In the present study,in vitrodigestibility and structure of soybean protein isolates (SPIs) prepared from five soybean varieties were investigated in simulated gastric fluid (SGF), using FT-IR microspectroscopy and SDS-PAGE. The result indicated thatβ-conformations were prone to be hydrolyzed by pepsin preferentially and transformed to unordered structure duringin vitrodigestion, followed by the digestion ofα-helix and unordered structure. A negative linear correlation coefficient was found between theβ-conformation contents of five SPIs and theirin vitrodigestibility values. The intensities of the protein bands corresponding to 7S and 11S fractions were decreased and many peptide bands appeared at 11~15 kDa during enzymatic hydrolysis.β-conglycinin was poorly hydrolyzed with pepsin, especially theβ-7S subunit. On the other hand, basic polypeptides of glycinin degraded slower than acidic polypeptides and represented a large proportion of the residual protein after digestion. 11S-A₃of all SPIs disappeared after 1 h digestion. Moreover, a significant negative linear correlation coefficient (r=-0.89) was found between theβ-7S contents of five SPIs and theirin vitrodigestibility values. These results are useful for further studies of the functional properties and bioactive properties of these varieties and laid theoretical foundations for the development of the specific functional soy protein isolate.

Raman spectroscopy is critically evaluated to establish the limits to which it may be used to detect changes in protein conformation upon extrusion. Rice bran protein (RBP) extruded with different temperatures (100, 120, 140, and 160°C, labeled as ERBP-) was considered. DSC showed that extrusion at 100°C increasedTDof RBP but decreased itsΔH, while, after extrusion treatment at 120°C, RBP completely denatured. A progressive increase in unordered structure and a general decrease inα-helix structure andβ-sheet structure of extruded RBP were observed from Raman study. Meanwhile the content of unordered structure increased up to 140°C and then decreased at 160°C, while the trend ofα-helix andβ-sheet content was opposite, which was contributed to the composite effect of formation of some more protein aggregation and protein denaturation. Extrusion generally induced a significant decrease in Trp band near 760 cm⁻¹but an increase at 160°C. No significant difference was observed in Tyr doublet ratios between controlled RBP samples and extruded RBP below 160°C, whereas Tyr doublet ratios of extruded RBP decreased at 160°C. Intensity of the band assigned toCHnbending decreased progressively and then increased as extrusion temperature increased, indicating changes in microenvironment and polarity.