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Cereal Chem. 76(1):116-121Weight-average molecular weight (M w ) and chain length of eight amylopectins isolated from one Australian, two United States, and five Korean wheats were measured using multiangle laser light scattering (MALLS) and refractive index (RI) detectors operated in a microbatch mode, and in a high-performance size-exclusion chromatography (HPSEC) mode. The M w of amylopectins measured in the microbatch mode ranged from 29 × 10 6 to 349 × 10 6 . Three amylopectins (Geuru, Tapdong and WW) showed significantly high M w values over 200 × 10 6 . The M w measured by HPSEC mode with MALLS-RI detectors (42 × 10 6 to 73 × 10 6 ) were sig-nificantly less than those obtained in the microbatch mode with exception of dark northern spring hard wheat (DNS) amylopectin, indicating the possible variation of M w by the analysis mode. Root-mean square of the radius of gyration (R g ) also was greater when the microbatch mode was used (122-340 vs. 95-116 nm). Chain length distributions of debranched amylopectins of different cultivars, measured by the HPSEC-MALLS-RI system, were similar. Weight average degrees of polymerization (DP w ) of A, B 1 , and larger B chains (B ≥2 ) had ranges of 13-22, 26-46, and 58-73, respectively, and mass ratios of A and B chains ranged from 0.7 to 1.1.Publication no. C-1999-0107-03R.

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 (Mw) and radius of gyration (Rg) 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 (Pv) decreased from 97 to 56 RVU by 25 h irradiation. Average Mw and Rg 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, Pv 53.2 RVU, amylopectin Mw 50 × 106). After 15 h under air, however, all measured values increased (ΔH 16.8 J/g, Pv 65.5 RVU, amylopectin Mw 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.