What are melanoidins in coffee?

Melanoidins are high-molecular-weight brown polymers produced during the later stages of the Maillard reaction in roasting. They are responsible for much of the characteristic brown colour of roasted coffee and contribute to body, mouthfeel, and the bitter-sweet complexity of darker roast profiles. Melanoidins also have antioxidant properties and play a role in the texture of espresso crema.

As roasting progresses, the initial Maillard reaction products - small aromatic molecules contributing to the characteristic roasted aroma - continue reacting, polymerising into larger and larger molecules. These melanoidin polymers are largely insoluble and contribute to the perceived body and coating sensation of brewed coffee rather than volatile aroma. Darker roasts contain more melanoidins, which is one reason dark roasts have heavier body but less aromatic complexity than light roasts.

For roasters, melanoidins are part of the explanation for why roast level significantly affects body and mouthfeel: a longer roast produces more melanoidins, creating a heavier, more viscous brew; a shorter light roast retains more low-molecular-weight Maillard products that contribute to aromatic complexity but less body. In espresso specifically, melanoidins contribute to crema stability - they help the foam persist longer than it would from the coffee's CO₂ content alone.