Effects of molecular size and chain profile of waxy cereal amylopectins on paste rheology during retrogradation

Abstract

Amylopectins were purified from five waxy cereal starches (corn, sorghum, barley, wheat, and rice) in amorphous state by ethanol precipitation from dimethyl-sulfoxide (DMSO) solution followed by removing any amylose complexes with butanol in water. Their molecular weights ( M w) were determined by size-exclusion chromatography in tandem with multi-angle laser light scattering and refractive index detectors. Chain length distributions were determined by debranching followed by size-exclusion chromatography coupled with a refractive index detector. Amylopectin pastes (15% dry solids) were also analyzed using a dynamic rheometer while cooling to 4 °C and storing for 20 days, and the relations between the structural and rheological characteristics were investigated. The weight-average molecular weights ( M w) of the waxy cereal amylopectins ranged from 204.4 × 10 6 to 344.4 × 10 6 g/mol, and the average chain length (CL w) ranged from 26.8 to 30.4. Among the tested starches, waxy rice had the largest amylopectin molecules and the longest B ⩾ 2 chains. The order of chain length of B ⩾ 2 chains among the samples were waxy rice > waxy corn = waxy sorghum > waxy barley = waxy wheat. But there were no difference in the CL w of total chains among the amylopectins. Storage ( G′) and loss ( G″) moduli of amylopectin pastes (15% solids) significantly increased during the cooling period from 95 to 4 °C, which was more than the increase during cold storage for 20 days at 4 °C. Among the waxy samples, waxy rice displayed the greatest moduli increases both during the initial cooling and during the cold storage. Among the structural parameters measured, the M w of amylopectin and CL w of B ⩾ 2 chains correlated positively with complex modulus( G ∗) increases.