Physico-chemical, nutritional, and flavor properties of soybean extracts processed by rapid-hydration hydrothermal cooking
The continuous steam infusion cooking process known as rapid-hydration hydrothermal cooking (RHHTC) was explored as a means of producing edible aqueous extracts of soybeans. RHHTC processing has two opportunities for lipoxygenase to develop off-flavors in soymilk: during grinding and storage of full-fat soy flours and during slurrying of soy flours prior to steam infusion. Attempts were made to reduce both possibilities;In the first study, the effects of moisture content and particle size on lipid and flavor deterioration of full-fat soy flours were investigated. Soybeans with moisture contents equivalent to or less than 11% could be stored up to 24 hours without significant lipid oxidation, hydrolytic rancidity or off-flavor development;In the second study, rapid slurrying and hot water slurrying prior to cooking were approaches used to reduce the action of lipoxygenase. Hot water (>80°C) slurrying was preferred in the RHHTC processing and it was not feasible to eliminate off-flavor development completely by mere rapid slurrying prior to steam infusion. The recoveries of solids and protein soymilk cooked at 154°C gradually increased with time up to maximum yield followed by a decrease in recovery. Maximum yields of solids were 84.2% for cold water slurrying and 80.6% for hot water slurrying in RHHTC processing compared to 39.7% for traditional soymilk. RHHTC soymilks were extraordinarily high in viscosity. Soymilk processed with hot water slurrying had lower viscosity than those slurried in cold water. High viscosity did not always result in high yield. Loss of available lysine was generally less in all RHHTC conditions studied than in traditional soymilk. Longer water contact times gave darker colored soymilk. Hot water slurrying resulted in lighter colored soymilk. Both RHHTC soymilks were highly digestible and maximum in vitro digestibilities occurred at the same points where solids yield and viscosity were also maximum. About an 88% reduction in trypsin inhibitor activity occurred at 19-26 sec of cooking when employing cold water slurrying and at 14-18 sec for hot water slurrying;Optimum RHHTC processing conditions were 18 sec of cooking at 154°C with 11 sec of water contact time using hot water slurrying (>80°C). Yield and nutritional and flavor properties of RHHTC soymilk were better than those values for traditional soymilk. However, RHHTC soymilk was much more viscous than traditional soymilk.