Fermentation

In the technical sense, fermentation means transformations carried out by micro-organisms. These include alcoholic fermentation in beer, wine and cider, lactic acid fermentation for example in kefir or kvass, and acetic acid fermentation, which together with yeasts shapes kombucha. Cocoa beans are also fermented by microbes after harvest, as are post-fermented teas such as pu-erh. The microbes change flavour, acidity and shelf life; fermented drinks can contain small amounts of alcohol, which must be considered for non-alcoholic claims.

For black tea, by contrast, "fermentation" is only a colloquial name. After withering and rolling, the plant's own enzymes (mainly polyphenol oxidase) react with oxygen from the air and convert catechins into theaflavins and thearubigins. This turns the leaf coppery brown and creates the typical aroma. Technically this is an enzymatic oxidation in which no micro-organisms are involved. Green tea is not oxidised, oolong only partially.