment, their capacity to maintain high cell densities, adherence to human or mouse intestinal cells, stimulation of immune mediators, and antimicrobial activity.
Main Fermented Vegetable Products Lactic acid fermentation of vegetables has an industrial significance only for cabbages, cucumbers, and olives. In the Mediterranean area, the industrial production of fermented vegetables is mainly limited to sauerkraut and table olives. Pickled cucumbers are among the most popular pickled foods. Additionally, niche markets exist for other pickled vegetables, which are fermented only slightly before consumption and several varieties of vegetables (e.g., artichokes, capers, garlic, peppers, okra, cauliflower, green tomatoes, and eggplants) are subjected to fermentation at the local level. A list of traditional fermented vegetable products from different regions of the world is reported in Table 3. Because of the commercial significance and extensive literature already available, table olives are not considered in this review.
Sauerkraut Sauerkraut is a vegetable food widely consumed in many European countries. It has usually been prepared by spontaneous lactic fermentation of shredded cabbage (Brassica oleracea L. variety capitata), both by manufacturers and in households. Fresh cabbage is trimmed of outer leaves and shredded and successively mixed with salt to obtain a final concentration of approximately 2% NaCl (wt/wt). The cabbage is quickly surrounded by brine and then covered with plastic sheeting draped over the tank. Water is added to improve anaerobic conditions and prevent contact with air, which may cause a loss of microbiological quality. After 24–48 h, anaerobic conditions are established thanks to the exhaustion of oxygen and CO2 production from heterofermentative lactic acid bacteria. The combination of salt concentration and temperature (18 C) allows a spontaneous lactic acid fermentation, although variations of these conditions are not uncommon and affect the competitiveness of the naturally present microorganisms. The microbial species typically isolated during sauerkraut fermentation are Leuc. mesenteroides, P. pentosaceus, L. brevis, and L. plantarum. Leuc. mesenteroides and Weissella spp. typically dominate the early stages of fermentation because they are present in larger cell numbers and have faster growth compared with the other lactic acid bacteria in cabbage juice. The increase of lactic acid concentration inhibits the multiplication of Leuc. mesenteroides while it promotes the growth of acid-tolerant species, such as L. brevis and in some cases L. curvatus, Lactobacillus sakei, E. faecalis, Lactococcus lactis subsp. lactis, and P. pentosaceus. L. plantarum becomes predominant in the latter stage, about 5–7 days after the beginning of fermentation, contributing to a further decrease of the pH value (w3.5). The end products resulting from both the stages of fermentation
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may include mannitol and acetic acid (w1% each) and lactic acid, which may exceed 2%, depending on how long the homolactic fermentation is allowed to continue.
Kimchi Kimchi is a name for various Korean traditional closely related fermented vegetable products. Kimchi is similar to sauerkraut and contains mainly Chinese cabbages (Brassica pekinensis) and radish, but other ingredients such as garlic, green onion, ginger, red pepper, mustard, parsley, and carrot may be added. After soaking with water, vegetables are cut and placed in a salt solution of 5–7% for 12 h or 15% for 3–7 h in order to increase the salt content (w2.0–4.0% of the total weight). Then vegetables are rinsed several times with fresh water and drained. Kimchi fermentation is carried out by various autochthonous microorganisms; thus, microbiota of kimchi can vary on the basis of ingredients, but it is also affected by temperature (ranging from 5 to 30 C) and salt concentration. Among the 200 bacteria isolated from kimchi, Leuc. mesenteroides and P. pentosaceus were those mainly involved in the first stage of fermentation. The dominant species of Lactobacillus in the later stages of kimchi fermentation vary according to the fermentation temperature; L. plantarum and L. brevis dominated fermentations carried out at 20–30 C, whereas Lactobacillus maltaromicus and Lactobacillus bavaricus dominated at 5–7 C. The composition of microbiota affects the kinds of acid synthesized and the final taste of kimchi. The increase of lactic acid inhibits the growth of Leuc. mesenteroides and promotes the development of acid-tolerant species, such as L. brevis and in some cases L. curvatus, L. sakei, and E. faecalis. The final stage of fermentation is dominated by L. plantarum, which allows a further decrease of the pH and lactic acid synthesis. The best tasting kimchi is obtained before overgrowth of L. plantarum and L. brevis, at an optimal pH of 4.5. The concentration of NaCl (w3%, wt/vol) and a temperature of w10 C are the optimal parameters to reach a level of lactic acid in the range 0.4–0.8% and a pH of 4.2–4.5. Kimchi was recognized as a health-promoting food, thanks to the physiological effects of its ingredients and end-products of the fermentation. Because of its nutritional properties, kimchi was mentioned by Health magazine in its list of the top-five ‘World’s Healthiest Foods’ (http://eating.health.com/2008/02/01/worlds-healthiest-foodskimchi-korea/) as one of the most popular functional foods in the world. Leuc. mesenteroides was found to be important in the early stage of fermentation of many other vegetable-based foods, such as Dhamuoi, a Vietnamese food similar to kimchi and the Turkish Tur¸s¸u, made with a wide variety of different vegetables and fruits. To manufacture Tur¸s¸u, vegetables such as cucumbers, cabbages, green tomatoes, green peppers, and fruits such as melons are pressed in to containers and added to a brine containing 10–15% of NaCl (wt/vol). After vinegar addition, the pickles are left to ferment at w20 C for 4 weeks. The species mainly associated with tur¸s¸u fermentation are L. plantarum, Leuc. mesenteroides, L. brevis, P. pentosaceus, and E. faecalis. After a first predominance of Leuc. mesenteroides, the fermentation is continued mostly by L. plantarum, followed in the latter stage by yeasts, such as Torulopsis, Hansenula, and Saccharomyces. Carrots, turnips, bulgur flour, and sourdough are the ingredients of S¸algam, a lactic acid fermented beverage very
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Examples of traditional fermented vegetable products from different regions of the world
Product
Main ingredients
Main lactic acid bacteria involved
Country
Sauerkraut Cucumbers Capers
Cabbage, salt Cucumbers, vinegar, salt Capers, water, salt
Kimchi
Cabbage, radish, salt, spices, and other vegetables (ginger, pepper, garlic, onion) Cabbage and other vegetables Mustard Mustard leaf, salt Local cabbages, mustard leaves, cauliflower leaves, Radish roots Cucumber Mustard leaves Cucumbers, cabbage, green tomatoes, green peppers, and other vegetables Black/violet carrots, turnip, bulgur flour, sourdough, salt, and water Red grape juice, black mustard seeds
Leuconostoc mesenteroides, Lactobacillus brevis, Lactobacillus plantarum Pediococcus pentosaceus, L. plantarum L. plantarum, Lactobacillus pentosus, Lactobacillus fermentum, L. brevis, Lactobacillus paraplantarum, Enterococcus faecium, P. pentosaceus Leuc. mesenteroides, Leuc. pseudomesenteroides, L. plantarum, L. brevis, Lactobacillus curvatus, Lactobacillus sakei Leuc. mesenteroides, L. plantarum L. brevis, Pediococcus spp. L. plantarum L. plantarum, L. casei subsp. casei, L. casei subsp. pseudoplantarum, L. fermentum, P. pentosaceus L. fermentum, L. plantarum, L. brevis, Leuc. fallax L. plantarum, L. brevis, Leuc. fallax, Pediococcus spp. L. brevis, L. plantarum L. plantarum, Leuc. mesenteroides, L. brevis, P. pentosaceus, E. faecalis L. plantarum, L. paracasei subsp. paracasei, L. fermentum, L. brevis
Europe, USA USA, Asia Mediterranean countries (Greece, Italy, Spain, Turkey, Morocco) Korea
L. paracasei subsp. paracasei, L. casei subsp. pseudoplantarum, L. pontis, L. brevis, L. acetotolerans, L. sanfranciscensis.
Turkey
Dhamuoi Burong mustasa Dakguadong Gundruk Sinki Khalpi Pak-Gard-Dong Tur‚s¸u S¸algam Hardaliye
Vietnam Philippines Thailand Eastern Himalaya Eastern Himalaya Eastern Himalaya Thailand Turkey Turkey
Adapted from Ciafardini and Di Cagno. Olive da mensa ed altri prodotti vegetali. In: Farris, A., Gobbetti, M., Neviani, E., and Vincenzini, M. (Eds.), Microbiologia dei prodotti alimentari, CEA – Casa Editrice Ambrosiana, Milan, Italy, pp. 365–382, ISBN: 978-8808-18246-3.
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Table 3
FERMENTED FOODS j Fermented Vegetable Products popular in Turkey. The manufacture of this beverage involves two steps of fermentation; in the first step, bulgur flour, sourdough, and salt are mixed with water and left to ferment by lactic acid bacteria and yeasts for 3–5 days at room temperature. In the second step, vegetables and water are added to the fermented product of the first step and fermentation takes place in wooden barrels for 3–10 days. S¸algam fermentation seems dominated mainly by L. plantarum, Lactobacillus paracasei subsp. paracasei, L. fermentum, and L. brevis. Yeasts such as S. cerevisiae are also known to contribute to flavor development.
Pickles Pickles are often produced by lactic acid fermentation of vegetables and fruits. Industrial-scale processes for the manufacture of many types of pickles were developed, but they are still carried out even at the domestic scale, although so far olives, cucumbers, and cabbages are the only one fermented in large volumes.
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especially Greece, Italy, Turkey, Morocco, and Spain. Fermentation of caper fruits is often done using traditional artisanal ways. The fruits are collected during the months of June and July and are immersed in tap water, where the fermentation takes place for approximately 5–7 days in a temperature range from 23 to 43 C. Successively, fermented capers are placed in brine and distributed for consumption. Fermentation of caper fruits, like other vegetable fermentation, is carried out by spontaneous lactic acid bacteria colonizing the raw material and processing environment. The predominance of L. plantarum in caper fermentation is important for the rapid production of lactic acid and fast acidification, allowing better preservation conditions of the fermented product. Overall, Leuconostoc species do not take part in caper fermentation. In contrast with other vegetable fermentations, the absence of Leuconostoc was attributed to the rapid decrease of pH, because this bacterium is not able to grow below pH 4.5, and to the high temperature of fermentation, which can reach 40 C.
Minor Fermentations and Traditional Products Cucumbers Fully ripe cucumbers are washed and drained and eventually sliced. Manufacturing usually consists of four stages: (1) selection of regular-shaped cucumbers, (2) dipping into brine (5–7% of NaCl) inside plastic or glass containers, (3) spontaneous lactic acid fermentation, and (4) packaging. Significant reductions in salt concentration (to 4% or less) may be possible using blanched cucumbers to reduce the initial microbiota. Sometimes, calcium chloride is added on the surface to allow a crisp texture during storage. As soon as the brine is produced, fermentation starts and lasts for 2–3 weeks, depending on the temperature (usually 20–27 C) until a final pH in the range 3.1–3.5 is reached. Homofermentative lactic acid bacteria, such as P. pentosaceus and L. plantarum, are the main bacteria responsible for fermentation, while Leuc. mesenteroides is inhibited by high concentrations of sodium chloride. Maintenance of structural integrity of whole cucumbers during brine fermentation is important for the quality of the product. However, CO2 can be produced as the result of cucumber respiration when they are submerged in brine and by malolactic fermentation carried out by L. plantarum. Because gaseous spoilage (bloater damage) may lead to serious economic losses, cucumbers are purged with air during the fermentation period to remove CO2 from the tank, even if this can increase the risk of molds and yeasts development. Currently, industrial fermentations are carried out by spontaneous lactic acid bacteria, and starter cultures of L. plantarum are rarely employed. A product made of fermented cucumbers named Khalpi is popular in Himalayan region. Cucumbers for manufacture of Khalpi are cut into pieces and sun dried for 2 days. After this step, cucumbers are put into a bamboo vessel and then left to ferment at room temperature for 3–7 days. The microbiota of Khalpi is characterized mainly by L. plantarum and L. brevis. Pediococci and Leuconostocs species, such as Leuc. fallax, were also found.
Capers Caper berries are the fruits of Capparis species (mainly Capparis spinosa L.), a Mediterranean shrub cultivated for its buds and fruits. Fermented capers are typical of Mediterranean countries,
The main reasons for the growing interest for pickling is the improvement of the nutritional, physiological, and preservation characteristics that this processing may bring, therefore lactic acid fermentation of vegetables with high nutritional potential was considered in these last years. Sweet potato-lacto pickles were proved to be suitable for fermentation and commercialization in small-scale industries. The use of a brine containing 10% of NaCl (wt/vol) and fermentation with selected strains of L. plantarum obtained good sensory properties. Pickled garlic is increasing in its popularity among consumers because of its particular organoleptic properties. Relatively short-term spontaneous fermentation was shown to improve the healthpromoting properties of pickled garlic, especially polyphenol content and antioxidative potential. Besides semi-industrial productions, there is a broad range of fermented foods that remains scantly documented. Indigenous vegetable-fermented foods were consumed for thousands of years and are strongly linked to culture and traditions, especially in rural households and village communities, where they can make a significant contribution to the daily diet. For instance, soy sauce is extensively consumed around the world and is a fundamental ingredient in several Asian countries. Fermentation is a useful technique, especially in developing countries where the preservation of foods may be difficult. Fermented pickles are particularly important in Asian and African countries where a variety of fruits and vegetables, such as lemons, lime, banana, mango, durian, and beetroots, and other local leaves are produced in a traditional way. A better knowledge of the processing of indigenous fermentations could be useful to avoid the loss of this important variability. Some of the most well-known fermented pickles originate in Asian countries. Pak-Gard-Dong is a fermented mustard (Brassica juncea) leaf product made in Thailand. Mustard leaves are washed, wilted, mixed with salt, and left to ferment packed into containers for 12 h. Water is then drained and a solution containing 3% of sugar is added. Mustard leaves are left to ferment for 3–5 days at room temperature. Microorganisms mainly associated with fermentation are L. brevis and L. plantarum. A very similar product is Hum-choy, produced in the south of China from local leafy vegetables. After washing and draining, the leaves are
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covered in salt and sun dried and successively put into pots, to allow fermentation which usually lasts for w4 days. Pickled radish is also common in Asian countries, such as Korea, where it exists in a variety of different products. Pickled radish tap roots are named Sinki and are traditionally consumed in India, Nepal, and Bhutan. Preparation of Sinki is similar to those of other pickles, in which vegetables are washed, drained, and sun dried before fermentation. The fermentation in this case can last up to 12 days at 30 C and is commonly started by L. fermentum and L. brevis, followed by L. plantarum.
Lactic Acid Fermentation of Vegetable Juices The high concentration of health-promoting compounds of vegetables and fruits favors also the manufacture of fermented juices with improved nutritional properties and agreeable sensory characteristics. Vegetable-based beverages produced by controlled fermentation of lactic acid bacteria are new products, answering to the consumer’s demand for minimally processed foods characterized by high nutritional value, rich flavor, and enhanced shelf life. In these products, high proportions of protective substances contained in the raw material are preserved. Moreover, during the fermentation, lactic acid bacteria may produce additional health-promoting components and allow for a better preservation. The technological options for the production of fermented vegetable juices mainly include the following three: (1) spontaneous fermentation by autochthonous lactic acid bacteria, (2) fermentation by starter cultures added into raw vegetables, and (3) fermentation of mild heat-treated vegetables by starter cultures. Lactic acid bacteria microbiota of spontaneously fermented vegetable juices is mainly represented by the genera Lactobacillus, Leuconostoc, and Pediococcus, whereas the starter cultures most widely used were L. plantarum. Hardaliye, a traditional fermented beverage from Turkey obtained from red grape juice and crushed black mustard seeds, is an example of a spontaneous fermented juice in which L. paracasei subsp. paracasei and Lactobacillus casei subsp. pseudoplantarum dominate the lactic acid bacteria microbiota, which may be made up of several different species, including L. brevis, Lactobacillus acetotolerans, Lactobacillus sanfranciscensis, and Lactobacillus vaccinostercus. Overall, it was shown that fermented vegetable juices with optimal characteristics may be achieved by selecting lactic acid bacteria strains suitable for the fermentation of individual raw materials, according to factors such as the specific dependence on the supply of nutrients for growth and the chemical and physical environment. Vegetables such as cabbage, carrot, tomato, and spinach have proven to be suitable for the manufacture of fermented juices because of their content of fermentable carbohydrates. Fermentation of cabbage into sauerkraut juice is mostly obtained by spontaneous microbiota, although as with other fermented vegetables, the use of the starters was required to obtain a uniform product. In addition to the choice of the starter culture, to improve the quality and sensory properties of sauerkraut juice, the addition of other ingredients and the mixture with other juices was also considered. Carrot juice is one of the most common vegetable juices that can be strongly improved by lactic acid bacteria fermentation. Fermented carrot juice is microbiologically
stable, with good sensory properties and potentially high nutritional value. The use of selected starter cultures for carrot juice can improve juice yield thanks to the activity of pectinolytic enzymes and its iron solubility, and it also enables it to obtain higher mineral availability. The demonstration of suitability of tomatoes as the raw material for the manufacture of lactic acid fermented juice has opened new perspectives for this product. Commercially available tomato juices are usually subjected to thermal processing, which induces undesirable changes of color, flavor, and nutritional value. On the contrary, processing of tomatoes into fermented juice influences nutritional and sensory properties. Besides amino acids and vitamin and mineral content, lactic acid bacteria fermentation may also increase the amount of other phytonutrients that positively affect human health, having antitumor and antioxidant properties. The preservation of pigments and the synthesis of healthpromoting substances, such as aglycones and b-carotene, after lactic acid fermentation was proven in beetroots and sweet potatoes.
Innovative Vegetable-Based Fermented Products Innovation in food technology plays an important role in the improvement of the nutritional quality of products, possibly enhancing the hedonistic aspects of food intake. For instance, smoothies, originally consisting of purely fresh fruits and vegetables, were first introduced in the 1960s in the United States and reemerged in the 2000s. Recently, a novel protocol for the manufacture of fermented smoothies was set up. White grape juice and Aloe vera extract were mixed with red (cherries, blackberries prunes, and tomatoes) or green (fennels, spinach, papaya, and kiwi) fruits and vegetables and were fermented by mixed autochthonous starters, including strains of L. plantarum, W. cibaria, and L. pentosus. Lactic acid fermentation by selected starters positively affected the content of antioxidant compounds and enhanced the sensory attributes of the smoothies. The consumer demand for nondairy beverages with high functionality is growing as a consequence of the ongoing trend of vegetarianism and the increasing prevalence of the lactose intolerance. Some probiotic lactic acid bacteria are able to grow in vegetables and fruits, even if different species can show different sensitivities toward the pH of the substrate, postacidification of fermented products, metabolism products, temperature, processing, and gastrointestinal tract conditions. The major part of the studies considered the use of the probiotic species L. acidophilus, L. plantarum, L. casei, Lactobacillus rhamnosus, and Lactobacillus delbrueckii, and in a few cases, Leuc. mesenteroides and species of the genus Bifidobacterium as well. Tomato, carrot, cabbage, artichokes, and red beet juices were proven to be particularly suitable for probiotic fermentation, allowing a rapid growth of the strains and viable cell population above w8 log cfu g1. Nevertheless, the properties of the juice may be affected by probiotic microorganisms as shown by the fermentation of orange juice by L. plantarum, which provoked unsuitable sensory properties. Thus, a proper selection of the fruit matrices, probiotic strains, and eventually the addition of other ingredients seems fundamental to achieve optimal healthy and sensory properties.
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Conclusion Intrinsic microbiota of raw vegetables and fruits may represent a source of metabolic tracts that deserve increasing interest. Selection of autochthonous lactic acid bacteria and their use as starters may have important applicative repercussions. Thus, further insight is needed to explore the potential of the intrinsic autochthonous microbiota and consequently exploit the vegetables and fruits through lactic acid fermentation.
See also: Cocoa and Coffee Fermentations; Fermented Foods: Origins and Applications; Fermented Foods: Fermentations of East and Southeast Asia; Probiotic Bacteria: Detection and Estimation in Fermented and Nonfermented Dairy Products; Starter Cultures; Starter Cultures: Importance of Selected Genera; Starter Cultures Employed in Cheesemaking; Fruit and Vegetables: Introduction; Advances in Processing Technologies to Preserve and Enhance the Safety of Fresh and Fresh-Cut Fruits and Vegetables; Sprouts; Fruit and Vegetable Juices.
Further Reading Buckenhüskes, H.J., 1997. Fermented vegetables. In: Doyle, P.D., Beuchat, L.R., Montville, T.J. (Eds.), Food Microbiology: Fundamentals and Frontiers, second ed. ASM Press, Washington DC, pp. 595–609.
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Ciafardini, Di Cagno, 2012. Olive da mensa ed altri prodotti vegetali. In: Farris, A., Gobbetti, M., Neviani, E., Vincenzini, M. (Eds.), Microbiologia dei prodotti alimentari, CEA – Casa Editrice Ambrosiana. Milan, Italy, pp. 365–382. ISBN: 978-880818246-3. Di Cagno, R., Surico, R.F., Paradiso, A., et al., 2009. Effect of autochthonous lactic acid bacteria starters on health-promoting and sensory properties of tomato juices. International Journal of Food Microbiology 128, 473–483. Di Cagno, R., Cardinali, G., Minervini, G., et al., 2010. Taxonomic structure of the yeasts and lactic acid bacteria microbiota of pineapple (Ananas comosus L. Merr.) and use of autochthonous starters for minimally processing. Food Microbiololgy 27, 381–389. Do Espírito Santo, A.P., Perego, P., Converti, A., Oliveira, M.N., 2011. Influence of food matrices on prebiotic viability: a review focusing on the fruity bases. Trends in Food Science and Technology 22, 377–385. Karovicova, J., Kohajdová, Z., 2003. Lactic acid fermented vegetable juices. Horticultural Science 30, 152–158. Kabaka, B., Dobson, A.D.W., 2011. An introduction to the traditional fermented foods and beverages of Turkey. Critical Reviews in Food Science Nutrition 51, 248–260. Lee, C.H., 1997. Lactic acid fermented foods and their benefits in Asia. Food Control 8, 259–269. Pérez-Pulido, R., Omar, N.B., Abriouel, H., et al., 2005. Microbiological study of lactic acid fermentation of caper berries by molecular and culture-dependent methods. Applied and Environmental Microbiology 71, 7872–7879. Plengvidhya, V., Breidt, F.J., Fleming, H.P., 2004. Use of RAPD-PCR as a method to follow the progress of starter cultures in sauerkraut fermentation. International Journal of Food Microbiology 93, 287–296. Tamang, J.P., Tamang, B., Schillinger, U., et al., 2005. Identification of predominant lactic acid bacteria isolated from traditionally fermented vegetable products of the Eastern Himalayas. International Journal of Food Microbiology 105, 347–356.
Introduction Minimally processed fresh vegetables and fruits have a short shelf life because they are subjected to rapid microbial spoilage, and, in some cases, to pathogen microorganisms as a result of their contact with soil during cultivation and harvesting. Cooking, pasteurization, and other similar processes, as well as the addition of preservatives, are traditional technology options that may guarantee safe vegetables; however, these methods would bring about a number of not always desirable changes in the physical characteristics and chemical composition of vegetables. To decrease such drawbacks, some novel technologies are considered, including high-hydrostatic pressure processing, ionization radiation, pulsed-electric fields, new packaging systems, and the use of natural antimicrobial compounds. Among the various technological options, lactic acid fermentation, as the traditional biopreservation method for the manufacture of finished and half-finished foods, may be considered to be a simple and valuable biotechnology for maintaining or improving the safety, nutritional, sensory, and shelf-life properties of vegetables and fruits. Lactic acid fermentation has an industrial significance that goes beyond simple preservation, and it is used to improve and develop characteristic sensory and nutritional properties, to enhance digestibility, to destroy undesirable components, to convey probiotics, and to develop new products. However, most of the fermented vegetables undergo a spontaneous fermentation, which may be responsible for undesirable variations of the sensory properties or may occur too slowly to inhibit spoilage and pathogen microorganisms. In addition, selection of starter cultures within the autochthonous microbiota of vegetables and fruits should be recommended because autochthonous cultures may ensure better performance compared with allochthonous strains. On the basis of these considerations, during the past decade, lactic acid fermentation of vegetables and fruits generated an increasing interest.
Microbiota of Raw Vegetables and Fruits Each particular type of vegetable provides a unique environment in terms of type, availability, and concentration of substrate, buffering capacity, competing microorganisms, and perhaps natural plant antagonisms. Nevertheless, analyses by molecular methods showed that each species of vegetables and fruits harbors a dominant and constant microbiota. The cell density of the microbial groups may vary depending on the plant species, temperature, and harvesting conditions. The microbiota of fruits is essentially represented by yeasts and fungi, which can cause discoloration and generate unpleasant odors and flavors and, in extreme cases, synthesize compounds that are toxic to the consumer. Inhibition of growth of yeasts is one of the main objectives for minimally processed products
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based on fruits. Because of their faster growth, yeasts generally anticipate the colonization by fungi. Overall, the microbial population of vegetables and fruits is estimated to fluctuate between 5 and 7 log cfu g1. The number of yeasts may range between 2 and 6 log cfu g1. Dominant yeasts, such as Cryptococcus spp., Candida spp., Saccharomyces spp., and Rhodotorula spp. were found in banana; Hansenula, Kloeckera, Candida, Pichia, and Saccharomyces spp. were found in cocoa; Saccharomyces cerevisiae, Candida krusei, and Debaryomyces hansenii were found in melon pod; Pichia guilliermondii and Hanseniaspora uvarum were found in pineapple, and Candida, Cryptococcus, Kloeckera, Rhodotorula, and Kluyveromyces spp. were found in other tropical fruits (e.g., pitanga, umbu, and acerola).
Taxonomic Structure of Lactic Acid Bacteria Microbiota of Vegetables and Fruits Overall, lactic acid bacteria are a small part (2–4 log cfu g1) of the autochthonous microbiota of raw vegetables and fruits, and their cell density is mainly influenced by the species of vegetables, temperature, and harvesting conditions. Recently, the lactic acid bacteria microbiota of raw carrots, marrows, and French beans was characterized. The raw vegetables used in this study harbored autochthonous lactic acid bacteria at cell densities of w2.7–3.0 log cfu g1. The following species were identified for each vegetable: carrots, Leuconostoc mesenteroides, Lactobacillus plantarum, and Weissella soli/W. koreensis; French beans, Enterococcus faecalis, Pediococcus pentosaceus, and Lactobacillus fermentum; and marrows, L. plantarum. Carrots and French beans had the most heterogeneous composition of autochthonous lactic acid bacteria, and L. plantarum was the only species found in marrows. In fact, L. plantarum is considered as an ubiquitous and metabolic versatile bacterium largely found in fruits and vegetables. Leuconostoc spp., including Leuc. mesenteroides subsp. mesenteroides, mainly dominated the early spontaneous fermentation of carrots. Pediococci and lactobacilli, mainly L. plantarum, were identified in many raw vegetables and, especially, in fermented vegetable juices. L. plantarum, Weissella cibaria/confusa, Lactobacillus brevis, P. pentosaceus, Lactobacillus spp., and Enterococcus faecium/faecalis were identified in raw tomatoes, and Lactobacillus curvatus, Leuc. mesenteroides, L. plantarum, and W. confusa were identified in red and yellow peppers. Several species of lactic acid bacteria, such as L. fermentum, L. brevis, and E. faecalis were identified in melon pod. Pineapple fruits harbored autochthonous lactic acid bacteria belonging to L. plantarum and Lactobacillus rossiae at cell densities of w5.5 log cfu g1. Several cultivars of sweet cherry fruits harbored a low population of lactic acid bacteria at a cell density lower than that usually found in other fruits and vegetables (w2.5 log cfu g1). L. plantarum, Pediococcus acidilactici, P. pentosaceus, and Leuc. mesenteroides subsp. mesenteroides were the only species identified in the eighth cultivars of sweet cherry. A similar trend was also found for other fruits,
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Table 1 Lactic acid bacteria isolated from raw or spontaneously fermented vegetables and fruits Lactic acid bacteria species
Source
Lactobacillus plantarum
Tomatoes, marrows, carrots, cucumbers, eggplants, sauerkraut, red beets, capers, kimchi, pineapple, plums, kiwi, papaya, fennels, cherries, cabbages, grape must Capers, papaya, eggplants, cucumbers Pineapple French beans, red beets, capers, eggplants Peppers, sauerkraut, kimchi Kimchi Tomatoes, sauerkraut, capers, eggplants, cabbages, cucumbers, grape must Cider, cabbages, capers Cider Cider White cabbages, carrots, peppers, cucumbers, eggplants, lettuce, sauerkraut, kimchi, cherries, grape must Carrots Peppers, tomatoes, blackberries, papaya
Lactobacillus pentosus Lactobacillus rossiae Lactobacillus fermentum Lactobacillus curvatus Lactobacillus sakei Lactobacillus brevis Lactobacillus paraplantarum Lactobacillus collinoides Lactobacillus casei Leuconostoc mesenteroides subsp. mesenteroides
Weissella soli Weissella confusa, Weissella cibaria Enterococcus faecalis, Enterococcus faecium Oenococcus oeni Pediococcus pentosaceus Pediococcus parvulus
French beans, tomatoes, capers Cider, grape must French beans, tomatoes, cucumbers, sauerkraut, capers, cherries, cabbages Cider
Adapted from Ciafardini, Di Cagno, 2012. Olive da mensa ed altri prodotti vegetali. In: Farris, A., Gobbetti, M., Neviani, E., and Vincenzini, M. (Eds.), Microbiologia dei prodotti alimentari, CEA – Casa Editrice Ambrosiana, Milan, Italy, pp. 365–382, ISBN: 978-8808-18246-3.
such as blackberries, prunes, kiwifruits, and papaya, where L. plantarum, Lactobacillus pentosus, and W. cibaria were the only species identified. The same species were commonly identified within the microbiota of cucumber, olives, pumpkins, peppers, carrots, persimmons, and eggplants when spontaneously fermented. Examples of lactic acid bacteria isolated from raw or spontaneously fermented vegetables and fruits are shown in Table 1.
Fermentation of Vegetable Products Spontaneous Fermentation In raw vegetables and fruits, lactic acid fermentation may take place spontaneously, when anaerobic conditions, water activity, moisture, salt concentration, and temperature are favorable to the growth of the autochthonous lactic acid bacteria. Spontaneous fermentation may be optimized through back slopping, commonly used for sauerkraut production (i.e., inoculation of the raw material with a small quantity of a previously performed successful fermentation). Hence, back slopping results in dominance of the best adapted strains and represents a way of using a selected starter culture to shorten the fermentation and to reduce the risk of fermentation failure. When occurring spontaneously, the fermentation of vegetables
and fruits is frequently characterized by a succession of heteroand homo-fermentative lactic acid bacteria, together with or without yeasts, which are responsible for multistep fermentation processes. The spontaneous lactic acid fermentation of raw vegetables may also require some days (4–6 days) before the pH value undergoes a significant decrease. Consequently, in some cases, spontaneous fermentation may be responsible for undesirable variations of the sensory and rheological properties of fresh vegetables and fruits or it may occur too slowly to inhibit spoilage and pathogen microorganisms. Both from hygiene and safety point of views, the use of starter cultures is recommended, as it would lead to a rapid inhibition of spoilage and pathogenic bacteria, and to a processed product with consistent sensory and nutritional properties. The use of starter cultures is increasing in vegetable fermentation. Contrarily, to other fermented foods (e.g., dairy, meat, and baked goods), only few cultures are used for fruit and vegetable fermentations, with L. plantarum being the most frequently used.
Commercial or Allochthonous Lactic Acid Bacteria Starter Cultures The use of commercial starter cultures was considered a breakthrough in the processing of fermented vegetables and fruits, resulting in a high degree of control over the fermentation process and standardization of the product. Some examples of the use of commercial lactic acid bacteria starters may be found in the literature. The controlled fermentation of peeled and blanched garlic using a starter culture of L. plantarum LP91 in comparison with unblanched started garlic was studied. The effect of the starter on the sensory properties of fermented garlic was also evaluated. Starter grew well in blanched garlic (w9 log cfu g1 after 2 days of fermentation) and lactic acid was the main synthesized metabolite. On the contrary, its growth was inhibited in unblanched garlic, and ethanol, fructose, and the appearance of an undesired green pigment were found. From a sensory point of view, started and unstarted garlic did not show significant differences. However, in most of the cases commercial starter cultures do not correspond to autochthonous strains, whereas, the selection of starter cultures within the autochthonous microbiota of vegetables and fruits should be recommended because autochthonous cultures may ensure better performance compared with allochthonous strains. Commercial or allochthonous starter cultures may exhibit different limitations, such as low flexibility with regard to the desired properties and functionality of the product and low diversity of metabolic activities.
Autochthonous Starter Although lactic acid bacteria represent a small part of the microbiota of vegetables and fruits, they may have various functions: (1) to exert intrinsic antagonistic activity toward spoilage and pathogen microorganisms, (2) to deliver health relevant microorganisms to the gastrointestinal tract, and (3) to supply autochthonous lactic acid bacteria suitable to be reused as starters. Recently, it was shown that the use of selected autochthonous lactic acid bacteria starters compared with allochthonous or spontaneous fermentation guaranteed the
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4.3
4.2
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log cfu g –1
8
6
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4.0
3.9
2 0
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Tomato juice without starter inoculum (kinetic of acidification) Tomato juice without starter inoculum (kinetic of growth) Tomato juice fermented with the autochthonous starter Lactobacillus plantarum POM35 (kinetic of acidification) Tomato juice fermented with the autochthonous starter Lactobacillus plantarum POM35 (kinetic of growth) Tomato juice fermented with the allochthonous starter Lactobacillus plantarum LP54 (kinetic of acidification) Tomato juice fermented with the allochthonous starter Lactobacillus plantarum LP54 (kinetic of growth)
Figure 1 Kinetics of growth and acidification of tomato juice fermented with autochthonous and allochthonous starters. Adapted from Ciafardini and Di Cagno. Olive da mensa ed altri prodotti vegetali. In: Farris, A., Gobbetti, M., Neviani, E., and Vincenzini, M. (Eds.), Microbiologia dei prodotti alimentari, CEA – Casa Editrice Ambrosiana, Milan, Italy, pp. 365–382, ISBN: 978-8808-18246-3.
prolonged shelf life of fermented vegetables and fruits and maintained agreeable nutritional, rheological, and sensory properties. Autochthonous strains always had better performances than allochthonous strains. Allochthonous strains showed, in particular, longer latency phases of growth and acidification with respect to selected autochthonous strains. Figure 1 shows the representative kinetics of growth and acidification of unstarted and fermented tomato juice with autochthonous and allochthonous strains of L. plantarum. When fermented with selected autochthonous strains of L. plantarum, Leuc. mesenteroides, and P. pentosaceus, carrots, French beans, or marrows were characterized by rapid decrease of pH, marked consumption of fermentable carbohydrates, and inhibition of Enterobacteriaceae and yeasts. Autochthonous strains of L. plantarum, L. curvatus, and W. confusa were used as mixed starters to ferment red and yellow peppers. Compared with unstarted vegetables, the rapid decrease of pH and the marked consumption of fermentable carbohydrates inhibited the growth of enterobacteria and yeasts. After 30 days of storage at room temperature, started vegetables positively differentiated also for higher firmness and color indexes with respect to unstarted red and yellow peppers. Recently, a protocol was set up for the minimal processing of pineapple to increase its shelf life and to maintain agreeable sensory and
nutritional features. After 30 days of storage at 4 C, pineapple fruits started with selected autochthonous strains of L. plantarum and L. rossiae had a number of lactic acid bacteria up to 1 000 000 times higher than the other processed pineapples as well as the lowest number of yeasts. The highest antioxidant activity and firmness, better preservation of the natural colors, and preference for odor and overall acceptability were also found. Fermentation of sweet cherry (Prunus avium L.) puree added by stem infusion by selected autochthonous lactic acid bacteria starters showed that, despite the hostile environment (e.g., low pH, presence of phenolic compounds), the selected autochthonous strains of P. pentosaceus and L. plantarum used as starters grew well, showing peculiar metabolic traits. Consumption of carbohydrates and the lactic acid fermentation was limited, whereas consumption of organic acids (e.g., malic acid) and free amino acids was evident, especially, throughout storage. Both lactic acid bacteria remained viable during 60 days of storage at cell numbers that exceeded those of potential probiotic beverages, presumably on the basis of these metabolic activities. Overall, the use of autochthonous lactic acid bacteria starters to ferment vegetables and fruits ensured better preservation of different properties: natural colors, firmness, antioxidant activities, and other health-promoting
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Raw vegetables and fruits
Table 2 Main metabolic traits to consider for the selection of starters for vegetable fermentation Criteria
Isolation, typing, and identification of autochthonous lactic acid bacteria
Pro-technological
Selection of single or mixed starters
Processing into puree, juices, or pieces eventually followed by a thermal treatment (e.g., 70 °C for 5 min)
Inoculum of the starter at a final cell density of ca. 7 log cfu g–1
Lactic acid fermentation (25–30 °C for 15–24 h)
Storage under refrigeration up to 40 days
Figure 2 Example of biotechnology protocol to ferment raw vegetables and fruits. Adapted from Ciafardini and Di Cagno. Olive da mensa ed altri prodotti vegetali. In: Farris, A., Gobbetti, M., Neviani, E., and Vincenzini, M. (Eds.), Microbiologia dei prodotti alimentari, CEA – Casa Editrice Ambrosiana, Milan, Italy, pp. 365–382, ISBN: 978-8808-18246-3.
compounds. This effect may be the consequence of modification of the profile of organic acids and of the synthesis of lactic and acetic acids and free amino acids. These modifications might have had direct (pH) or indirect (redox potential) repercussions on the activity of endogenous browning enzymes, oxidation, and sensory properties (color, flavor, and aroma) of vegetables and fruits. An example of biotechnology protocol to ferment raw vegetables and fruits is reported in Figure 2. The main criteria for selecting starters to be used for vegetable fermentation are the (1) rate of growth, (2) rate and total production of acids which, in turn, affect the changes of pH, and (3) environmental adaptation and tolerance. Major metabolic traits to be considered for the selection of autochthonous lactic acid bacteria starters for vegetables and fruits fermentation are shown in Table 2. The criteria for selection of starters may be divided into three main categories: (1) technological, (2) nutritional, and (3) sensory. Predominance of
Sensory Nutritional
Metabolic traits Velocity of growth Velocity of acidification Salt tolerance Low pH values tolerance Capacity of growth at low pH values Completeness of fermentation Developing of malolactic fermentation Mild acid producing or fermentation Tolerance to high concentration of polyphenolic compounds Synthesis of antimicrobial compounds Synthesis of exopolysaccharides Pectinolytic activity Heterofermentative metabolism Synthesis of aroma precursor compounds Synthesis of biogenic compounds Synthesis of exopolysaccharides
Adapted from Ciafardini and Di Cagno. Olive da mensa ed altri prodotti vegetali. In: Farris, A., Gobbetti, M., Neviani, E., and Vincenzini, M. (Eds.), Microbiologia dei prodotti alimentari, CEA – Casa Editrice Ambrosiana, Milan, Italy, pp. 365–382, ISBN: 978-8808-18246-3.
growth by a species of lactic acid bacteria is influenced by the chemical and physical environment in which it has to compete. L. plantarum, which predominates the later stage of vegetable fermentation because of its high acid tolerance and metabolic versatility, seems a likely choice when homolactic fermentation is desired. Moreover, robustness of autochthonous starters throughout fermentation and storage processes, able to achieve high cell numbers (w8.0–9.0 log cfu g1), is an indispensable prerequisite to ensure hygiene, safety, and potential probiotic properties of the product. The synthesis of exopolysaccharides is another metabolic tract to be considered for selection, especially for juices and puree. In addition, the capacity of lactic acid bacteria to synthesize protopectinases, which may enhance the viscosity of fruit matrices, may be an important characteristic for starters. Growth and viability of lactic acid bacteria, in particular L. plantarum and pediococci, were frequently shown on plant materials when polyphenolic compounds were abundant. L. plantarum had the metabolic capacity to degrade some phenolic compounds and other chemical compounds strictly related. Starter cultures with probiotic properties were largely considered in these last years, especially for the production of nondairy probiotic foods. More than other food ecosystems, raw fruits and vegetables possess intrinsic chemical and physical parameters that, for some traits, mimic those of the human gastrointestinal tract. The extremely acidic environment, buffering capacity, high concentration of indigestible nutrients (e.g., fiber, inulin, and fructooligosaccharides (FOS)) and antinutritional factors (e.g., tannins and phenols) are the main characteristics of raw fruits and some vegetables. In most of the cases, the autochthonous microbiota of fruits and vegetables has to colonize and adhere to surfaces, and exerts antagonistic activity toward spoilage and pathogenic microorganisms. A large number of autochthonous lactic acid bacteria isolated from carrots, French beans, cauliflower, celery, tomatoes,
FERMENTED FOODS j Fermented Vegetable Products pineapples, kimchi, several ethnic fermented vegetables of the Himalayas, and Japanese pickles belonging to the genera Lactobacillus, Weissella, Pediococcus, Enterococcus, and Leuconostoc were characterized for their probiotic potential, showing their adaptability to gastrointestinal environment, their capacity to maintain high cell densities, adherence to human or mouse intestinal cells, stimulation of immune mediators, and antimicrobial activity.
Main Fermented Vegetable Products Lactic acid fermentation of vegetables has an industrial significance only for cabbages, cucumbers, and olives. In the Mediterranean area, the industrial production of fermented vegetables is mainly limited to sauerkraut and table olives. Pickled cucumbers are among the most popular pickled foods. Additionally, niche markets exist for other pickled vegetables, which are fermented only slightly before consumption and several varieties of vegetables (e.g., artichokes, capers, garlic, peppers, okra, cauliflower, green tomatoes, and eggplants) are subjected to fermentation at the local level. A list of traditional fermented vegetable products from different regions of the world is reported in Table 3. Because of the commercial significance and extensive literature already available, table olives are not considered in this review.
Sauerkraut Sauerkraut is a vegetable food widely consumed in many European countries. It has usually been prepared by spontaneous lactic fermentation of shredded cabbage (Brassica oleracea L. variety capitata), both by manufacturers and in households. Fresh cabbage is trimmed of outer leaves and shredded and successively mixed with salt to obtain a final concentration of approximately 2% NaCl (wt/wt). The cabbage is quickly surrounded by brine and then covered with plastic sheeting draped over the tank. Water is added to improve anaerobic conditions and prevent contact with air, which may cause a loss of microbiological quality. After 24–48 h, anaerobic conditions are established thanks to the exhaustion of oxygen and CO2 production from heterofermentative lactic acid bacteria. The combination of salt concentration and temperature (18 C) allows a spontaneous lactic acid fermentation, although variations of these conditions are not uncommon and affect the competitiveness of the naturally present microorganisms. The microbial species typically isolated during sauerkraut fermentation are Leuc. mesenteroides, P. pentosaceus, L. brevis, and L. plantarum. Leuc. mesenteroides and Weissella spp. typically dominate the early stages of fermentation because they are present in larger cell numbers and have faster growth compared with the other lactic acid bacteria in cabbage juice. The increase of lactic acid concentration inhibits the multiplication of Leuc. mesenteroides while it promotes the growth of acid-tolerant species, such as L. brevis and in some cases L. curvatus, Lactobacillus sakei, E. faecalis, Lactococcus lactis subsp. lactis, and P. pentosaceus. L. plantarum becomes predominant in the latter stage, about 5–7 days after the beginning of fermentation, contributing to a further decrease of the pH value (w3.5). The end products resulting from both the stages of fermentation
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may include mannitol and acetic acid (w1% each) and lactic acid, which may exceed 2%, depending on how long the homolactic fermentation is allowed to continue.
Kimchi Kimchi is a name for various Korean traditional closely related fermented vegetable products. Kimchi is similar to sauerkraut and contains mainly Chinese cabbages (Brassica pekinensis) and radish, but other ingredients such as garlic, green onion, ginger, red pepper, mustard, parsley, and carrot may be added. After soaking with water, vegetables are cut and placed in a salt solution of 5–7% for 12 h or 15% for 3–7 h in order to increase the salt content (w2.0–4.0% of the total weight). Then vegetables are rinsed several times with fresh water and drained. Kimchi fermentation is carried out by various autochthonous microorganisms; thus, microbiota of kimchi can vary on the basis of ingredients, but it is also affected by temperature (ranging from 5 to 30 C) and salt concentration. Among the 200 bacteria isolated from kimchi, Leuc. mesenteroides and P. pentosaceus were those mainly involved in the first stage of fermentation. The dominant species of Lactobacillus in the later stages of kimchi fermentation vary according to the fermentation temperature; L. plantarum and L. brevis dominated fermentations carried out at 20–30 C, whereas Lactobacillus maltaromicus and Lactobacillus bavaricus dominated at 5–7 C. The composition of microbiota affects the kinds of acid synthesized and the final taste of kimchi. The increase of lactic acid inhibits the growth of Leuc. mesenteroides and promotes the development of acid-tolerant species, such as L. brevis and in some cases L. curvatus, L. sakei, and E. faecalis. The final stage of fermentation is dominated by L. plantarum, which allows a further decrease of the pH and lactic acid synthesis. The best tasting kimchi is obtained before overgrowth of L. plantarum and L. brevis, at an optimal pH of 4.5. The concentration of NaCl (w3%, wt/vol) and a temperature of w10 C are the optimal parameters to reach a level of lactic acid in the range 0.4–0.8% and a pH of 4.2–4.5. Kimchi was recognized as a health-promoting food, thanks to the physiological effects of its ingredients and end-products of the fermentation. Because of its nutritional properties, kimchi was mentioned by Health magazine in its list of the top-five ‘World’s Healthiest Foods’ (http://eating.health.com/2008/02/01/worlds-healthiest-foodskimchi-korea/) as one of the most popular functional foods in the world. Leuc. mesenteroides was found to be important in the early stage of fermentation of many other vegetable-based foods, such as Dhamuoi, a Vietnamese food similar to kimchi and the Turkish Tur¸s¸u, made with a wide variety of different vegetables and fruits. To manufacture Tur¸s¸u, vegetables such as cucumbers, cabbages, green tomatoes, green peppers, and fruits such as melons are pressed in to containers and added to a brine containing 10–15% of NaCl (wt/vol). After vinegar addition, the pickles are left to ferment at w20 C for 4 weeks. The species mainly associated with tur¸s¸u fermentation are L. plantarum, Leuc. mesenteroides, L. brevis, P. pentosaceus, and E. faecalis. After a first predominance of Leuc. mesenteroides, the fermentation is continued mostly by L. plantarum, followed in the latter stage by yeasts, such as Torulopsis, Hansenula, and Saccharomyces. Carrots, turnips, bulgur flour, and sourdough are the ingredients of S¸algam, a lactic acid fermented beverage very
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Examples of traditional fermented vegetable products from different regions of the world
Product
Main ingredients
Main lactic acid bacteria involved
Country
Sauerkraut Cucumbers Capers
Cabbage, salt Cucumbers, vinegar, salt Capers, water, salt
Kimchi
Cabbage, radish, salt, spices, and other vegetables (ginger, pepper, garlic, onion) Cabbage and other vegetables Mustard Mustard leaf, salt Local cabbages, mustard leaves, cauliflower leaves, Radish roots Cucumber Mustard leaves Cucumbers, cabbage, green tomatoes, green peppers, and other vegetables Black/violet carrots, turnip, bulgur flour, sourdough, salt, and water Red grape juice, black mustard seeds
Leuconostoc mesenteroides, Lactobacillus brevis, Lactobacillus plantarum Pediococcus pentosaceus, L. plantarum L. plantarum, Lactobacillus pentosus, Lactobacillus fermentum, L. brevis, Lactobacillus paraplantarum, Enterococcus faecium, P. pentosaceus Leuc. mesenteroides, Leuc. pseudomesenteroides, L. plantarum, L. brevis, Lactobacillus curvatus, Lactobacillus sakei Leuc. mesenteroides, L. plantarum L. brevis, Pediococcus spp. L. plantarum L. plantarum, L. casei subsp. casei, L. casei subsp. pseudoplantarum, L. fermentum, P. pentosaceus L. fermentum, L. plantarum, L. brevis, Leuc. fallax L. plantarum, L. brevis, Leuc. fallax, Pediococcus spp. L. brevis, L. plantarum L. plantarum, Leuc. mesenteroides, L. brevis, P. pentosaceus, E. faecalis L. plantarum, L. paracasei subsp. paracasei, L. fermentum, L. brevis
Europe, USA USA, Asia Mediterranean countries (Greece, Italy, Spain, Turkey, Morocco) Korea
L. paracasei subsp. paracasei, L. casei subsp. pseudoplantarum, L. pontis, L. brevis, L. acetotolerans, L. sanfranciscensis.
Turkey
Dhamuoi Burong mustasa Dakguadong Gundruk Sinki Khalpi Pak-Gard-Dong Tur‚s¸u S¸algam Hardaliye
Vietnam Philippines Thailand Eastern Himalaya Eastern Himalaya Eastern Himalaya Thailand Turkey Turkey
Adapted from Ciafardini and Di Cagno. Olive da mensa ed altri prodotti vegetali. In: Farris, A., Gobbetti, M., Neviani, E., and Vincenzini, M. (Eds.), Microbiologia dei prodotti alimentari, CEA – Casa Editrice Ambrosiana, Milan, Italy, pp. 365–382, ISBN: 978-8808-18246-3.
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Table 3
FERMENTED FOODS j Fermented Vegetable Products popular in Turkey. The manufacture of this beverage involves two steps of fermentation; in the first step, bulgur flour, sourdough, and salt are mixed with water and left to ferment by lactic acid bacteria and yeasts for 3–5 days at room temperature. In the second step, vegetables and water are added to the fermented product of the first step and fermentation takes place in wooden barrels for 3–10 days. S¸algam fermentation seems dominated mainly by L. plantarum, Lactobacillus paracasei subsp. paracasei, L. fermentum, and L. brevis. Yeasts such as S. cerevisiae are also known to contribute to flavor development.
Pickles Pickles are often produced by lactic acid fermentation of vegetables and fruits. Industrial-scale processes for the manufacture of many types of pickles were developed, but they are still carried out even at the domestic scale, although so far olives, cucumbers, and cabbages are the only one fermented in large volumes.
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especially Greece, Italy, Turkey, Morocco, and Spain. Fermentation of caper fruits is often done using traditional artisanal ways. The fruits are collected during the months of June and July and are immersed in tap water, where the fermentation takes place for approximately 5–7 days in a temperature range from 23 to 43 C. Successively, fermented capers are placed in brine and distributed for consumption. Fermentation of caper fruits, like other vegetable fermentation, is carried out by spontaneous lactic acid bacteria colonizing the raw material and processing environment. The predominance of L. plantarum in caper fermentation is important for the rapid production of lactic acid and fast acidification, allowing better preservation conditions of the fermented product. Overall, Leuconostoc species do not take part in caper fermentation. In contrast with other vegetable fermentations, the absence of Leuconostoc was attributed to the rapid decrease of pH, because this bacterium is not able to grow below pH 4.5, and to the high temperature of fermentation, which can reach 40 C.
Minor Fermentations and Traditional Products Cucumbers Fully ripe cucumbers are washed and drained and eventually sliced. Manufacturing usually consists of four stages: (1) selection of regular-shaped cucumbers, (2) dipping into brine (5–7% of NaCl) inside plastic or glass containers, (3) spontaneous lactic acid fermentation, and (4) packaging. Significant reductions in salt concentration (to 4% or less) may be possible using blanched cucumbers to reduce the initial microbiota. Sometimes, calcium chloride is added on the surface to allow a crisp texture during storage. As soon as the brine is produced, fermentation starts and lasts for 2–3 weeks, depending on the temperature (usually 20–27 C) until a final pH in the range 3.1–3.5 is reached. Homofermentative lactic acid bacteria, such as P. pentosaceus and L. plantarum, are the main bacteria responsible for fermentation, while Leuc. mesenteroides is inhibited by high concentrations of sodium chloride. Maintenance of structural integrity of whole cucumbers during brine fermentation is important for the quality of the product. However, CO2 can be produced as the result of cucumber respiration when they are submerged in brine and by malolactic fermentation carried out by L. plantarum. Because gaseous spoilage (bloater damage) may lead to serious economic losses, cucumbers are purged with air during the fermentation period to remove CO2 from the tank, even if this can increase the risk of molds and yeasts development. Currently, industrial fermentations are carried out by spontaneous lactic acid bacteria, and starter cultures of L. plantarum are rarely employed. A product made of fermented cucumbers named Khalpi is popular in Himalayan region. Cucumbers for manufacture of Khalpi are cut into pieces and sun dried for 2 days. After this step, cucumbers are put into a bamboo vessel and then left to ferment at room temperature for 3–7 days. The microbiota of Khalpi is characterized mainly by L. plantarum and L. brevis. Pediococci and Leuconostocs species, such as Leuc. fallax, were also found.
Capers Caper berries are the fruits of Capparis species (mainly Capparis spinosa L.), a Mediterranean shrub cultivated for its buds and fruits. Fermented capers are typical of Mediterranean countries,
The main reasons for the growing interest for pickling is the improvement of the nutritional, physiological, and preservation characteristics that this processing may bring, therefore lactic acid fermentation of vegetables with high nutritional potential was considered in these last years. Sweet potato-lacto pickles were proved to be suitable for fermentation and commercialization in small-scale industries. The use of a brine containing 10% of NaCl (wt/vol) and fermentation with selected strains of L. plantarum obtained good sensory properties. Pickled garlic is increasing in its popularity among consumers because of its particular organoleptic properties. Relatively short-term spontaneous fermentation was shown to improve the healthpromoting properties of pickled garlic, especially polyphenol content and antioxidative potential. Besides semi-industrial productions, there is a broad range of fermented foods that remains scantly documented. Indigenous vegetable-fermented foods were consumed for thousands of years and are strongly linked to culture and traditions, especially in rural households and village communities, where they can make a significant contribution to the daily diet. For instance, soy sauce is extensively consumed around the world and is a fundamental ingredient in several Asian countries. Fermentation is a useful technique, especially in developing countries where the preservation of foods may be difficult. Fermented pickles are particularly important in Asian and African countries where a variety of fruits and vegetables, such as lemons, lime, banana, mango, durian, and beetroots, and other local leaves are produced in a traditional way. A better knowledge of the processing of indigenous fermentations could be useful to avoid the loss of this important variability. Some of the most well-known fermented pickles originate in Asian countries. Pak-Gard-Dong is a fermented mustard (Brassica juncea) leaf product made in Thailand. Mustard leaves are washed, wilted, mixed with salt, and left to ferment packed into containers for 12 h. Water is then drained and a solution containing 3% of sugar is added. Mustard leaves are left to ferment for 3–5 days at room temperature. Microorganisms mainly associated with fermentation are L. brevis and L. plantarum. A very similar product is Hum-choy, produced in the south of China from local leafy vegetables. After washing and draining, the leaves are
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covered in salt and sun dried and successively put into pots, to allow fermentation which usually lasts for w4 days. Pickled radish is also common in Asian countries, such as Korea, where it exists in a variety of different products. Pickled radish tap roots are named Sinki and are traditionally consumed in India, Nepal, and Bhutan. Preparation of Sinki is similar to those of other pickles, in which vegetables are washed, drained, and sun dried before fermentation. The fermentation in this case can last up to 12 days at 30 C and is commonly started by L. fermentum and L. brevis, followed by L. plantarum.
Lactic Acid Fermentation of Vegetable Juices The high concentration of health-promoting compounds of vegetables and fruits favors also the manufacture of fermented juices with improved nutritional properties and agreeable sensory characteristics. Vegetable-based beverages produced by controlled fermentation of lactic acid bacteria are new products, answering to the consumer’s demand for minimally processed foods characterized by high nutritional value, rich flavor, and enhanced shelf life. In these products, high proportions of protective substances contained in the raw material are preserved. Moreover, during the fermentation, lactic acid bacteria may produce additional health-promoting components and allow for a better preservation. The technological options for the production of fermented vegetable juices mainly include the following three: (1) spontaneous fermentation by autochthonous lactic acid bacteria, (2) fermentation by starter cultures added into raw vegetables, and (3) fermentation of mild heat-treated vegetables by starter cultures. Lactic acid bacteria microbiota of spontaneously fermented vegetable juices is mainly represented by the genera Lactobacillus, Leuconostoc, and Pediococcus, whereas the starter cultures most widely used were L. plantarum. Hardaliye, a traditional fermented beverage from Turkey obtained from red grape juice and crushed black mustard seeds, is an example of a spontaneous fermented juice in which L. paracasei subsp. paracasei and Lactobacillus casei subsp. pseudoplantarum dominate the lactic acid bacteria microbiota, which may be made up of several different species, including L. brevis, Lactobacillus acetotolerans, Lactobacillus sanfranciscensis, and Lactobacillus vaccinostercus. Overall, it was shown that fermented vegetable juices with optimal characteristics may be achieved by selecting lactic acid bacteria strains suitable for the fermentation of individual raw materials, according to factors such as the specific dependence on the supply of nutrients for growth and the chemical and physical environment. Vegetables such as cabbage, carrot, tomato, and spinach have proven to be suitable for the manufacture of fermented juices because of their content of fermentable carbohydrates. Fermentation of cabbage into sauerkraut juice is mostly obtained by spontaneous microbiota, although as with other fermented vegetables, the use of the starters was required to obtain a uniform product. In addition to the choice of the starter culture, to improve the quality and sensory properties of sauerkraut juice, the addition of other ingredients and the mixture with other juices was also considered. Carrot juice is one of the most common vegetable juices that can be strongly improved by lactic acid bacteria fermentation. Fermented carrot juice is microbiologically
stable, with good sensory properties and potentially high nutritional value. The use of selected starter cultures for carrot juice can improve juice yield thanks to the activity of pectinolytic enzymes and its iron solubility, and it also enables it to obtain higher mineral availability. The demonstration of suitability of tomatoes as the raw material for the manufacture of lactic acid fermented juice has opened new perspectives for this product. Commercially available tomato juices are usually subjected to thermal processing, which induces undesirable changes of color, flavor, and nutritional value. On the contrary, processing of tomatoes into fermented juice influences nutritional and sensory properties. Besides amino acids and vitamin and mineral content, lactic acid bacteria fermentation may also increase the amount of other phytonutrients that positively affect human health, having antitumor and antioxidant properties. The preservation of pigments and the synthesis of healthpromoting substances, such as aglycones and b-carotene, after lactic acid fermentation was proven in beetroots and sweet potatoes.
Innovative Vegetable-Based Fermented Products Innovation in food technology plays an important role in the improvement of the nutritional quality of products, possibly enhancing the hedonistic aspects of food intake. For instance, smoothies, originally consisting of purely fresh fruits and vegetables, were first introduced in the 1960s in the United States and reemerged in the 2000s. Recently, a novel protocol for the manufacture of fermented smoothies was set up. White grape juice and Aloe vera extract were mixed with red (cherries, blackberries prunes, and tomatoes) or green (fennels, spinach, papaya, and kiwi) fruits and vegetables and were fermented by mixed autochthonous starters, including strains of L. plantarum, W. cibaria, and L. pentosus. Lactic acid fermentation by selected starters positively affected the content of antioxidant compounds and enhanced the sensory attributes of the smoothies. The consumer demand for nondairy beverages with high functionality is growing as a consequence of the ongoing trend of vegetarianism and the increasing prevalence of the lactose intolerance. Some probiotic lactic acid bacteria are able to grow in vegetables and fruits, even if different species can show different sensitivities toward the pH of the substrate, postacidification of fermented products, metabolism products, temperature, processing, and gastrointestinal tract conditions. The major part of the studies considered the use of the probiotic species L. acidophilus, L. plantarum, L. casei, Lactobacillus rhamnosus, and Lactobacillus delbrueckii, and in a few cases, Leuc. mesenteroides and species of the genus Bifidobacterium as well. Tomato, carrot, cabbage, artichokes, and red beet juices were proven to be particularly suitable for probiotic fermentation, allowing a rapid growth of the strains and viable cell population above w8 log cfu g1. Nevertheless, the properties of the juice may be affected by probiotic microorganisms as shown by the fermentation of orange juice by L. plantarum, which provoked unsuitable sensory properties. Thus, a proper selection of the fruit matrices, probiotic strains, and eventually the addition of other ingredients seems fundamental to achieve optimal healthy and sensory properties.
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Conclusion Intrinsic microbiota of raw vegetables and fruits may represent a source of metabolic tracts that deserve increasing interest. Selection of autochthonous lactic acid bacteria and their use as starters may have important applicative repercussions. Thus, further insight is needed to explore the potential of the intrinsic autochthonous microbiota and consequently exploit the vegetables and fruits through lactic acid fermentation.
See also: Cocoa and Coffee Fermentations; Fermented Foods: Origins and Applications; Fermented Foods: Fermentations of East and Southeast Asia; Probiotic Bacteria: Detection and Estimation in Fermented and Nonfermented Dairy Products; Starter Cultures; Starter Cultures: Importance of Selected Genera; Starter Cultures Employed in Cheesemaking; Fruit and Vegetables: Introduction; Advances in Processing Technologies to Preserve and Enhance the Safety of Fresh and Fresh-Cut Fruits and Vegetables; Sprouts; Fruit and Vegetable Juices.
Further Reading Buckenhüskes, H.J., 1997. Fermented vegetables. In: Doyle, P.D., Beuchat, L.R., Montville, T.J. (Eds.), Food Microbiology: Fundamentals and Frontiers, second ed. ASM Press, Washington DC, pp. 595–609.
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Ciafardini, Di Cagno, 2012. Olive da mensa ed altri prodotti vegetali. In: Farris, A., Gobbetti, M., Neviani, E., Vincenzini, M. (Eds.), Microbiologia dei prodotti alimentari, CEA – Casa Editrice Ambrosiana. Milan, Italy, pp. 365–382. ISBN: 978-880818246-3. Di Cagno, R., Surico, R.F., Paradiso, A., et al., 2009. Effect of autochthonous lactic acid bacteria starters on health-promoting and sensory properties of tomato juices. International Journal of Food Microbiology 128, 473–483. Di Cagno, R., Cardinali, G., Minervini, G., et al., 2010. Taxonomic structure of the yeasts and lactic acid bacteria microbiota of pineapple (Ananas comosus L. Merr.) and use of autochthonous starters for minimally processing. Food Microbiololgy 27, 381–389. Do Espírito Santo, A.P., Perego, P., Converti, A., Oliveira, M.N., 2011. Influence of food matrices on prebiotic viability: a review focusing on the fruity bases. Trends in Food Science and Technology 22, 377–385. Karovicova, J., Kohajdová, Z., 2003. Lactic acid fermented vegetable juices. Horticultural Science 30, 152–158. Kabaka, B., Dobson, A.D.W., 2011. An introduction to the traditional fermented foods and beverages of Turkey. Critical Reviews in Food Science Nutrition 51, 248–260. Lee, C.H., 1997. Lactic acid fermented foods and their benefits in Asia. Food Control 8, 259–269. Pérez-Pulido, R., Omar, N.B., Abriouel, H., et al., 2005. Microbiological study of lactic acid fermentation of caper berries by molecular and culture-dependent methods. Applied and Environmental Microbiology 71, 7872–7879. Plengvidhya, V., Breidt, F.J., Fleming, H.P., 2004. Use of RAPD-PCR as a method to follow the progress of starter cultures in sauerkraut fermentation. International Journal of Food Microbiology 93, 287–296. Tamang, J.P., Tamang, B., Schillinger, U., et al., 2005. Identification of predominant lactic acid bacteria isolated from traditionally fermented vegetable products of the Eastern Himalayas. International Journal of Food Microbiology 105, 347–356.