Biology 103 - Microbes and You

Lecture 24 Outline

Pickling/sauerkraut (succession of species)


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Sauerkraut, succession of species, and lactic acid fermentation
fermented vegetables are all basically made in the same way
conditions are created that inhibit the growth of pathogens and unwanted microbes and enhance the growth of the desired microbes
for sauerkraut, cabbage is first shredded, releasing cell contents like sugars
the cabbage is then salted to about 2.5% and packed in crocks and stored
high salt concentrations kill many microbes and many microbes cannot live in the anaerobic conditions of the crock
only the bacteria that are found on the natural cabbage leaves are added to the mixture
first coliform bacteria and other aerobes begin to grow
these quickly use up the oxygen creating anaerobic conditions where facultative anaerobes begin the fermentation process, generating gases, volatile acids, and some lactate (lactic acid)
as lactate levels rise, this favors microbes that can grow with high lactate concentrations and low pH values
the coliform bacteria are *succeeded* by lactic acid bacteria
first the Leuconostoc arise and continue to ferment, accumulating lactate to 1%
at this high level of lactate, Lactobacillus succeeds the Leuconostoc as the dominant bacteria
fermentation continues and lactic acid accumulates to 1.5-2%
the Lactobacillus also removes the remaining bitter mannitol from the sauerkraut
commercial sauerkraut is 1.7% lactate giving the product its distinctive tart flavor

Lactic acid fermentation
in aerobic respiration, glucose (or other sugars) are broken down via glycolysis to small molecules
these can then enter the Krebs cycle where oxygen is used to burn the carbon compounds to carbon dioxide releasing energy for ATP production
under anaerobic conditions, the Krebs cycle cannot run
aerobic respiration produces dozens of ATP per glucose molecule respired
anaerobic metabolism must rely on glycolysis to produce ATP, but only 2 ATP per molecule of glucose is produced
glycolysis converts the glucose (6 carbons) in two pyruvate molecules (3 carbons)
lactic acid fermentation then takes the pyruvate and converts it into lactate
this is a reduction reaction and converts NADH to NAD
during the conversion of glucose to pyruvate, NAD is reduced to NADH
in the absence of oxygen it is difficult to regenerate NAD through oxidation
thus, pyruvate is used to oxidize NADH to NAD
the reoxidizing of reducing equivalents is a critical feature of fermentation

Olives
this story is similar to the sauerkraut tale
olives are harvested and salted to 5-10% and packed in crocks
most bacteria are killed, but the lactic acid bacteria proliferate
again the population is dominated by Leuconostoc and Lactobacillus
fermentation takes place over 6-10 months and the lactic acid concentration rises to 7%
near the end of the fermentation process salt-tolerant acid-tolerant yeasts begin to grow as well
green olives are treated with lye (NaOH) before fermenting
black olives are treated with lye after fermentation
lye removes compounds that give raw olives a very bitter taste

Pickles
the fermentation of cucumber pickles is a little bit different
here, the salt is added gradually over several weeks up to 16%
initially, a mixed microbial community of generally soil bacteria like Pseudomonas and Bacillus proliferate
as the salt concentration increases, the microbial community begins to shift to the lactic acid bacteria
Lactobacillus predominates until the salt concentration reaches about 10%
at this point salt-tolerant yeasts begin to grow and eventually take over the community
the yeasts lower the lactate concentration somewhat and add other flavor compounds
the sourness of a pickle is determined by its lactic acid content
the pickling process is rather tricky and it is easy to get a bad batch of pickles
this is due in part to the low salt concentrations early in the fermentation, which can allow undesirable organisms to proliferate

Dill pickles are made differently
here 7.5-8.5% salt is added to start, along with dill for flavor, and vinegar (acetic acid)
indigenous soil bacteria grow first
as the lactate concentration increases, Leuconostoc takes over and there is a significant community of Streptococcus as well
remember that Streptococcus are found on teeth surfaces and ferment sugars to lactic acid which then dissolves the calcium in teeth
when the lactate concentration gets high enough, Lactobacillus begins to predominate the community
dill pickles, commonly called vinegar pickles, at about 1-1.5% lactic acid

Poi
this is another fermented product, but it is made from taro stems
initially, Pseudomonads predominate the fermentation
then Lactobacillus takes over
finally a fungus Geotrichum is added to give some characteristic flavors


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