Fermentation is a general term used to describe any processes where a change arises because of the action of living microorganisms and their enzymes on a food or drink. The best known processes are the actions of yeasts on sugar solution in alcohol fermentation and the rising of bread dough, and of bacteria that make lactic acid in kimchi, sauerkraut and yogurt. Bacteria and moulds can be the active organism, as well as yeasts. Fermentation is also used to describe the oxidative actions of enzymes in turning fresh tea leaves black.
Fermentation is utilised in alcohol-making - brewing, wine-making - and food preparation - bread, cheese, sauerkraut, yoghurt. Fermentation is also part of the process of making coffee, cocoa, some sausages and vanilla. These uses fall into two main categories:
- Preservation: this converts food from a short shelf life product to one with a longer life-span, e.g. converting milk to cheese or yoghurt.
- Digestibility: this converts ingredients from their original state that is indigestible into a useful and digestible form, e.g. wheat and soya beans into bread and tempeh.
The control of fermentation depends on adjusting the environment (food material, acidity, saltiness, temperature etc) that the organism or organism will grow at most effectively, or what unwanted organisms grow most slowly at or not at all; hence, cold (fridges) and freezing (freezer) are used to reduce fermentation of dairy products, for example. In general, it is usually more than one organism that is active rather than just a single one, so making a good fermented product is about achieving the best balance of types of organism, which is particularly important for flavour. For example, particular bacteria produce small quantities of chemicals to give a creamy taste in milk.