Functional properties, physicochemical interactions, and in vitro digestibility of starch blends and starch-blend extrudates

Abstract

Starch is one major component in many food products, the techno-functional characteristics of which are crucial for the appearance, palatability, and consumer acceptance of starchy food products. Blending and extrusion of different native starches to improve the techno-functional properties have attracted much attention recently due to the “clean-label” feature and the relatively low processing costs. In Study I, high-amylose maize starch (HA7) was blended with wheat (WHE) or potato (POT) starch at 1:1 ratio (dwb). Interestingly, the formation of a large amount of amylose-lipid complexes (ALC) was observed in HA7-WHE blend, partly accounting for its slow retrogradation during cold storage. The starch blends demonstrated good resistance to thermal degradation and thixotropic breakdown during pasting at 120°C. After cooking at 95°C, the gel strength of HA7-POT blend was higher than the theoretical value; and after cooking at 120°C, HA7-WHE and HA7-POT blends also formed gels stronger than anticipated. Importantly, it was found that reduced digestibility of cooked starches could be achieved through simple blending. Subsequently, the starch blends and three individual starches were extruded under two sets of extrusion conditions, namely “mild” and “extreme”, to explore if extrusion could further diversify the functional profiles and in vitro digestibility. Extrusion decreased the amylose contents of starch blends and individual starches, and the mild and extreme conditions showed similar reducing effects, except for POT and HA7-POT blend. As expected, extrusion under the extreme condition degraded and/or gelatinized starches to greater extents. Starch extrudates (except for HA7 extrudates) showed instant viscosities, but their pasting viscosities were generally lower than those of their native counterparts, which could be attributed to starch gelatinization, loss of granular structure, and molecular degradation resulting from extrusion. These physicochemical changes also diminished the gelling ability of the starch samples. The information presented in this thesis will provide new directions for the food industry to produce native and pre-gelatinized starch ingredients with enhanced functional and nutritional properties.