Light‐Scattering Molecular Weights and Intrinsic Viscosities of Processed Waxy Maize Starches in 90% Dimethyl Sulfoxide and H₂O

Abstract

Cereal Chem. 74(5):687-691Waxy maize starch was treated by a variety of gentle and severe methods: direct dispersion-solubilization into 90% dimethyl sulfoxide (DMSO) and H 2 O solvent, extrusion followed by dispersion-solubilization of the ground exudate into solvent, or jet-cooking or stirred autoclaving of an aqueous starch slurry followed by transfer into solvent. Intrinsic viscosities [η] and multiangle light-scattering measurements were made in 90% DMSO-H 2 O. A Mark-Houwink relation, [η] = (0.28-1.2) M w 0.29 ± 0.04 , was obtained over a molecular weight range of ≈30-700 million. However, there was a large amount of scatter in the data when [η] were >140 mL/g. The power law relationship R g ∝ M w 0.41±0.04 was noted between radii of gyration and molecular weights. We infer from our data that over the entire range of M w distributions, the amylopectin existed in solution as relatively compact molecules or aggregates and that in the higher molecular weight region, the size and possibly the shape of the "dissolved" amylopectin was highly sensitive to the method of dispersal and treatment.The purposes of this study were to: 1) characterize the solution state of amylopectin from processed waxy maize starches by obtaining relationships between weight-average molecular weights (M w ), intrinsic viscosities ([η]), and radii of gyration (R g ) in the solvent 90% dimethyl sulfoxide (DMSO) and H 2 O (w/w); and 2) consider effects of processing methods on the molecular parameters of waxy maize amylopectin. A commercial multiangle laser light-scattering detector (MALLS) was used in a batch mode to examine these very high molecular weight biopolymers. Amylopectins are the high molecular weight, highly branched polysaccharide components of starches. The sensitivity of amylopectins to high or moderate shear degradation has been deduced from [η] and light-scattering measurements (Whitnauer et al 1955, Banks et al 1972. In this study, we showed that dispersal techniques involving low shear, time, and temperature also strongly influence the solution state of the dispersed amylopectin.In a study of the effects of thermomechanical processing on the viscosity behavior of corn starches (Dintzis and Bagley 1995), a lack of a unique correlation was noted between the intrinsic viscosity ([η]) and the shear viscosity (η) of the dispersed starch. In addition, there was greater variation in values of these parameters when waxy maize starch was prepared by gentle procedures rather than by severe treatments. These results, which we interpreted as evidence for nonuniform degradation and dissociation of the amylopectin when waxy maize starch granules were gently dispersed and solubilized, led to the present study.The M w for amylopectins obtained by light-scattering before 1975 had a range of ≈(10-450) × 10 6 . There was concern in that era, that is valid today, that "microgel" particles might make significant contributions to high molecular weight values (Sitaramaiah and Goring 1962, Banks andGreenwood 1975). Recent reports of amylopect...

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