Degradation of starch by polarised light

Aqueous suspensions of either original corn starch (30%, w/w), or starch he ated for 2 h at 120 degreesC in water, were illuminated by linearly polaris ed visible (>500 nm) light for 5-25 h. Molecular weights ((M) over bar (w)) and radii of gyration ((R) over bar (g)) for amylopectin, intermediate and amylose fractions of illuminated starches were measured by high-pressure s ize exclusion chromatography, coupled with multi-angle laser light scatteri ng and refractive index detectors. Average molecular weight ((M) over bar ( w)) and radii of gyration ((R) over bar (g)) of amylopectin fraction for or iginal starch was 2.08 X 10(8) and 214 nm, respectively. Similar values wer e found for samples that were illuminated for 5h (2.14 x 10(8) and 211 nm). After 25 h of illumination decrease in (M) over barw (5.71 x 10(7)) and (R ) over bar (g) (122 nm) was observed. Prolonged illumination, for 50 h, led to an increase in both (M) over barw (9.38 x 10(7)) and (R) over bar (g) ( 150 nm) values. The molecular weight and radius of gyration of the amylopec tin fraction of the original corn starch heated for 2 h at 120 degreesC (7. 90 x 10(7) and 133 nm, respectively) were lower than for the original starc h. Illumination of preheated starch led, after first 5 h, to a further decr ease in (M) over bar (w) (4.87 x 10(7)) and (R) over bar (g) (101 nm), foll owed by a rise in both values after 25 (8.97 x 10(7); 146 nm) and 50 h (9.6 9 x 10(7); 147) of illumination. The differences in the pasting properties of the original sample, and samples illuminated for 5 as well as 50 h were negligible; only the starch sample illuminated for 25 h produced gels of es sentially different rheology. illumination of preheated starch produced mor e significant changes in the rheology of corresponding pastes, regardless o f the period of sample illumination. The effect of illumination upon the th ermal properties of starch, could be observed solely as the decrease in the onset temperature for the sample illuminated for 50 h. (C) 2001 Elsevier S cience Ltd. All rights reserved.