Effect of Toasting and Extrusion at Different Shear Levels on Soy Protein Interactions

The effect of toasting and extrusion at different shear levels on protein interactions in soybean meal was studied by extraction methods using buffers containing urea and dithiothreitol (DTT). It is suggested that noncovalent interactions were the main forces in protein structure formation during the toasting process but are less important during extrusion. After extrusion, both noncovalent interactions and disulfide bonds may be involved during low-shear extrusion. At higher shear levels, other covalent cross-linking reactions may also occur. After extrusion, mainly polypeptides of glycinin were found in the protein fractions obtained after extraction with DTT, especially the acidic polypeptide. In combination with in vitro protein digestibility results, it was concluded that glycinin is less digestible compared with β-conglycinin. It appeared that after toasting and especially after extrusion, an increasing amount of still active trypsin inhibitors could be detected after extraction with DTT, urea, and both urea and DTT, respectively. This suggests that trypsin inhibitors were embedded in the protein matrix and were thereby protected against heat inactivation.