Effect of Cysteine on the Functional Properties and Microstructures of Wheat Flour Extrudates

The effect of the concentration of added cysteine (0, 0.25, 0.5, 0.75, 1.0, and 1.5% (w/w)) on the functional properties and microstructures of wheat flour extrudates was studied in order to elucidate the relationship of the disulfide cross-linking, the extrudate microstructure, and the functional property of proteins through the high-temperature short-time extrusion. The added cysteine markedly affected the functional properties and microstructures of the extrudates during twin-screw extrusion. After extrusion, the disulfide content in the extrudate (including in both protein and cystine) increased by nearly two times. The expansion ratio of the extrudates decreased by 25%. The bulk density of the extrudates first dropped by 50% and then remained constant. The expansion volume of the extrudates first increased by 25% and then returned back. The water-holding capacity of ground extrudates decreased by 16%. The oil-absorption capacity of ground extrudates first increased by 135% and then remained constant. The "whiteness" value of the extrudates increased by 3.9 color units. The gumminess and cohesiveness of the extrudates increased. For the microstructures of the extrudates, the cell size decreased and the cell wall thinned; also, the cell surface morphology changed. The information generated in this study could be applied to predict and control the functional properties and textural characteristics of extruded products. Keywords: Cysteine; extrusion; disulfide cross-linking; microstructure; functional properties