Starter culture
The chosen lactic acid bacteria added to milk at the start of cheesemaking to sour it at a known pace.
Also: starter, culture, ferment
A starter culture is a measured dose of lactic acid bacteria added to the milk before rennet. The bacteria turn lactose into lactic acid, which helps rennet set the milk, controls how much whey the curd releases, protects against spoilage and sets the stage for ripening. Starters are chosen to match the cheese: mesophilic strains for cheddar, Gouda and soft cheeses, thermophilic ones for Alpine and Italian cooked cheeses, and gas-making strains for small eyes. Most dairies buy freeze-dried cultures from specialist suppliers; traditional makers keep their own as a mother culture or rely on the bacteria already in raw milk. Paneer and chhena need no starter because they are set with acid directly.