Degradation of starch by polarised light

Aqueous suspensions of either original corn starch (30%, w/w), or starch heated for 2 h at 120 degreesC in water, were illuminated by linearly polarised 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 size exclusion chromatography, coupled with multi-angle laser light scattering and refractive index detectors. Average molecular weight ((M) over bar (w)) and radii of gyration ((R) over bar (g)) of amylopectin fraction for original starch was 2.08 X 10(8) and 214 nm, respectively. Similar values were 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 amylopectin 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 starch. Illumination of preheated starch led, after first 5 h, to a further decrease in (M) over bar (w) (4.87 x 10(7)) and (R) over bar (g) (101 nm), followed by a rise in both values after 25 (8.97 x 10(7); 146 nm) and 50 h (9.69 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 essentially different rheology. illumination of preheated starch produced more significant changes in the rheology of corresponding pastes, regardless of the period of sample illumination. The effect of illumination upon the thermal properties of starch, could be observed solely as the decrease in the onset temperature for the sample illuminated for 50 h. (C) 2001 Elsevier Science Ltd. All rights reserved.