Effect of steam-cooking on (Poly)phenolic compounds in purple yam and purple sweet potato tubers

Abstract

This study investigated the influences of steam-cooking on (poly)phenolic compounds and colors in purple yam (Dioscorea alata L.) and purple sweet potato (Ipomoea batatas) tubers cooked at the household level. Steam-cooking did not significantly change the contents of total phenolic compounds, flavonoids, anthocyanins and antioxidant capacity measured by ABTS radical scavenging assay in purple yam (P≥0.05), but lowered the antioxidant capacity measured by Crocin assay (P<0.05). The liquid chromatograph– mass spectrometer–Ion Trap–Time of Flight (LCMS-IT-TOF) indicated the degradation of high MW alatanin B to lower MW alatanins due to the loss of glucose units. Purple sweet potato, however, responded to steam-cooking differently from purple yam (P<0.05). Steam-cooking drastically increased total phenolic compounds, flavonoids, anthocyanins, antioxidant capacities, and retained the vivid reddish-purple color of cooked purple sweet potato (P<0.05). The stability of polyphenolic compounds in starchy tubers against steam-cooking at the household level was, in part, due to the different contents of indigenous phenolic compounds in the raw tubers.