TIME-COURSE OF PPO-RELATED BROWNING OF YAMS

The time course of polyphenol oxidase (PPO) activity and tissue browni ng was investigated in five Dioscorea species. In three of the yam spe cies, 86% of the browning was associated with the peak of PPO activity . In D dumentorum and D rotundata, only 9 and 40%, respectively, of br owning was PPO-related, while most of the tissue browning occurred aft er PPO activity had declined to the control value. The browning incide nce was temperature-dependent such that the optimal temperature for PP O activity (30 degrees C) also coincided with that of maximal browning in all the yam species expect in D dumentorum. Classical inhibitors o f PPO prevented the browning in tissues where the enzymic mechanism pr edominated while failing to reduce browning in D dumentorum and D rotu ndata where most of tissue browning was not enzyme associated. The res ults put the incidence of yam discolouration in proper perspective in terms of the timescale of PPO and non-PPO-catalysed discolouration.