Fermentation, in a biochemist’s sense, is how microbes get energy from sugar without using oxygen: they break glucose down to pyruvate (glycolysis) and then turn the pyruvate into end products such as lactic acid, or ethanol and carbon dioxide. In food, the word is broader. An international expert panel defines fermented foods as “foods made through desired microbial growth and enzymatic conversions of food components” (Marco et al., 2021). That covers yogurt, sauerkraut, kimchi, bread, beer, wine, vinegar, miso, cheese and hundreds of traditional foods made with the help of bacteria, yeasts and molds (Tamang et al., 2016).
The Main Types
- Lactic acid fermentation: lactic acid bacteria turn sugars into lactic acid. Homofermentative species make mostly lactic acid; heterofermentative ones also make acetic acid or ethanol and carbon dioxide – which is why some ferments fizz (Gänzle, 2015). Sauerkraut, kimchi, pickles, yogurt and sourdough all depend on them. The old genus Lactobacillus was split into 25 genera in 2020, which is why you now see names like Lactiplantibacillus plantarum and Levilactobacillus brevis (Zheng et al., 2020).
- Alcoholic fermentation: yeasts, above all Saccharomyces cerevisiae, turn sugars into ethanol and carbon dioxide – wine, beer, cider and the rise in bread.
- Acetic acid fermentation: acetic acid bacteria oxidize ethanol to acetic acid. This needs oxygen, which is why vinegar and kombucha are made in open vessels.
- Mold fermentation: molds such as Aspergillus oryzae (koji, for miso, soy sauce and sake) and Rhizopus (tempeh) release enzymes that break down starch and protein.
- Bacillus fermentation: Bacillus subtilis and relatives make alkaline, sticky ferments such as natto and cheonggukjang.
Many foods use several at once. Kombucha and water kefir are made by communities of yeasts and bacteria; in one water kefir study, sugar was almost gone within a day and the main products were ethanol and lactic acid (Laureys & De Vuyst, 2014).
Succession: Microbes Take Turns
A spontaneous ferment isn’t a single microbe at work. Different species dominate at different stages as the food gets more acidic and the sugars change. In kimchi, Leuconostoc and Weissella lead early and more acid-tolerant lactobacilli take over later (Jung et al., 2014). That’s why taste changes from day to day, and why temperature and time matter so much.
What Controls a Ferment
- Salt draws water from vegetables and favors salt-tolerant lactic acid bacteria over many spoilage microbes. Vegetable ferments typically use about 2–3% by weight (NCHFP).
- Temperature sets the pace and the winners: vegetable ferments work best around 18–24°C, yogurt cultures at about 40–45°C.
- Oxygen: keeping vegetables under brine favors lactic acid bacteria and keeps out molds; vinegar needs air.
- Acidity: as acid builds, the pH falls and fewer microbes can grow – the main reason a good ferment keeps.
Safety
Acidity is the key safety line. US rules treat pH 4.6 or below as the threshold for acidified foods (21 CFR 114), because the bacterium that causes botulism can’t grow below it. Fermentation usually gets vegetables well below that, but only if it works: weigh the salt, keep things submerged, and follow tested methods. Low-acid ingredients stored moist and airless without acid – garlic in oil or honey, for example – are a botulism risk, and you can’t see, smell or taste the toxin (CDC). Fuzzy mold on soft foods, sliminess and putrid smells mean discard.
Fermented Doesn’t Always Mean Probiotic
Many fermented foods are baked, pasteurized, filtered or cooked, so their microbes are dead: bread, most beer and wine, shelf-stable sauerkraut, and cooked miso soup. A food is only “probiotic” if it contains defined live microbes shown to benefit health; most fermented foods haven’t been tested that way (Marco et al., 2021). Vinegar pickles aren’t fermented at all. The best human evidence so far is a 10-week study in which a diet high in fermented foods increased gut microbiome diversity and lowered inflammatory markers (Wastyk et al., 2021) – promising, but a small trial.
Try It Yourself
The quickest way to see all this is to make something: sauerkraut (lactic), yogurt (lactic, warm), sourdough (yeasts and bacteria together), kombucha (yeast, bacteria and acetic acid) or tempeh (mold). If something goes wrong, our troubleshooting guide can help.