Effect of cysteine on the molecular weight distribution and the disulfide cross-link of wheat flour proteins in extrudates

The changes in solubility, molecular size, and free sulfhydryl and disulfide contents of wheat proteins as affected by additions of cysteine during wheat flour extrusion processing were investigated. An increase in the concentration of added cysteine resulted in increases in the solubility and sulfhydryl and disulfide contents of wheat proteins in the wheat flour extrudates. The addition of cysteine as a reducing agent resulted in a decrease in the molecular size and therefore an increase in the solubility of the cross-linked wheat proteins by weakening the extensive disulfide-mediated cross-linking of wheat proteins during extrusion processing. The oxidation of two SH groups into an S-S bond between the protein and the added cysteine resulted in an increase in the disulfide content of the wheat proteins. The interchange between an SH group of an added cysteine and an S-S bond afa protein resulted in an increase in SH content in wheat proteins. A schematic pathway to illustrate the mechanism of all these changes is proposed.