EFFECT OF TOASTING AND EXTRUSION AT DIFFERENT SHEAR LEVELS ON SOY PROTEIN INTERACTIONS

The effect of toasting and extrusion at different shear levels on prot ein interactions in soybean meal was studied by extraction methods usi ng buffers containing urea and dithiothreitol (DTT). It is suggested t hat noncovalent interactions were the main forces in protein structure formation during the toasting process but are less important during e xtrusion. After extrusion, both noncovalent interactions and disulfide bonds may be involved during low-shear extrusion. At higher shear lev els, other covalent cross-linking reactions may also occur. After extr usion, mainly polypeptides of glycinin were found in the protein fract ions obtained after extraction with DTT, especially the acidic polypep tide. In combination with in vitro protein digestibility results, it w as concluded that glycinin is less digestible compared with beta-congl ycinin. It appeared that after toasting and especially after extrusion , an increasing amount of still active trypsin inhibitors could be det ected after extraction with DTT, urea, and both urea and DTT, respecti vely. This suggests that trypsin inhibitors were embedded in the prote in matrix and were thereby protected against heat inactivation.