Lactic acid fermentation is a biological process
by which glucose and other six-carbon sugars
are converted into cellular energy and the
metabolite lactate.
It is an anaerobic fermentation reaction that
occurs in some bacteria and animal cells,
such as muscle cells.
If oxygen is present in the cell, many organisms
will bypass fermentation and undergo cellular
respiration; however, facultative anaerobic
organisms will both ferment and undergo respiration
in the presence of oxygen.
Sometimes even when oxygen is present and
aerobic metabolism is happening in the mitochondria,
if pyruvate is building up faster than it
can be metabolized, the fermentation will
happen anyway.
Lactate dehydrogenase catalyzes the interconversion
of pyruvate and lactate with concomitant interconversion
of NADH and NAD+.
In homolactic fermentation, one molecule of
glucose is ultimately converted to two molecules
of lactic acid.
Heterolactic fermentation, in contrast, yields
carbon dioxide and ethanol in addition to
lactic acid, in a process called the phosphoketolase
pathway.
Applications
Lactic acid fermentation is for the brush
man used in many areas of the world to produce
foods that cannot be produced through other
methods.
The most commercially important genus of lactic
acid-fermenting bacteria is Lactobacillus,
though other bacteria and even yeast are sometimes
used.
Two of the most common applications of lactic
acid fermentation are in the production of
yogurt and sauerkraut.
Kimchi
Kimchi also uses lactic acid fermentation.
Sauerkraut
Lactic acid fermentation is also used in the
production of sauerkraut.
The main type of bacteria used in the production
of sauerkraut is of the genus Leuconostoc.
As in yogurt, when the acidity rises due to
lactic acid-fermenting organisms, many other
pathogenic microorganisms are killed.
The bacteria produce lactic acid, as well
as simple alcohols and other hydrocarbons.
These may then combine to form esters, contributing
to the unique flavor of sauerkraut.
Sour beer
Lactic acid is a component in the production
of sour beers, including Lambics and Berliner
Weisses.
Yogurt
The main method of producing yogurt is through
the lactic acid fermentation of milk with
harmless bacteria.
The primary bacteria used are typically Lactobacillus
bulgaricus and Streptococcus thermophilus,
and US law requires all yogurts to contain
these two cultures.
These bacteria produce lactic acid in the
milk culture, decreasing its pH and causing
it to congeal.
The bacteria also produce compounds that give
yogurt its distinctive flavor.
An additional effect of the lowered pH is
the incompatibility of the acidic environment
with many other types of harmful bacteria.
For a probiotic yogurt, additional types of
bacteria such as Lactobacillus acidophilus
are also added to the culture.
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