Time Course of PPO-Related Browning of Yams

The time course of polyphenol oxidase (PPO) activity and tissue browning was investigated in five Dioscorea species. In three of the yam species, 86% of the browning was associated with the peak of PPO activity. In D dumentorum and D rotundata, only 9 and 40%, respectively, of browning was PPO‐related, while most of the tissue browning occurred after PPO activity had declined to the control value. The browning incidence was temperature‐dependent such that the optimal temperature for PPO activity (30°C) also coincided with that of maximal browning in all the yam species expect in D dumentorum. Classical inhibitors of PPO prevented the browning in tissues where the enzymic mechanism predominated while failing to reduce browning in D dumentorum and D rotundata where most of tissue browning was not enzyme associated. The results put the incidence of yam discolouration in proper perspective in terms of the timescale of PPO and non‐PPO‐catalysed discolouration.