Around 1876, German physiologist Wilhelm Kühne (1837–1900) proposed that the term “enzyme” be used to denote phenomena that occurred during fermentation. The word itself means “in yeast” and is derived from the Greek en, meaning “in,” and zyme, meaning “yeast.”

Enzymatic reactions can build up or break down specific molecules. The specific molecule an enzyme works on is the substrate; the molecule that results from the reaction is the product. For example, for people who are not lactose intolerant, lactose is the substrate, lactase is the enzyme, and the products are glucose and galactose.