Molecular Distribution and Pasting Properties of UV-Irradiated Corn Starches

Suspension (30 %, w/w) of corn starch (25 % amylose) in water was irradiated by UV-light with wavelength greater than 250 nm at 25°C, under a stream of nitrogen or air, for time intervals ranging from 5 to 25 h. Effects of the irradiation on the transition enthalpy and temperature for melting, 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 exclusion chromatography coupled with multiangle laser light scattering and differential refractive index detectors (HPSEC-MALLS-RI). In the case of starch irradiated under nitrogen, the transition enthalpy (ΔH) 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 × 10⁶ and 144 nm, and 2.0 × 10⁶ and 104 nm, respectively, were decreased by irradiation to 32.2 × 10⁶ and 93.7 nm, and 0.7 × 10⁶ and 83.6 nm by the irradiation for 15 h under nitrogen, respectively. When the starch was irradiated with aeration, sharp drops of all measured parameters were observed in 5 h of irradiation (ΔH 11.4 J/g, P_v 53.2 RVU, amylopectin M_w 50 × 10⁶). After 15 h under air, however, all measured values increased (ΔH 16.8 J/g, P_v 65.5 RVU, amylopectin M_w 63.1 × 10⁶). Molecular size distribution profiles confirmed oxidative the photodegradation in the early stage (up to 5 h), and cross-linking reactions in the late stage (5—15 h) of irradiation under aeration.