Summary Soy protein isolates (SPIs) treated with sodium sulphite (Na2SO3) under low temperature (50 °C) extrusion were investigated to create a system where the disulphide bonds were destroyed. The physicochemical properties of the extrudates were characterised after the addition of 0.0%, 1.5% and 3.0% (w/w) Na2SO3. Under these conditions, free sulfhydryl content significantly increased (from 98.46 to 449.64 μmol g−1), specific mechanical energy during extrusion significantly increased (from 426.91 to 593.39 kJ kg−1), degradation temperature reduced (from 315.7 to 302.0 °C), protein solubility of the water extraction increased incrementally (from 5.41% to 20.17%) and water absorption capacity decreased (from 3.51% to 2.69%). Moreover, the addition of Na2SO3 caused a blue shift in the tryptophan fluorescence spectra of soluble SPI. These results suggest that Na2SO3 breaks disulphide bonds in SPI, which accelerates protein molecule denaturation to promote peptide disentanglement; consequently, SPI become more hydrophobic and less thermally stable.

Backgrounds and objectives The protein from various parts of the wheat kernel has different physicochemical properties, which is one of the reasons for the different processing characteristics of the corresponding flour made from these parts. In this study, wheat kernels were pearled into seven fractions from the outermost to the innermost layers to evaluate the quantitative and conformational properties of protein fractions from different pearling fractions. Findings The crude protein and protein fraction content was lower in the corresponding pearling fractions that were closer to the center of the endosperm. The albumin and SDS‐soluble protein contents in the total protein from the pearling fractions exhibited a similar trend, whereas the dry gluten content exhibited an opposite trend. The average molar mass (Mw), polydispersity index (Mw/Mn), and density (ρ) of the SDS‐soluble protein were decreased, but there was no significant difference in the Rgz. And the G′ plateau values of glutenin macropolymer (GMP) were increased. Conclusions Protein conformation of the inner endosperm was looser than the other pearling fractions. Loose network wrapped more protein that led to the increased proportion of dry gluten. And it may increase aggregate entanglement that attributed to GMP became more elastic. Significance and novelty The changes in protein conformation in pearling fractions of wheat kernels further explained changes in physicochemical properties of protein from different pearling fractions.

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