Cereal Chem. 70:260-268   |  VIEW ARTICLE

Chemical and Molecular Properties of Irradiated Starch Extrudates.

A. S. Sokhey and R. Chinnaswamy. Copyright 1993 by the American Association of Cereal Chemists, Inc. 

Corn starch samples containing 0, 25, 50, and 70% amylose were gamma-irradiated at 0 (native), 5, 10, 20, and 30 kGy. All starch samples were extrusion cooked at 140 C barrel temperature, 140 rpm screw speed, and 18% moisture content (db) using a C. W. Brabender single-screw extruder. Starches irradiated at a 20- kGy dosage were extrusion cooked with and without hydrogen peroxide, potassium persulfate, or ceric ammonium nitrate. The quantity of free radicals produced on the starch increased with increasing irradiation dosages (0-30 kGy). Stability of the free radicals was greater for high-amylose starches than for those with low amylose. Extrusion-cooked starches had traces of free radical activity. Acidity of the irradiated starches increased (pH decreased) with increasing irradiation dosages. Gel permeation chromatographic separation of variously treated starches gave three fractions. Fraction I, mostly amylopectin, eluted at the void volume, whereas fraction II, mostly amylose, eluted at the latter part of the gel. Fraction 0, degraded products of amylopectin and amylose, mostly eluted closer to the total volume of the gel. Fraction I quantities of irradiated starches decreased with increasing irradiation dosages, whereas fraction II and III quantities correspondingly increased. Native starches with 0% amylose exhibited more than a fourfold decrease in fraction I content, whereas 70% amylose native starches showed less than a twofold decrease due to increasing irradiation dosages from 0 to 30 kGy. Extrusion cooking accelerated the degradation of fraction I for 0% amylose starches more than for 70% amylose starches. Both 2.5 and 5% concentrations of chemical additives caused excessive degradation of fraction I of starches irradiated at 20 kGy, consequently increasing reducing powers. Ceric ammonium nitrate caused the highest decrease in the iodine binding capacity of the starches. Fraction I clearly suffered more degradation due to irradiation, extrusion, addition of chemical agents, or a combination of these.