1. Because fresh vegetables are highly perishable, fermentation has been utilized for centuries to preserve these foods.
2. As early as the third century BC, the Chinese described the preservation of vegetables by fermentation.
3. Vegetable preservation is achieved by:
a. The addition of salt
b. The fermentative conversion of plant sugars to organic acids by LAB.
4. The degradation of all readily available carbohydrate is important to prevent undesirable secondary fermentations by acid tolerant yeasts.
5. Fermented vegetables continue to provide a significant portion of the human diet in Asia and parts of Europe and also in developing countries where canned or frozen vegetables are not widely available.
6. Preservation by fermentation is simple and energy efficient. Fermented vegetables can often be stored for a year or more without serious loss of flavor or texture.
In general, the procedure used to ferment vegetables:
1-remove dirt from vegetables (donŐt want to scrub them too much or you may lose some of the necessary bacteria)
2-add salt or brine
3-incubate anaerobically at a mesophilic temperature for several weeks.
Lactic Acid Bacteria (LAB)
1. LAB are primarily responsible for vegetable fermentation.
2. Their initial concentration is small; 0.01-0.1% of the total microflora.
3. Under the proper conditions, these organisms will overcome the other dominant microbiota within a few days.
4. LAB rapidly convert plant sugars (primarily glucose, fructose, and sucrose) to lactic and acetic acids which allows them to overwhelm all other organisms.
5. Vegetable fermentations are often characterized by a succession of lactic species, which is determined by each speciesŐ sensitivity to acid and salt.
The most important fermented vegetables in the United States are pickles, sauerkraut, and olives. Lesser amounts of other vegetables such as carrots, cauliflower, celery, onions, and hot peppers are fermented, usually in a mixed fermentation.
1. Cucumbers are the #1 fermented vegetable in the United States
2. Produced primarily in MI, WI, N.C., OH and CA.
3.A pickle is defined as an immature cucumber, properly prepared without taking up any metallic compound other than NaCl, and preserved in any kind of vinegar, with or without spices and sugar.
Pickling cucumbers are harvested while still immature. Cucumbers should be delivered to the fermentation plant as soon as possible after harvest because respiration in the tissues promotes the growth of undesirable softening organisms. Cucumbers are sorted and size-graded to obtain uniformity.
Three major types of pickled cucumbers are produced:
1. Fresh pack pickles, about 50% of the U.S. market, are not a fermented product.
2. Salt stock undergo a complete lactic acid fermentation in wood, plastic, or fiberglass tanks which will hold up to one ton of cucumbers.
3. Dill pickles are fermented in a dill-flavored, spiced, salt brine to generate their distinctive flavor and aroma.
Key determinants on the microbiology of pickles:
The composition and evolution of the dominant microbiota in pickles and other fermented vegetables is primarily influenced by the following:
1. Natural microbiota of the cucumbers. When properly handled (i.e. not washed excessively, of treated with antimicrobials), all vegetables will have lactic acid bacteria as a minor part of their natural biota (.01-.1% of total MO).
a. The initial stage of cucumber fermentation contains a wide variety of different bacteria, yeasts, and molds, which greatly outnumber the lactic acid bacteria.
b. Therefore, the most important stage of the process is the initiation of the fermentation, which lasts only 2-3 days. During this time, the numbers of LAB and oxidizing yeasts increase rapidly while undesirable bacteria are eliminated.
c. The key to vegetable fermentation then is to establish conditions, which promote the growth of lactic acid bacteria over all other microorganisms.
2. Salt concentration.
a. Dill pickles are usually fermented in a low-salt brine of 5% or less.
b. Salt stock pickles are fermented in tanks with a brine solution of 5-8% NaCl.
c. At salt levels below 5%, the fermentation is initiated by Leuconostoc mesenteroides whose early growth is more rapid than other LAB species.
1. Leuconostoc produces CO2 and organic acids, which drop the pH and inhibit undesirable microorganisms and enzymes that may damage the product.
2. The CO2 replaces air and creates an anaerobic condition, which also inhibits growth of undesirable aerobes.
3.The anaerobic environment and stimulatory activity of carbon dioxide promotes the growth of other lactic acid bacteria.
4. Heterofermentative Lactobacillus brevis and the homofermentative Lactobacillus plantarum and Pediococcus cerevisiae begin to grow rapidly and produce lactic acid, carbon dioxide, ethanol, and acetic acid which can contribute to the flavor of the product.
At higher salt levels (5-8% NaCl), the sequence of lactic microflora begins with the heterofermentative bacterium Lactobacillus brevis.
The fermentation is usually complete within 20 to 30 days and the more acid tolerant lactobacilli are predominant.
3. Fermentation temperature. The speed of the fermentation is governed by the temperature of the brine and the concentration of salt.
a. Optimum temperature for vegetable fermentations is between 21oC and 26.7oC.
b. At the end, total acidity may be as high as 0.9% lactic acid with a pH as low as 3.3.
4. Availability of fermentable carbohydrate.
a. Vegetables do not generally contain high levels of mono- and disaccharide sugars which can be easily fermented by most microorganisms.
b. As a consequence, most of the readily available carbohydrate is depleted during the fermentation.
c. Because residual sugar can exist and thus contribute to undesirable secondary fermentations by acid tolerant yeasts or lactobacilli, many pickle products undergo pasteurization (74oC for 15 min) in their glass containers before they are sold.
Defects of Pickles:
-pectinolytic or cellulolytic enzymes may be secreted by contaminating microorganisms.
a. These enzymes degrade cucumber outer tissues and result in damage that ranges from a general loss of texture or firmness to "slippery" pickles whose skin slips off.
b. Though produced by a wide variety of bacteria, yeasts and molds, pectolytic enzymes are inhibited < pH 5.0. As a result, his defect arises from poor acid production during the initial stages of fermentation.
c. Molds often grow and secrete softening enzymes into cucumber flowers. If all flowers are not removed from the fermentation tank, softening of the product is possible.
d. Enzymic contamination caused by flowers can be minimized by draining the brine once and replacing it with new brine. This reduces the amount of enzyme.
2. Gaseous spoilage:
One defect is termed "bloaters" i.e. pickles that float on the brine or are hollow or have large air spaces in the interior - due to formation of gas inside the pickles.
a-during the early stages of the fermentation, coliforms and certain halophilic bacteria can produce hydrogen gas and carbon dioxide.
b-fermentation of carbohydrates by yeasts produces gas
c-respiration by the cucumber itself can produce gas.
Methods to control gas production include:
a. piercing of the cucumber
b. purging carbon dioxide away with nitrogen gas. Nitrogen does not cause undesirable reactions nor does it stimulate aerobic microbes.
Bloaters are not a complete loss since they may be used in cut pickle and relish products at an economic loss of about 50%.
Second most common fermented vegetable in the U.S.
Sauerkraut is obtained by the full fermentation, chiefly lactic, of properly prepared and shredded cabbage in the presence of not less than 2% nor more than 3% salt.
When the fermentation is complete, sauerkraut contains no less than 1.5% acid expressed as lactic acid.
Method of Manufacture
1. Prior to making sauerkraut, the cabbage heads are wilted for two or more days to improve shredding since fresh heads fracture too easily.
2. The heads are then trimmed to remove the outer broken or dirty leaves and washed lightly to remove soil bacteria.
3. The cabbage is then sliced into long shreds .16 to .08 cm in width.
4. The shreds are mixed with 2-2.5% dry salt and packed into plastic lined concrete vats that may be as large as 12 to 14 feet in diameter and 8 feet deep. The shredded cabbage is packed firmly without crushing to reduce air pockets.
5. The vats are then covered with plastic sheets that are weighted with water to provide anaerobic conditions.
6. Respiration of the cabbage tissue and microorganisms quickly uses up residual oxygen in the tank.
7. If the seal is not airtight, aerobic bacteria, yeasts and molds grow on the surface of the kraut and produce undesirable effects.
1. Natural microbiota. Cabbage initially contains about 106 microbes/gram and this number includes bacteria yeasts and molds. Within two days of fermentation at 21oC, 90% of the microflora are lactic acid bacteria, and the pH drops from 6.2 to 4.8.
2. Salt. Like pickles, salt has an important role in the production of sauerkraut. Most producers use between 2.2 and 2.5%, which serves several functions:
a-extracts water from the shredded cabbage through osmosis thus forming the fermentation brine which contains carbohydrates and other nutrients needed for growth of the lactic acid bacteria.
b-suppresses the growth of some undesirable bacteria and influences the type and extent of lactic fermentation.
c-contributes to the flavor and texture of the sauerkraut
At the salt levels used in sauerkraut, fermentation is initiated by Leuconostoc mesenteroides and the succession of lactic acid bacteria proceeds as described for pickles. The anaerobic environment these bacteria help to create prevents oxidation of ascorbic acid and color in the cabbage.
Another important feature of the sauerkraut fermentation is that mannitol in the cabbage is fermented by the lactobacilli which keeps the product from tasting bitter.
3. Fermentation Temp. The optimum fermentation temperature is 18.3-21.1oC. Above 26.7oC, pediococci and Enterococcus faecalis initiate a rapid homolactic fermentation, which results in a raw or sour product.
4. Carbohydrate levels. The sugar content of cabbage is about 3-6%.
a. After four weeks at 21oC, the sugar content of the cabbage is totally depleted.
b. The fermentation is completed in 1 to 2 months depending on the quantity of fermented materials, concentration of salt, and temperature used.
c. The final product may have as much as 1.7% lactic acid, 0.25% acetic acid, a final pH of 3.6 or less, from 2.0 to 2.5% NaCl, and, hopefully, less than 0.13% ethanol. A higher ethanol content indicates the growth of yeasts and a lower acetic acid content indicates a depressed heterolactic fermentation.
Raw sauerkraut is packed in barrels or plastic pouches and is highly perishable so it must be kept under continuous refrigeration.
Sauerkraut canned in metal or in glass is pasteurized at 74oC to destroy the lactic acid bacteria and yeasts. Canned sauerkraut rarely spoils.
Defects and Spoilage - Most result from oxygen getting into the vat.
-surface discoloration due to autooxidation
-loss of acidity, off-flavors, colors, texture, and odors caused by growth of aerobic bacteria, molds and yeasts.
Slimy or ropy kraut has been observed for many years. It usually is caused by dextran formation by Leuconostoc mesenteroides. The slime is usually found at an intermediate stage of fermentation but with time, the dextrans are usually utilized by other LAB.
In rare instances, some strains of L. brevis will produce a water-soluble, heat-stable red pigment. It appears that production of the pigment is pH dependent, occurring at higher pHs of about 5.5 and studies have linked this defect to conditions where the pH does not fall properly
Several varieties of olives are produced in the U.S., but the market is dominated by canned ripe olives, a minimally fermented product (70% of total). Commercial olive production is confined almost entirely to the Sacramento and San Joaquin valleys of CA.
There five main varieties of olives produced in California:
Mission variety: with 20% or more oil.
Ascolano variety: with less than 15% oil - used to produce ripe olives.
Seveillano variety: with 15% oil is used for both ripe and green olives
Manzanillo variety: 16-18% oil, is an all-purpose olive.
Barouni variety: sold as a fresh product.
Once harvested, olives are destemmed, sorted, and size-graded. They are either processed immediately (or stored in salt brine for future processing) or made into fermented Sicilian or Spanish green olives.
a. Olives may be treated with lye (NaOH) prior to processing or fermentation in order to hydrolyze the bitter component in the olive, oleuropein.
b. The concentrations of lye and the number of treatments will vary depending on the particular type of olive and the manufacturers preference.
c. Lye is allowed to penetrate to the pit when ripe olives are desired and about two-thirds of the way in green olives.
d. The time of exposure varies from 4-7 h depending on the size of the olive and the temperature. Above 27oC, a strong lye solution may cause blistering of the olive skin by dissolving the pectins.
After the lye treatment, olives are washed several times to remove the base. To prevent softening, the olive flesh must be kept below pH 8.0. If there is too much residual lye, the final pH of the product will be too high.
Olives contain glucose, fructose, sucrose, and mannitol in concentrations between 3.7 and 7.5%. When olives are lye treated, as much as 65% of the sugars may be lost so glucose or sucrose are often added to the olives after the lye treatment.
Five main types of olives are sold in the U.S.:
1. Ripe black olives (most popular variety): No fermentation is involved. The black ripe olives are harvested when green with a red blush and made dark purple by oxidation of polyphenols in the flesh. Oxidation is achieved by treating the olives in lye solution (1-2% NaOH) with aeration. After oxidation treatment, the olives are rinsed, packed, and heat-treated at ll6oC for 60 min.
2. Green ripe olives. Also not fermented. Green-ripe canned olives are processed and canned at harvest time (called direct or fresh-cured). Absence of a lye treatment makes these olives quite bitter.
Sicilian-type, Spanish-type, and Greek type, are true fermented varieties.
Microbiology of olive fermentations:
Sicilian-type olives. Placed in 5-8% brine without lye treatment so this variety is quite bitter. Open fermentation tanks are filled much in the manner that cucumbers are. The olives are often needled so that their surfaces do not shrivel in the brine due to osmotic changes. Fermentation takes about 30 days at 15.6-21oC with Pediococcus and L. plantarum as the dominant lactic acid bacteria. Yeasts appear in the first two weeks and continue throughout the fermentation. The total acidity of the final product usually ranges between 0.2-0.7% as lactic acid. Final pH is about 3.6.
Spanish-type olives. Treated with 0.9 and 2.6% lye, which is allowed to penetrate about 3/4 of the way to the pit. After lye treatment, the olives are washed in water for 24 h with 3-4 water changes to remove residual base. These olives are fermented in 5-10% brine, which is often acidified with lactic acid to a pH of 4.5-5. Because of the high salt concentration, the initial and critical stage of the fermentation, which allows lactic acid bacteria to dominate, may require up to 14 days. At 10% NaCl, the only lactic acid bacterium present will be L. plantarum. Final pH is about 3.6 with an acidity of 0.4-0.6%. During the fermentation, sugar is exhausted and the olives turn the characteristic "olive green".
Greek type. Also treated with lye but aeration included in lye step to obtain the oxidized black color. The olives are washed then placed in a high salt brine (7-10% initial then increased to 15%). Because of the high salt level, these olives do not undergo a lactic fermentation. Instead, they appear to undergo fermentation by salt tolerant yeasts.
1. Gassy spoilage
-Characterized by blisters resulting from the production and accumulation of gasses which cause separation of the skin from the flesh and by the formation of gas pockets which may extend to the pits.
-Mainly coliforms, some Bacillus and yeasts may also cause gassy deterioration.
-Control - sanitation, controlled reduction of pH by acidification, and pasteurization.
2. Malodorous fermentations: Three main types, all are caused by bacteria.
a. Butyric acid fermentation
characterized by the formation of butyric acid which makes the olives taste like rancid butter. Most of the cultures responsible are related to Clostridium butyricum. This defect generally starts in the initial stages of the fermentation.
b. Hydrogen sulfide fermentation
caused by production of hydrogen sulfide gas (rotten eggs). Black brines may occur if iron is present as a result of iron sulfide formation. Desulfovibrio aestuarii, a halophile has been associated with this defect. Preacidification to below pH 5.5 will control this defect. Spoilage may be remedied by replacing the brine and then aerating violently to oxidize the hydrogen sulfide.
c. Zapatera spoilage
associated with a cheesy or sagey odor which sometimes develops into a foul fecal-like stench. This type of spoilage occurs when the desirable lactic acid fermentation is stopped before the pH of the brine has reached a value of 4.5 or less. Clostridium and Propionibacterium have been implicated in this defect. To control this type of spoilage, acidification must continue to pH 3.8 or below.
Softening spoilage is due to the pectolytic activity of bacteria, molds, and yeasts. High levels of cellulolytic enzymes cause sloughing spoilage characterized by skin rupture and sloughing. Olive softening can also be caused by intensive lye treatment, frosting, or heating. Difficult to differentiate these types of softening from microbial softening.
a. Outside of the United States, fermented vegetables are an important part of the diet of many peoples, especially in the Orient.
b. One example is Korean Kimchi. Korean people consume 50-500 g/day of kimchi, a fermented blend of radishes, turnips, onions, and Chinese cabbage. Sweet or sour peppers are often included to provide additional flavor.
c. 3% brine is used in the fermentation which occurs at 10-20oC.
d. Lactic acid bacteria responsible for this fermentation include L. mesenteroides, S. faecalis, L. brevis, L. plantarum, and P. cerevisiae.
e. Because the ingredients do not contain as much sugar as some other vegetables, the final pH is 4.2-4.5 (0.8% total acidity). The product also has notable carbonation due to the prevalence of heterolactic species.