Molecular distribution and pasting properties of UV-irradiated corn starches

Suspension (30 %, w/w) of corn starch (25 % amylose) in water was irradiate d by UV-light with wavelength greater than 250 nm at 25 degrees C, under a stream of nitrogen or air, for time intervals ranging from 5 to 25 h. Effec ts of the irradiation on the transition enthalpy and temperature for meltin g, and the pasting viscosity profile of the irradiated starch were examined . Weight-average molecular weight (M-w) and radius of gyration (R-g) of the irradiated starch molecules were measured by high performance size exclusi on chromatography coupled with multiangle laser light scattering and differ ential refractive index detectors (HPSEC-MALLS-RI). In the case of starch i rradiated under nitrogen. the transition enthalpy (Delta H) decreased with increasing irradiation time whereas the melting temperature was not changed . Similarly, the peak paste viscosity (P-v) decreased from 97 to 56 RVU by 25 h irradiation. Average M-w and R-g of amylopectin and amylose fractions, which were 93 x 10(6) and 144 nm, and 3.0 x 10(6) and 104 nm, respectively , were decreased by irradiation to 32.2 x 10(6) and 93.7 nm, and 0.7 x 10(6 ) and 83.6 nm by the irradiation for 15 h under nitrogen, respectively. Whe n the starch was irradiated with aeration, sharp drops of all measured para meters were observed in 5 h of irradiation (Delta H 11.4 J/g, P-v 53.2 RVU, amylopectin M-w 50 x 10(6)). After 15 h under air, however, all measured v alues increased (Delta H 16.8 J/g, P-v 65.5 RVU, amylopectin M-w 63.1 x 10( 6)). Molecular size distribution profiles confirmed oxidative the photodegr adation in the early stage (up to 5 h), and cross-linking reactions in the late stage (5-15 h) of irradiation under aeration.