Role of hydroxycinnamates in coffee melanoidin formation

Melanoidins are the high molecular weight brown end products of the Maillard reaction. They are formed during heat processing of foods like coffee, bread, malt, and beef. A chemical definition of these food polymers is still impossible, despite several efforts to determine their structure. In the last years, the interest in research on melanoidins has increased due to their biological activities. Coffee brew is one of the main sources of melanoidins in human diet. Various melanoidin fractions were obtained by applying chromatographic separation techniques or specific isolation procedures, allowing, a partial view on structural features and diversity of coffee brew melanoidins. Different melanoidin populations can be found with respect to total carbohydrate contents and their structural features. In this paper, the recent advances in research on coffee melanoidin structures and formation mechanisms are reviewed. The participation of hydroxycinnamates in melanoidin formation, especially true for coffee melanoidins, is a hypothesis older than three decades, but only recently more consistent data have been obtained for their presence. Although the role of hydroxycinnamates in melanoidin formation is not yet completely understood, it was demonstrated that the interaction between phenolic compounds and melanoidins can be of non-covalent or of covalent nature. The most likely linkage point is through the protein fragments incorporated in the coffee melanoidin during the roasting process, although carbohydrates, such as arabinose, seem to be possible binding sites for the chlorogenic acid derivatives on these brown structures, too.