O‐dihydroxyphenoloxidase action on natural polyhydric phenolics and enzymic browning of edible yams

Abstract

AbstractTen cultivars of five edible yam species were examined for o‐dihydroxyphenoloxidase (o‐DPOase) activity against yam extracts containing phenolic compounds, and for their propensity to brown when cut slices are exposed to air. (+)‐Catechin levels in Dioscorea cayenensis (260 mg kg−1) and D. bulbifera (240 mg kg−1) were similar to those in D. dumetorum (270 mg kg−1), but the latter contained less o‐DPOase and showed less tendency to browning. D. alata contained more (+)‐catechin than other yams, and one cultivar with a higher (+)‐catechin content (660 mg kg−1) showed more browning than another cultivar containing 430 mg kg−1 (+)‐catechin. Five cultivars of D. rotundata showed less tendency to brown, and had lower (+)‐catechin content (90–190 mg kg−1) than the other yams examined, but showed marked variation in o‐DPOase activity. o‐DPOase activity, assayed by recording the rate of oxygen consumption, varied between 75 units (1 unit=μmol O2 consumed min−1 100 g−1 fresh yam tissue) in D. dumetorum and 2380 units in one D. rotundata cultivar when a crude extract of phenolic compounds diluted to contain 10 mM (+)‐catechin was used as substrate and from 60 to 3480 units for these same two enzyme extracts when assayed using crude extracts of phenolic compounds (also adjusted to contain 10 mM (+)‐catechin) prepared from the same yam as the enzyme extract. However, in general, the o‐DPOase activities recorded using the D. alata extract showed little correlation with the activity when the phenolic extract was prepared from the same yam as the enzyme extract. Incubation of crude extracts containing phenolics with crude extracts of o‐DPOase at 20°C for 24 h resulted in a decrease in the quantity of (+)‐catechin estimated by h.p.l.c. Cyanidin released by hydrolysis with HCl under mild conditions was also measured. The level in D. alata was sufficiently high to account for ‘pinking’ when this species is boiled. It is concluded that the o‐DPOase‐catalysed oxidation of (+)‐catechin is largely responsible for the browning of yams. The possible influence of other factors, including the reducing agent ascorbic acid in moderating the rate and extent of browning observed, is discussed.