Extrusion of cross-linked hydroxypropylated corn starches - II. Morphological and molecular characterization

A series of cross-linked hydroxypropylated corn starches were extruded with a Leistritz. micro-18 co-rotating extruder. Extrusion process variables in cluding moisture (30, 35, and 40%), barrel temperature (60, 80, and 100 deg rees C), and screw design (low, medium, and high shear) were investigated. Scanning electron microscopy (SEM) of extruded starches showed a gel phase with distorted granules and granule fragments after extrusion at 60 degrees C. After extrusion at 100 degrees C only a gel phase was observed with no granular structures remaining. High performance size exclusion chromatograp hy (HPSEC) equipped with multiangle laser light-scattering (MALLS) and refr active index (RI) detectors showed extruded starches degraded to different extents, depending on extrusion conditions. The average molecular weight of the amylopectin of unextruded native corn starch was 7.7 x 10(8). Extrusio n at 30% moisture, 100 degrees C, and high shear reduced the molecular weig ht of amylopectin to 1.0 x 10(8). Hydroxypropylated normal corn starch extr uded at identical conditions showed greater decreases in amylopectin molecu lar weight. With the addition of cross-linking, the amylopectin fractions o f the extruded starches were less degraded than those of their native and h ydroxypropylated corn starch counterparts. Similarly, increasing moisture c ontent during extrusion lowered amylopectin degradation in the extruded sta rches. Increasing temperature during extrusion of cross-linked hydroxypropy lated starches at high moisture content (e.g., 40%) lowered amylopectin mol ecular weights of the extruded starches, whereas increasing extrusion tempe rature at low moisture content (30%) resulted in less degraded molecules. T his difference was attributed to the higher glass transition temperatures o f the cross-linked starches.