Melanoidins and (poly)phenols: an analytical paradox

We suggest that phenolic acid and flavonoid ((poly)phenol) contents in plant-based heat-treated foods and products are often overestimated when measuring total phenolic content (TPC) using the Folin–Ciocalteu or antioxidant assays owing to the presence of melanoidins. The Maillard reaction occurs between reducing sugars and amino acids during thermal treatment of foods and beverages, with the final steps resulting in the formation of melanoidins, brown/black-colored, nitrogen-containing, high molecular weight polymers. In addition, (poly)phenols in plant-based foods can become incorporated into melanoidins and affect their properties. Since both melanoidins and (poly)phenols exhibit some overlapping biological properties, it is difficult to determine the relative contribution of each, and so we highlight various analytical methods able to separate them. In heat-treated foods, we recommend that more specific methods are used to measure (poly)phenol contents to avoid overestimation or that the melanoidin fraction is removed before TPC measurements are made. • (Poly)phenols are incorporated into melanoidins in heat-processed plant-based foods. • Melanoidins, Maillard reaction products, and (poly)phenols exhibit some common biological properties. • Folin–Ciocalteu and antioxidant assays cannot distinguish between melanoidins, Maillard reaction products, and (poly)phenols. • (Poly)phenols in plant-based heat-treated foods are overestimated in the presence of melanoidins and Maillard reaction products.