Transcript Microbiology of sauerkraut fermentation

Vegetable Fermentation
Traditional fermentations
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Under appropriate conditions, most
vegetables will undergo a spontaneous lactic
acid fermentation
Example of natural microflora of plant:
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Anaerobes: 105-106; aerobes: 106-107
Coliforms: 104-105
LAB: 101-103
Yeasts: 101-103
Molds: 101-103
Vegetable fermentation steps
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Harvest
Wash
Trim, and shred or size
Brine
ferment
Making sauerkraut
sauerkraut is "acidic cabbage." It is the result
of a natural fermentation by bacteria
indigenous to cabbage in the presence of 2 to
3% salt. The fermentation yields lactic acid as
the major product. This lactic acid, along with
other minor products of fermentation, gives
sauerkraut its characteristic flavor and
texture.
Vegetable fermentations
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Harvest
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Wash
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Special crop varieties for fermented vegetables
Growth conditions and harvest time affect sugar levels
Minimal
Trim
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Remove damaged parts and core, shred, or sort by size
Key points for vegetable fermentation
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Natural fermentation
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No heat process to inactive other flora
Natural lactic acid bacteria to carry out fermentation
LAB minor population, but dominant in successful product
fermentation
Succession: the fermentation depends not on any single
organism, but a consortium of bacteria representing several
different genera and species. A given organism (or group of
organisms) initiates growth and becomes established for a
period of time. Due to accumulation of inhibitory
compounds, growth slows down and gives way to other
species that are less sensitive to those factors. (Fig. 7.3)
Bacteriophage may also have a role
Microbiology of sauerkraut fermentation
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A definite sequence of lactic acid bacterial species
required
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Initiated by the heterofermentative Leuconostoc
mesenteroides
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Followed by heterofermentative rods such as Lb. brevis,
homofermentative Lb. plantarum and Pediococcus
cerevisiae
Sauerkraut
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Leuconostoc mesenteroides
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Has relatively short lag phase and high growth rate at low
temp (15-18C)
Heterofermentative pathway (lactic acid, acidic acid, CO2,
ethanol)
Acidic environment (0.6%-0.8%, as lactic acid) inhibit nonlactic competitor and favors other LAB
Acid approaches 1.0%, inhibit L. mensenteroides (4-6
days)
Other homolactic bacteria
Acidity 1.6%, pH below 4.0, only L. plantarum can
grow
Final acidity 1.7%, pH 3.4-3.6 (Fig 7-2)
Microbiology of sauerkraut fermentation
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Leuc. mesenteroides
 Gas-forming
 Rapid growth
 Active over a wide range of temp and salt conc.
 Produce lactic acid, acetic acid, CO2, lower pH rapidly
 Limit undesirable M/O and enzymes that might soften the
cabbage shreds
 Creates anaerobic atmosphere, prevent oxidation of ascorbic
acid and darkening of natural color of the cut cabbage and
stimulates growth of LAB
Incidental M/O
 G- coliform and pseudomonad types usually undetectable in a
day or two
Microbiology of sauerkraut fermentation
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Lb. brevis, Lb. plantarum, Ped. cerevisiae increase
rapidly
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Contribute to the major end products including lactic
acid, acetic acid, carbon dioxide, ethanol
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Minor end products
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Volatile compounds: diacetyl, acetyladehyde,
sulphur compounds, ethyl butyrate, etc.
Microbiology of sauerkraut fermentation
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Control of salt of fermentation
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Control of temp of fermentation
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Brine, flavor, control the growth of M/O
2.25% salt, 18°C (65°F)
Temp increases, LAB sequence changes too
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Lenc retarded, Lb dominant
At 32°C and above, homofermentation dominant, flavor and
aroma deteriorated, reminiscent of acidified cabbage due to
LA, darkened readily
Defects & spoilage of sauerkraut fermentation
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Discoloration (autochemical oxidation)
Loss of acidity
Off-flavor and odors (moldy, yeasty, rancid)
Slimy
Softened kraut and pink-colored kraut
Due to aerobic growth of molds and/yeasts
Control-create anaerobiosis
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Shift in microbial community
Leuconostoc mesenteroides - dominant micro popln
@ 21oC grows well
Produces mannitol
Not inhibited by 2.5% salt
Up to 1% lactic acid accumulate
Yeast & various bacteria may grow as surface film
Continuing succession
Lactobacillus plantarum - produces acid (no gas)
[Lactic acid] reaches 1.5-2%
Growth removes mannitol (has bitter flavour)
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Fermentation can be STOPPED
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Residual sugar & mannitol
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Canning or refrigeration
after L. plantarum continues succession
L. brevis
Increase [Lactic acid] to 2.4%
Imparts bitter acid flavour
High Quality Sauerkraut:
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[Lactic acid] 1.7%
Low [diacetyl] contribute to flavour
To make sauerkraut the cabbage must be shredded to
produce a large surface area for the growth of the
microbes and to extract the plant juice nutrients which will
be metabolized by the microbes. Sodium chloride (table
salt) is added to a concentration of 3% to provide
OPTIMUM CONDITIONS FOR GROWTH of the desired
fermenting bacteria, to help EXTRACT the tissue juices,
and to INHIBIT the growth of microbes (molds) that would
ruin the cabbage.
The cabbage/salt mixture is weighted down to squeeze out
the juices and incubated at room temperature in covered
containers. The cover inhibits the entry of OXYGEN into
the mixture and allows ANAEROBIC FERMENTATION
occur. At the end of the fermentation period the pH should
be ~ 2.0 and the sauerkraut should contain about 1% lactic
acid.
The sauerkraut fermentation process utilizes the indigenous
population of bacteria in the raw cabbage to produce lactic
acid. This produces a low pH environment that allows few if
any other bacteria to survive. The lactic acid is also what
gives the kraut it's characteristic sour flavor. Salt is added to
the raw cabbage to draw out much of the water (drier
product keeps longer) and to inhibit salt-intolerant bacteria.
This allows the acid producing bacteria to get a strong foot
hold and dominate the population.
Throughout the fermentation, it is critical that
oxygen be excluded. The presence of oxygen
would permit the growth of some spoilage
organisms, particularly the acid-loving molds
and yeasts.
As no starter cultures are added to the system, this is
referred to as a wild fermentation. The normal flora of the
cabbage leaves is relied upon to include the organisms
responsible for a desirable fermentation, one that will
enhance preservation and organoleptic acceptability. The
floral succession is governed mainly by the pH of the
growth medium.
Pickle Production
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Any vegetable or fruit preserved by salt or acid
Most important: cucumber
1 billion Kg in the US used for pickles (half of the
crop)
Now more than half of the pickles are not fermented
(direct add acetic acid)
Types
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Fresh-packed (non-fermented)
Refrigerated (non-fermented)
Fermented (processed)
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Distinctive flavor and texture
Manufacture of fermented pickles
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Rely on salt, oxygen exclusion, anaerobiosis to
select for growth of
instead of dry salt
Salt conc. higher than that for sauerkraut
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Less diverse microflora
Brine at least 5% salt, some 7%-8%, up to 12%
Up to 2 months, end pH ~3.5, acidity 0.6%-1.2% (as lactic)
L. mensenteroides cannot grow
Initiated by L. plantarum and Pediococcus sp.
Brine condition inhibitory to coliforms and other non-LAB
De-salted after fermentation for further consumption
Can use starters (controlled fermentation) (Fig 7-5)
Defects
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Pickles
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Bloaters and floaters (Table 7-4)
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Excessive gas pressure, internal cavity formation
LAB (heterolactic, malolactic fermentation), coliforms, yeasts
Control: remove dissolved CO2 by flushing or purging with
nitrogen gas
Some can still be used
Destruction and softening
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Slippery, loses crispness and crunch
Cannot be used
Pectinolytic enzymes by microorganisms
Fungi
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Penicillium, fusarium, Alternaria, Aschyta, Cladosporium
Control: acidity