Physical properties of extruded strands of hydroxypropylated normal and high-amylose corn starch

Normal (25% amylose) and high-amylose (70% amylose) corn starches (CS and HA) were hydroxypropylated to 0.1 degree of molar substitution (MS) with propylene oxide in an alkaline-ethanol medium (70% ethanol). CS and hydroxypropylated corn starch (HPCS) were mixed on dry basis with water and glycerol at a weight ratio of 7:2:1, and HA and hydroxypropylated high amylose corn starch (HPHA) were mixed at 7:3:1. Stearic acid, glycerol monostearate, or lecithin (3%, based on starch) was added to each mixture to examine the effects on the physical properties of the extrudate. The starch mixtures were extruded at high shear (100 rpm) to nonexpanded strands using a corotating twin-screw extruder in a temperature range of 75-90 degrees C. HA, alone and with all additives, showed lower die swelling in extrusion than did CS, whereas HPCS and HPHA showed higher die swelling than the corresponding unmodified starches. Water absorption of all HA extrudates was lower than those of all CS extrudates (22-35% and 68-97%, respectively, at 25 degrees C). Hydroxypropylation increased the absorption for both starches. All extruded starches, regardless of additives, showed low solubility in water (0.1-1.0% for 2 hr at 25 degrees C). Differential scanning calorimetry indicated that during extrusion, the lipid additives formed a helical complex with amylose in CS and HA, but weakly with HPCS and HPHA. The extruded strands of HA, alone and with additives, exhibited higher tensile and bending strengths (37.1-58.4 and 2.16-5.07 MPa, respectively), compared to the CS strands (12.5-59.3 and 1.06-4.10 MPa, respectively) at the same moisture content (7.5-8.5%). Both tensile strength and percent of elongation of the starch strands were reduced by the presence of a lipid additive. Hydroxypropylation increased elongation and flexibility of the extrudates. HPHA exhibited the greatest mechanical strength and flexibility among the tested starches.