Effect of polyphenols on the browning reaction in barley paste after heating

To analyze the contribution of polyphenols in barley grain to the browning reaction, we defined the browning index as the decrease in the Hunter's W value of the paste during the heating process (90 degrees C, 2h) and analyzed the browning index, total polyphenol content and polyphenol composition. When Ichibanboshi grains were pearled to 90, 75. 60, 45%, both the browning index and total polyphenol content were highest ingrains pearled to 90%, and prodelphinidin B3 (PDB3), procyanidin B3 (PCB3) and the component in peak C were mainly found in the peripheral part of grains. The browning index, total polyphenol content and polyphenol composition depended on the pearling rate of grains. Among 9 hulless (pearled to 60%) and 6 hulled (pearled to 55%) barley lines and cultivars, the total polyphenol content was highly correlated with the browning index (r=0.845, P0.01), although proanthocyanidin-free mutants (ant 13-152 and ant 17-148) showed a relatively low browning index for their polyphenol content. Each line and cultivar showed a unique polyphenol composition. The contents of PDB3, PCB3, (+)-catechin and the component in peak C as well as the total polyphenol content in proanthocyanidin-free mutants were very low. The browning index was correlated with the contents of PDB3, PCB3 and the component in peak C significantly (P0.01). When each solution containing the component in peak C, PDB3, PCB3 and (+)-catechin was heated (90 degrees C, 1h), the color changed for red to orange, while the spectrum of solution containing the component in peak F didn't show any change during the heat treatment. These results suggest that some endogenous polyphenols, such as peak C, PDB3, PCB3 and (+)-catechin, can form brown products after heating and that their content and composition can influence the brewing reaction occurring in barley paste after heating