A substrate is the substance on which an enzyme or other catalyst acts — the raw material that the catalyst converts into products. Lactose is the substrate of the enzyme lactase, which splits it into glucose and galactose; starch is the substrate of amylase; proteins are the substrates of proteases. The fit between an enzyme and its substrate is highly specific, often compared to a lock and key, which is why each enzyme catalyses only its own reactions. In food science the concept underpins fermentation, where yeasts act on sugar substrates to produce alcohol and carbon dioxide; cheese ripening, where enzymes act on protein and fat substrates to develop flavour; and laboratory assays that measure enzyme activity.