Mother culture
A starter kept alive from batch to batch, often as yesterday's whey, that carries a dairy's own bacteria forward.
Also: whey starter, siero innesto, backslopping, natural starter
A mother culture is a starter a dairy keeps going itself rather than buying fresh each day. The simplest form is backslopping: saving some whey or soured milk from one day's making and adding it to the next day's milk. Parmigiano Reggiano is still made this way, with siero innesto, natural whey starter from the previous day's batch, incubated overnight. Many traditional cheeses of India, Mongolia, the Middle East and Africa rely on the same idea, using sour whey from the last batch to curdle new milk, as in kalari and byaslag. Mother cultures carry the dairy's own bacteria forward and give a local character that bought cultures cannot.