A reductase is an enzyme that catalyses the chemical reduction of an organic compound, meaning it adds electrons or hydrogen to its substrate, often with the help of cofactors such as NADH or NADPH. Reductases drive countless metabolic steps: they build fatty acids, synthesise cholesterol, convert folate to its active forms, and detoxify foreign compounds in the liver. The most famous is HMG-CoA reductase, the rate-limiting enzyme of cholesterol synthesis and the target of statin drugs. In food technology, reductases matter in fermentation, where microbial enzymes reduce compounds to create flavour, and in meat, where enzyme activity after slaughter influences colour and keeping quality. Each reductase is highly specific to its substrate.