is the gain of electrons or a decrease in oxidation state by an atom in a substance. The greater the degree of reduction, the lower the oxidation state of a given atom. A reduction always occurs in tandem with an oxidation. One reactant is reduced (gains electrons or decreases in oxidation state) as another is oxidized (loses electrons or increases in oxidation state). If a transfer of electrons occurs, the electron-donating substance is called a reducing agent or reductant. By donating electrons, it is itself oxidized.

Glucose and other sugars capable of effecting reduction are called reducing sugars. In monosaccharides, the carbonyl carbon (number one) in the linear form is oxidized to a carboxyl group. In polysaccharides, the end of a chain with a free anomeric carbon (that is, not linked to another sugar) is commonly called the reducing end. So, for example, lactose is a reducing sugar (because it has an anomeric carbon available for oxidation), whereas sucrose is not.

Reductants in beer protect it to a certain extent from the ravages of oxidation. Reductants are initially present in beer; chief among them are products of the Maillard reaction. Increasing boil length will increase formation of melanoidins and thereby reduction potential to an extent. Prolonged boils, however, can lead to faster staling through other mechanisms. It is therefore advised to boil within appropriate limits, neither minimizing nor overextending the boil. Caramel and other highly kilned malts will increase the reduction potential of the beer. Even in very pale beers, it was once common practice to add a small amount of highly kilned malt as an antioxidant. Polyphenols, sulfhydryls, nitrogen compounds, and hops also contribute to reduction potential.