Title : Boiling-induced changes in color attributes, total phenolic content, LC-UV-MS phenolic profile, and polyphenol oxidase activity in new water yam (Dioscorea alata) hybrids
Abstract:
Darkening linked to cooking is a highly undesirable quality defect in potatoes that occurs during cooling after cooking. We report boiling-induced darkening for the first time in yam. Six varieties were evaluated to determine the effects of boiling and subsequent cooling on color attributes and total phenolic content. Polyphenol oxidase activity (PPO), and detailed catechin and condensed tannin (i.e. procyanidins) profiles were monitored during boiling, characterized and quantified. Boiling decreased the L* value in all varieties. However, except in Gwadinra, the L* value increased after 1 h of cooling, unlike in potato. Total phenolic content decreased by 42 % after boiling, as PPO remained active during the first 11 min of cooking. PPO was denatured when the core temperature of the pulp reached 60 ℃. These findings suggest that cooking-related darkening in yam involves an enzymatic mechanism, whereas darkening in potato is primarily chemical. Although procyanidin content significantly decreased with boiling, the proportions of their subunits remained unchanged, indicating that their qualitative profile may remain stable after cooking. Of the 42 % decrease in total phenolic content associated with boiling, 19 % was attributed to leaching into the cooking water. Therefore, breeders should consider cooking-induced darkening as a selection criterion.
Keywords: boiling, after-cooking darkening, procyanidins, condensed tannins, leaching out, enzymatic oxidation.
