Biochemical composition of maturing soybean seeds and functional properties of 7S and 11S proteins

The quality and storage stability of Hack and Williams 82 soybeans at four maturation stages were studied. As compared to mature beans, trypsin inhibitor, urease and lipoxygenase (LA) activities were lower in immature seeds, but free fatty acid (FFA) was higher and oil color was greener. During storage for six months, LA decreased and FFA increased at a faster rate in immature soybeans. Both 7S and 11S proteins increased with maturation but the 7S/11S ratio decreased. There was no change in the ratio during storage.Functional properties of 7S and 11S isolated from defatted soy flour were also investigated. The effects of 7S/11S ratio, pH and heat on bound water, water imbibing and rheological characteristics of the proteins were the main focus.Bound water of 7S and 11S was characterized by sorption isotherm and pulsed NMR techniques. Water sorption capacity (WSC) of 7S was superior to that of 11S. There was no difference in WSC between native and heated proteins. According to the biphasic linear realtion of both isotherm and NMR data, two states of bound water, polymer and capillary water, were distinguished.Water imbibing capacity (WIC) of 7S and 11S was determined by a Baumann apparatus with an additional glass ring in the central chamber. WIC was influenced by pH, 7S/11S ratio, solute and heating. At the same pH, WIC of 7S was superior to that of 11S. Both proteins and their mixtures imbibed more water at pH 6.4 than pH 4.8. WIC decreased in the presence of sucrose and sodium chloride. Upon heating at 80 or 95$\sp\circ$C for 30 mins, WIC at pH 4.8 decreased but that at pH 6.4 increased.Rheological properties of 7S and 11S were studied using Haake viscometer. Rheological data of all protein dispersions (6 to 14% protein at both pH 4.8 and 6.4) were adequately described by power law equation. Consistency coefficient (a) and flow behavior (b) were influenced by concentration, pH, 7S/11S ratio and boating. At pH 4.8, the 11S showed higher "a" value but lower "b" value than the 7S. At pH 6.4, the results were opposite. Both proteins were more sensitive to heat at pH 4.8 than at pH 6.4.U of I OnlyETDs are only available to UIUC Users without author permissio