Fermentation
Fermentation is the process in which microbes such as yeast, bacteria, and moulds break down sugars and other carbohydrates, turning them into acids, gases, or alcohol. Cooks use it to raise bread, preserve vegetables, thicken milk into yogurt, and build deep flavours in foods like soy sauce, cheese, and chocolate.
How it works
Microbes need energy. When they digest sugars without relying on oxygen, they release by-products, and those by-products are what change our food:
- Carbon dioxide makes bread rise and beer sparkle.
- Alcohol gives wine and beer their kick and contributes aroma to bread.
- Lactic and acetic acids make food tangy and protect it from spoilage.
- Flavour compounds such as esters, aldehydes, and free amino acids create fruity, savoury, cheesy, or funky notes.
Main types of fermentation
| Type | Microbes | By-products | Foods |
|---|---|---|---|
| Alcoholic | Yeast | Ethanol, CO₂ | Bread, beer, wine, cider |
| Lactic acid | Lactic acid bacteria | Lactic acid | Yogurt, sauerkraut, kimchi, sourdough |
| Acetic acid | Acetobacter | Acetic acid | Vinegar, kombucha |
| Mould-driven | Aspergillus and others | Enzymes, sugars, amino acids | Soy sauce, miso, sake |
Mould fermentation works differently: moulds like Aspergillus oryzae (see koji) release enzymes that break down starches and proteins. That creates sugars for yeast to ferment and free glutamate for intense umami.
Why fermentation preserves food
As microbes produce acid, the food's pH drops. Most spoilage and disease-causing bacteria can't grow in strong acid. At pH 4.6 or below, Clostridium botulinum can't produce toxin, which is the safety line used in home canning too. Salt, alcohol, and the good microbes crowding out the bad ones add further protection.
Controlling a fermentation
- Temperature. Warmer means faster; cooler gives better flavour. Bread dough at 24 °C (75 °F) and sauerkraut at 18–22 °C (64–72 °F) are reliable starting points.
- Salt. In vegetable ferments, 2–3% salt by weight favours lactic acid bacteria over spoilage microbes. In bread, about 2% salt keeps yeast from racing.
- Oxygen. Most vegetable ferments need to stay submerged. Vinegar, in contrast, needs air.
- Time. Longer fermentation means more sourness and more complex flavour, up to a point.
Fermentation in bread
Bread fermentation is a mix of alcoholic and lactic fermentation. Commercial yeast does most of the work in a standard loaf. In sourdough, lactic acid bacteria contribute sourness and help bread keep longer. Long, cool rises, including overnight fridge proofs, give both more time to build flavour.
Questions
What are the main types of food fermentation?
Alcoholic fermentation by yeast makes bread, beer, and wine. Lactic acid fermentation by bacteria makes yogurt, sauerkraut, and kimchi. Acetic fermentation turns alcohol into vinegar, and mould-driven fermentation makes soy sauce and miso.
Is fermenting food at home safe?
Yes, when you control salt, temperature, and acidity. Most fermentations become acidic quickly, and a pH below 4.6 stops dangerous bacteria like Clostridium botulinum from growing.
What is the difference between fermented and pickled?
Fermented foods become sour because microbes produce acid over days or weeks. Quick pickles are made sour instantly by adding vinegar. Some foods, like lacto-fermented dill pickles, are both.
Does temperature affect fermentation?
Strongly. Warmth speeds microbes up but can produce harsher or less complex flavours. Cooler fermentation is slower and usually tastes rounder and more nuanced.