Fermentation

In food, fermentation means converting carbohydrates to alcohol or organic acids. (chemically, it refers to consuming sugar in the absence of oxygen, in any way or context)

This is typically done with yeasts, or bacteria, and under anaerobic conditions (without a ready supply of oxygen).

This can be done for

• mainly the ethanol product, e.g. in wine, beer, cider, sake
• mainly the side product, e.g. yeast in bread dough produces CO₂ that leavens it
• preservation, e.g. the lactic acid in sauerkraut, yoghurt, various pickled food
• the taste, e.g. in kimchi (various preserved food is also acquired as a taste)

...or a mix of the above