Melanoidins are brown and flavorsome pigments found in malts and malt products. Their structure varies and, in general, melanoidins from darker malts have higher molecular weights than those from pale malts (which are usually more aromatic). The structure of these brown pigments is largely unknown, but they are not the same as the melanins formed in some biological systems by the action of polyphenol oxidase on a substrate such as tyrosine. The formation of melanoidins is not catalyzed by enzymes, and most reactions do not require the presence of oxygen. All are formed by a series of complex processes called Maillard reactions (after the French chemist) and most of these are still imperfectly understood.
What we do know, however, is that reducing sugars interact with amino compounds (e.g., amino acids, simple peptides) to initially yield Schiff bases. These give rise to aldosamines and ketosamines by Amadori rearrangements. The latter may condense with another sugar molecule to form diketosamines, which are unstable and break down to give a range of products including hydroxymethylfurfural and reductones, and some of these products interact and polymerize to melanoidins. Reductones are useful because they consume oxygen and thus stabilize beer.
Melanoidin-producing (browning) reactions first occur during malt kilning and are then carried on during wort boiling. In malt, conditions favoring melanin formation include high temperatures, high moisture levels, and high amino acid and sugar concentrations. Browning reactions are, of course, carried to an extreme in roasted malts, such as chocolate, and in caramels. Because of the above- mentioned Maillard reactions, reducing sugar and amino nitrogen levels will, of course, be reduced during malt kilning and wort boiling. Flavor contributions by malt melanoidins can include bitter or burned flavors but also malty, toffee-like, bready, caramel, coffee, and roasted flavors. This being the case, melanoidins form a basis for the flavor profiles of many beer styles and are also among the major differences between flavors and aromas found in wine and those found in beer.