Fermented Vegetables

 Fermented Vegetables

Vegetables were preserved by fermentation from time immemorial. A wide range of vegetables and fruits including cabbages, olives, cucumber, onions, peppers, green tomatoes, carrots, okra, celery, and cauliflower are preserved. The fermentation process for vegetables result in nutritious foods that may be stored for extended periods without refrigeration. Before fermentation, fresh fruits and vegetables contain a variety of microorganisms, including aerobic spoilage bacteria, yeasts and molds. Their growth deteriorates the vegetable material, due to the degradative enzymes such as proteases, lipases, amylases, nucleases, etc released from these microbes.  Brining vegetables for fermentation results in the production of organic acids, CO₂ and a variety of antimicrobial compounds by lactic acid bacteria.  The production of acid quickly lowers the pH which inhibit the development of undesirable microorganisms and their enzyme activity, carbon dioxide produced provides anaerobic conditions which favors the stabilization of ascorbic acid and the natural color of the vegetables.

Vegetables are fermented through either of the following methods

·         Dry salted – vegetable is treated with dry salt and the juice from the vegetable forms the brine.  Generally, for 100 kg vegetable 3 kg of salt is needed. 

·         Brine salted – Brine solution of 15-20% salt is used in which the vegetables are kept. 

·         Non-salted – vegetables are fermented by lactic acid bacteria without addition of salt

Biochemistry of vegetable fermentations: During the fermentation of cucumbers, cabbage and olives, glucose and fructose are converted to lactic acid, acetic acid, ethanol, and CO₂ by LAB and yeasts. The primary pathway for homofermentative LAB involves the breakdown of six-carbon sugar (glucose) to give three-carbon lactic acid molecules. Heterofermentative organisms use a more complex metabolism. Glucose is initially converted to CO₂ and a five-carbon sugar phosphate, which is further degraded to lactic acid and a two-carbon compound, ethanol or acetic acid.

Cabbage Fermentations – Sauerkraut and Kimchi

Examples of commercial production of fermented cabbage are kimchi in Korea (made from the Chinese cabbage, Brassica rapa) and sauerkraut in the United States and Europe (made from Brassica oleracea).

Sauerkraut

Cabbage is sliced into thin pieces known as slaw and preserved in salt water or brine containing

about 2.5% salt at a temperature of about 15°C. The slaw must be completely immersed in brine to prevent it from darkening. Fermentation is initiated by Leuconostoc mesenteroides which is a heterofermentative lactic acid bacterium which can grow over a wide range of pH and temperature.  The CO₂ generated during fermentation will create an anaerobic condition that eliminate other microorganisms and encourages the growth of other lactic acid bacteria such as L. brevis, L. plantarum, Pediococcus cerevisiae, etc. Coliforms and pseudomonads disappear.  Lactic acid bacteria produce compounds such as lactic and acetic acids, ethanol, and volatile compounds such as diacetyl, acetaldehyde, acetal, isoamyl alcohol, n-hexanol, ethyl lactate, ethyl butarate, and iso amyl acetate which contribute to the flavor of sauerkraut

Sauerkraut fermentations are done in large fermentation tanks that contain 100 tons or more of shredded or chopped cabbage. Cabbage contains 4 to 5% sugar (2.5% glucose and 2% fructose).  The shredded cabbage is dry salted and conveyed to fermentation tanks, brine with an NaCl concentration of about 2 to 3% will be formed in the fermentation tanks. During the first 24 to 48 h, carbon dioxide and lactic and acetic acids are produced through the initial heterolactic fermentation. The volatile acetic acid contributes to the flavor and aroma. Heterofermentative microorganisms die off after one week and are replaced by the acid-tolerant homofermentative microorganisms. Sauerkraut may be stored for up to 1 year in fermentation tanks until it is processed for food service or sale.

It is reported that two heterolactic species, L. mesenteroides and Lactobacillus brevis, and two homolactic species, Pediococcus cerevisiae and Lb. plantarum are the bacteria present in the fermentation.

The salt serves a number of purposes in fermented cabbage

(i) it extracts moisture from the shredded cabbage by osmosis to form the brine in which the fermentation take place

(ii) it inhibits some of the natural microflora of the cabbage which may otherwise cause spoilage and helps to selective growth of the lactic acid bacteria

(iii) it helps to maintain the crisp texture of the cabbage by withdrawing water and inhibiting endogenous pectinolytic enzymes

(iv) It contributes to the flavour of the product.

Kimchi

Kimchi fermentation is microbiologically similar to sauerkraut fermentation, except the ingredients, flavor, and preparation methods. For cabbage kimchi, the fresh cabbage is cut in half lengthwise or quartered and initially soaked in brine of 5 to 10% NaCl to wilt the cabbage. The cabbage is then washed and drained. An aqueous paste of ground red pepper is prepared and mixed in with the cabbage leaves. Small amounts of additional ingredients, such as garlic, ginger, and jeotgal (a highly salted (20% NaCl) anchovy (a small fish)) and green onion are usually included.  Kimchi fermentation also has a biphasic heterofermentative and then homofermentative pattern of LAB succession, with a few prominent species (L. mesenteroides, P. cerevisiae, Lb. brevis, and Lb. plantarum).  After the fermentation, the final salt concentration is between 3 and 6%.

In rural areas, kimchi is traditionally packed into earthen jars and buried in the soil and in urban South Korea, kimchi is prepared using household kimchi refrigerators, which are small, programmable refrigerators that provide an initial 18°C fermentation period of a few days, followed by very cold refrigeration (1 to 2°C).

More than 65 different types of kimchi are identified based on differences in raw materials and processing.

The vitamin B content increases during sauerkraut and kimchi fermentations, and vitamin C and A are preserved.

Olive Fermentations

There are several methods for fermenting olives such as green table olives, natural black olives in brine, and canned ripe black olives.

Green table olives are treated with lye (1 to 3% NaOH) and then washed prior to being brined and fermented. The NaOH treatment reduce the natural bitterness of the fruit, due to the degradation of oleuropein, reduces the antimicrobial activity of the phenolic components of olives and makes the skin of the olive more permeable allowing sugar diffusion to improve fermentation. After NaOH treatment, the olives are washed and brined in 10% NaCl for fermentation. The initial microflora during fermentation include a variety of gram-positive bacilli (Bacillus species) and gram-negative enteric bacteria (Enterobacter, Citrobacter, Klebsiella, and Escherichia).  As organic acids accumulate and the pH decreases below 6, the LAB such as Lb. plantarum and Leuconostoc mesenteroides dominate the fermentation and yeast species such as Candida, Pichia, Saccharomyces, and others are also present. They contribute desirable flavor characteristics. Following fermentation, they are pitted and stuffed before sale.

Natural black olives are prepared by a slow fermentation without lye. For this black olives are picked in a ripened state and have a black color as well as a softer texture than green table olives. Fermentation slower due to the lack of NaOH treatment. Antimicrobial phenolic compounds diffuse into the brine and diffusion of sugars is also reduced, which slows fermentation and it may take months to complete. The microflora is usually yeasts of the genera Saccharomyces, Hansenula, Candida, Torulopsis, Debaryomyces, Pichia, Kluyveromyces, and Cryptococcus.

Ripe black olives are prepared by darkening olives through oxidation in an alkali followed by washing and canning.  These are prepared from green or semiripened olives and are brined without an initial NaOH treatment. Following storage in brine for up to 1 year, the olives are subjected to oxidation treatments in the presence of 1 to 2% NaOH which blackens the olives.  They are then washed with water to remove NaOH and then canned in 1 to 3% NaCl brine. Sterilization is needed for black olives to prevent botulism, because the pH is generally above 4.6.

Sinki

This is a non-salted fermented radish tap root product traditionally consumed in some north-eastern states of India, Nepal and Bhutan. Fermentation is carried out by Lactobacillus plantarum or Lactobacillus brevis.  It is the most popular pickle in Nepal.

Fresh radish roots are washed and wilted in sun light for 1-2 days.  They are shredded, washed and packed in a jar and kept for fermentation at 30^oC for about 12 days.  During this period, L fermentum and L brevis followed by L plantarum carry out fermentation and drop the pH to 3.3.  After fermentation, it is sundried to remove moisture content and processes in a clay line pit for 2-3 months.

Gundruk

This is a non-salted fermented vegetable made from the leafy vegetables such as Mustard, Cauliflower and raddish. This is used in Nepal.  Shredded leaves are packed in an earthen pot with wamwater covering the leaves in a warm place for 5-7 days.  After fermentation it is sundried.  Pediococcus pentosaceus, Lacobacillus cellobiosus and Lacobacillus plantarum  are the organisms involved. 

Sunki

This is a non-salted fermented vegetable prepared from the leaves of Otaki Turnip in Japan.  The otaki-turnip is boiled and mixed with a wild small apples and dried sunki from previous year and kept for fermentation for 1-2 months at a low temperature, preferably during winter season.  The microorganisms involved in fermentation are Lactobacillus plantarum, Lactobacillus brevis, Bacillus coagulans and Pediococcus petosaceus

Kawal

This is fermented leaf preparation from a Wild African Legume Cassia obtusifolia.  This is highly rich in protein and used a meat analogue.  The leaves are not washed, ground into a paste and kept in earthen ware pot for 14 days.  The contents are mixed once in three days.  Microorganisms involved are Bacillus subtilis, Propionibacterium, Lactobacillus plantarum, Candida krusei, Sacharomyces, Rhizopus, etc.

 

Cucumber Fermentations (Pickling)

The word “pickle” usually refers to a pickled cucumber.

Cucumber pickles may be prepared unfermented, partially fermented or fully fermented.  They are then heated so that the interior of the cucumber reach 73.9^OC for 15 minutes.  Cucumbers (Cucumis sativus) for pickling are harvested before they are mature. Mature cucumbers are generally not fermented since they are large, ripen easily and are full of mature seeds.  Cucumber fermentations typically undergo a homolactic acid fermentation by Lb. plantarum and related LAB, which does not result in production of carbon dioxide. 

Most commercial cucumber fermentations depend upon growth of the LAB that are naturally present on the surface of cucumbers. Sometimes starter cultures are used to enhance product consistency.  (Starter cultures consist of microorganisms that are inoculated directly into food materials to carry out fermentation to cause desired and predictable changes in the food product).

There are two types of fermented pickles – Salt or Salt-stock pickle and dill pickle. 

Salt or Salt-stock pickle

Immature cucumbers are washed, packed in barrels or tanks and brined.  Sometimes 1% glucose is added.  Depending upon the level of salt, two methods are there, high salt method and low salt method.  In the low salt method, a low amount of salt is added and the concentration is gradually increased until enough concentration is reached to prevent contaminating bacteria.  8% brine is added initially and 9 lb salt per 100 lb cucumber are added.  In high salt method, 10.5% brine is added initially and 9 lb salt per 100 lb cucumber are added. 

During the primary fermentation for about two or three days, most of the unwanted bacteria disappear and the lactics and yeasts proliferate. Initially there will be Pseudomonas, Flavobacterium, Bacillus spp, Leuconostoc mesenteroides, streptococcus fecalis, etc.  In the final stages, after 10 to 14 days, Lactobacillus plantarum and L. brevis, followed by Pediococcus, are the major organisms.  The traditional process takes 6-9 week for completion. 

Commercial cucumber fermentations are commonly done in 30,000- to 40,000-liter, open-top, fiberglass tanks exposed to sunlight.  The UV radiation in sunlight kill aerobic surface yeasts that can metabolize lactic acid produced by the fermentation. Cucumbers are covered with salt brine (6% NaCl) and held below the brine surface. Calcium chloride (0.1 to 0.4%) is added to maintain the firm, crisp texture of the fermented cucumbers during fermentation and storage. After fermentation cucumbers may be stored in the fermentation tanks for 1 year or more. Fermented cucumbers are washed to remove excess salt and then packed with an appropriate cover liquor which contains acetic acid and spices in addition to residual lactic acid. Further microbial growth is prevented by the organic acids, low pH, and lack of fermentable sugars.

Dill pickles

These are named so because they are flavored by the addition of a herb, dill and some other spices, garlic and onion may also be added.  Here a lower concentration of salt is used and the brine is acidified using vinegar.  The low salt content allows more microbial production of acids. 

Overnight dill pickle

These are prepared by slow acid fermentation at low temperature in a low acidified brine.  The brined cucumbers are held at 3.3^OC when they undergo slow lactic acid fermentation and 0.3-0.6% acid is developed. These are having short keeping time. 

Genuine dill pickles

A brine containing 7.5-8.5% salt is added to cucumber and the concentration of salt in the finished pickle will be 3.5-4.5%.  Temperature between 15-30^OC is used.  The final acidity ranges from 1-1.5%.

Pickle defects and spoilage

·         Shriveling results from physical damage due to too much strong salt, vinegar or sugar solution.

·         Hollow pickles are formed when the cucumbers are left to stand for some time after harvesting and before fermenting.

·         Floaters or Bloaters - Some heterofermentative LAB form carbon dioxide during malolactic fermentation which result in the creation of bloaters.  These are fermented cucumbers with undesired internal gas pockets.

·         Slippery pickles – when cucumbers are exposed to air, capsulated bacteria may grow.

·         Soft pickles are the result of pectinolytic enzymes from molds or cucumber flowers.

·         Black pickles develop due to the formation of hydrogen sulfide by bacteria.

·         Ropy pickle brine is due to the growth of certain gram negative capsulated bacteria.

 

 

References

Food Microbiology: Fundamental and Frontiers, 4^th Edition, Michael P Doyle and Robert L Buchanan

Modern Industrial Microbiology and Biotechnology, Nduka Okafor, Science Publishers

Food Microbiology, Third Edition, Martin R. Adams and Maurice O. Moss University of Surrey, Surrey, Guildford, UK.

Food Microbiology, William C Frazier, Dennis C Westoff, K N Vanitha

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