Characterisation and comparison of phenols, flavonoids and isoflavones of soymilk and their correlations with antioxidant activity

Summary To investigate the relationship between bioactive compounds and antioxidant property, total phenolic content (TPC), total flavonoid content (TFC), individual and total isoflavone content of soymilk and their correlations with oxygen-radical-absorbing capacity (ORAC), ferric reducing antioxidant power (FRAP) and 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging capacity values were evaluated and compared. Results showed that TPC, TFC, isoflavones (including concentration and profiles) and antioxidation activity significantly varied among ten tested soybean cultivars. Significant correlations were established between DPPH and TFC ( r = 0.553, P < 0.01), DPPH and TPC ( r = 0.753, P < 0.01), FRAP and TFC ( r = 0.599, P < 0.01) and FRAP and TPC ( r = 0.616, P < 0.01). Positive linear correlations were found between subtotal, total isoflavones and ORAC. Unlike other isoflavone monomers, aglycone isoflavones correlated positively with DPPH and FRAP significantly. DPPH well correlated with FRAP, whereas neither DPPH nor FRAP correlated with ORAC. TPC, TFC, individual and total isoflavone values are potentially useful for soymilk antioxidant activity assessment.