Physicochemical properties of selected root and tuber starches and characterization of extruded, chemically modified corn starches

The physicochemical properties of normal potato, waxy potato, yam and sweet potato starches were examined and compared. Normal potato and waxy potato starches displayed the B-type x-ray diffraction pattern, whereas yam and sweet potato displayed the CA- and C-type, respectively. X-ray diffraction patterns of Naegeli dextrins of normal potato and waxy potato remained the B-type, but those of yam and sweet potato changed to the A-type. 31P-NMR showed the phosphorus contents of the starches to be primarily phosphate monoesters with no detectable phospholipid in any of the four starches. Normal potato and waxy potato starches displayed lower proportions (13 and 14.8%, respectively) of short branch chains of chain length dp 6--12 than did yam and sweet potato starches (17.1 and 19.0%, respectively). Normal potato displayed a larger proportion of long branch chains than did waxy potato amylopectin.A series of cross-linked hydroxypropylated corn starches were extruded. Extrusion process variables including moisture, barrel temperature, and screw design were investigated. Rapid Visco Analyzer pasting profiles of extruded starches displayed different hot paste viscosity and final viscosity depending on extrusion parameters. Increasing starch moisture content during extrusion and level of cross-linking increased starch viscosity (p < 0.0001). Whereas, increasing extrusion temperature and shear decreased starch viscosity (p < 0.0001).Scanning electron microscopy (SEM) of extruded starches showed a gel phase with distorted granules and granule fragments after extrusion at 60°C whereas after extrusion at 100°C only a gel phase was seen. High performance size exclusion chromatography 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. Extrusion at 30% moisture, 100°C, and high shear reduced the molecular weight of amylopectin to 1.0 x 108. With the addition of cross-linking, the amylopectin fraction of the extruded starches was less degraded than those of their native and hydroxypropylated corn starch counterparts. Similarly, increasing moisture content during extrusion lowered amylopectin degradation in the extruded starches. The glass transition temperatures of the starches were found to influence amylopectin degradation.