Vegetable
Fermentations
Introduction
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.
A. Pickles
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:
1. Softening:
-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.
Reasons:
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%.
2. Sauerkraut
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.
Sauerkraut Microbiology:
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
3. OLIVES:
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.
Lye Treatment
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.
Spoilage
Problems:
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.
3. Softening:
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.
4. Kimchi
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.