Food Browning as a Polyphenol Reaction

Abstract

With increasing emphasis being placed on consumer acceptance of foods, the retention of original color has become a matter of great importance. In this respect, the need to prevent the formation of undesirable color during processing has always been a challenge to food technologists. In many edible plant products, formation of several shades of colors from pink to bluish-black, generally termed “browning,” has been observed. Except in some food products, such as baked cereal food, black tea, and roasted coffee, these color changes or browning are considered unwelcome from the point of view of consumer acceptability. In recent years, polyphenols have received increasing attention from chemists and food technologists. Such compounds present in food have been found to take part in both enzymic and nonenzymic browning reactions. The specific enzymes that take part in browning reactions involving polyphenols have been known by different names but in general can be referred to as polyphenoloxidases. Enzyme chemists have been able to isolate, purify, and characterize polyphenoloxidase enzymes from several sources. The oxidation of polyphenolic substrates, secondary reactions, inhibition, and activation has also been investigated thoroughly during the last few decades. During the processing and storage of food products, especially fruits and vegetables, several nonenzymic changes leading to browning involving polyphenols have been noted. The common cause of darkening of many products is attributable to the interaction between polyphenols and heavy metals especially iron. Formation of colored anthocyanidin pigments has been suspected in others. Different aspects of food browning attributable to polyphenol reactions have been covered separately in several papers, however, a complete review embracing all facets of this problem is useful to both research workers and to the food industry.