Isoflavones, Flavan-3-ols, Phenolic Acids, Total Phenolic Profiles, and Antioxidant Capacities of Soy Milk As Affected by Ultrahigh-Temperature and Traditional Processing Methods

The objectives of this work were to assess antioxidant activities and phenolic compounds of soy milk as affected by traditional and ultrahigh-temperature (UHT) processing. Three soybean varieties were processed into raw soy milk and then cooked soy milk by indirect and direct UHT methods (both at 143 °C for 60 s) and traditional cooking (stove cooking and steam injection) methods (both at 100 °C for 20 min). Total phenolic content (TPC), total flavonoid content (TFC), phenolic acids, isoflavones, flavan-3-ols, and anthocyanins were quantified. DPPH free radical scavenging activity, ferric reducing antioxidant power (FRAP), and oxygen radical absorbance capacity (ORAC) were analyzed. As compared to the raw soy milk, all thermal processing significantly (p < 0.05) reduced TPC values and significantly (p < 0.05) increased TFC values for all soybean varieties. All processing methods significantly (p < 0.05) increased DPPH and FRAP values in the soy milk processed from yellow soybean varieties Proto and IA 2032. UHT processing increased their ORAC values, but traditional and steam processing reduced their ORAC values. However, in the case of the soy milk from black soybean, all processing reduced ORAC values as compared to the raw soy milk. None of processing affected total phenolic acids, chlorogenic, and trans-cinnamic acid, as well as (+)-catechin. However, all processing significantly (p < 0.05) affected contents of total isoflavones and individual isoflavones. Thermal processing caused significant (p < 0.05) increases in 7-O-β-glucosides and acetylglucosides, but caused significant (p < 0.05) decreases in malonylglucosides and aglycones. Indirect UHT processing transformed more isoflavones from malonylglucosides into 7-O-β-glucosides than the direct UHT did.