Influence of sulfonation on the properties of expanded extrudates containing 32% whey protein

Whey protein concentrate (WPC) was treated with sodium sulfite to achieve 4 levels of disulfide bond sulfonation (0%, 31%,54%, and 71% mole/mole). The WPCs were blended with cornstarch to a 32% (weigh/weight) protein content and extruded into an expanded product. Extrudates were collected at 160 degrees C and 170 degrees C and analyzed for physical (air cell diameter, expansion ratio, breaking strength, and density) and chemical (water adsorption index [WAI], water solubility index, moisture content, soluble protein, and carbohydrates) properties. The control and 54% sulfonated samples had larger expansion ratios and air cell diameters and smaller densities and breaking strengths than the 31% and 71% samples. Expansion increased at 170 degrees C in the sulfonated samples. The WAI was influenced by both sulfonation and temperature, whereas the other chemical properties (except moisture content) were influenced only by sulfonation level. Soluble protein and carbohydrate were highest in the control and 54% samples. The anomalous behavior of the 54% sample may have been the result of significant structural and functional changes of alpha-lactalbumin that are predicted to occur at approximately 50% sulfonation. Many functional properties of the WPCs were measured and were significantly correlated to the extrudate properties, particularly those related to protein unfolding and flexibility. The increased ability for the proteins to become unfolded during extrusion may have promoted protein-starch interactions, which led to decreases in expansion and overall quality. Disulfide bond content did influence the chemical and physical properties of an extruded-expanded whey protein products.