Abstract
Automated gel permeation chromatography (GPC) with application of the universal calibration concept was used to investigate the mechanism of extrusion-induced starch fragmentation in wheat. High and low protein flours were subjected to twin-screw extrusion and the effects of moisture, die temperature, screw speed, mass flow rate and protein content on starch structure and textural properties were investigated by non-aqueous GPC in dimethylacetamide (DMAC) and LiCl. This solvent system allowed for complete dissolution of the native and extruded starches. The use of refractive index and viscosity detectors enabled application of Mark-Houwink calculations to obtain quantitative size profiles of unprocessed and processed starch, and information describing branching patterns of the starch. Fragmentation was most pronounced in amylopectins of MW 10 7–10 8, which yielded fragments of MW 10 5–10 7. Of the operating parameters investigated, low die temperature and low moisture content led to extensive fragmentation. Methylation analysis showed only modest changes in linkage distributions, with little or no increase in terminal glucose indicating few fragmentation points relative to the total number of glycosidic linkages present. The lack of dextrins or oligosaccharides suggests that fragmentation occurs primarily in the B chains of amylopectin. Interactions between die temperature and moisture content were shown to significantly effect the hardness, cohesiveness, springiness, gumminess and chewiness of the extruded flours.
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