Fermentation is a metabolic process that releases energy from carbohydrates without the use of oxygen. It occurs in yeast and bacteria, but also in oxygen-starved muscle cells.
As a result of fermentation, carbohydrates can convert to acids (lactic acid, butyric acid, caproic acid, acetic acid, etc.), alcohol (butanol, ethanol, etc.) and gasses (carbon dioxide, hydrogen, methane).
The kinds of fermentation most commonly used by humans for practical purposes are:
- Alcoholic (ethanol) fermentation occurring in yeast – production of alcoholic beverages
- Acetic acid fermentation occurring in bacteria– production of vinegar
- Lactic acid fermentation a.k.a lacto-fermentation occurring in bacteria – production of lacto-fermented foods and beverages
So-called lacto-bacteria comprise hundreds of species, most of which belong to one genus, namely Lactobacillus. The genus was first studied in fermented milk, thus its name. However, the same bacteria produce the same lactic acid in the range of products other than milk.
Lactobacilli, like all bacteria, are omnipresent. This is why lacto-fermentation happens “spontaneously”; raw, unpasteurized food containing sugar e.g. cabbage will naturally turn into fermented food (sauerkraut) in the absence of oxygen. Lactobacilli, as oppose to most so-called spoilage bacteria (that cause the breakdown of proteins) are salt-tolerant. This is why adding salt will help to obtain sauerkraut rather than spoiled cabbage.
Lactobacilli thrive in low (acidic) pH, which is lethal for most putrefactive bacteria and fungi. For this reason, sauerkraut can be eaten years after it was made.
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