Rheological and Thermal Properties of Extruded Mixtures of Rice Starch and Isolated Soy Protein

Abstract

AbstractMany foods gain new mechanical, thermal and textural properties after being processed due to interactions between carbohydrates and proteins. This effect is characteristic for each foodstuff. The properties of extruded isolated soy protein (ISP) and rice starch were studied considering the following extrusion variables: starch proportion with respect to ISP (0–100%), pH (3–9), moisture content (20–30%) and temperature (140–180ºC). The following characteristics were measured: Water absorption index (WAI), water solubility index (WSI), glass transition temperature (Tg), melting temperature (Tm), viscosity at 90ºC and at 50ºC, storage (G′), loss modulus (G′′) and tan δ. The results indicate that the extruded starch exhibits higher WAI and WSI values than untreated starch. For extruded ISP these values are much lower than for untreated ISP. Extrudates with higher starch proportion had higher Tg and Tm values; pH has a significant effect (p<0.05), at pH 3 higher Tg values were observed, and at pH 9 higher values of Tm. The highest viscosities at 90ºC and 50ºC were observed for extrudates with a higher starch proportion and pH 9. Extruded mixtures showed a more elastic than viscous behavior and an extruded 1:1 blend of starch‐ISP exhibited the behavior of a viscous liquid.