Leaven

In cooking, a leaven is a substance that has a microbiological composition capable of causing fermentation. It is used to prepare raw food products for further culinary processing.
Starter cultures are mainly used for fermenting milk, accelerating the rise of dough, making drinks (kumis, beer, kvass, jelly).

Substances with an organic base containing fungi or enzymes are used as starter cultures.

Starter cultures for dairy products can be:

Enzymes formed by the third stomach of a calf or lamb (abomasum). This substance is the highest quality, but most expensive starter culture.
Fungal cultures. Most often it is a Bulgarian or Swiss stick, kefir or koumiss fungus. They help to obtain a product of standard quality.
Previously fermented finished products. They are more often used for fermentation at home, as they have a weaker effect.

Flour products are fermented by:

The inner part of the bark of some trees (alder, willow).
In order to achieve a quick but fragile effect on the food environment, wine vinegar, beer, dry sour grape wine are often used in cooking as a starter culture.