Cereal Chem. 70:543-547   |  VIEW ARTICLE

Determination of Molecular Size Distribution of Starch and Debranched Starch by a Single Procedure Using High-Performance Size-Exclusion Chromatography.

A. G. W. Bradbury and A. B. Bello. Copyright 1993 by the American Association of Cereal Chemists, Inc. 

A single procedure, using high-performance size-exclusion chromatography (HPSEC), was developed for the analysis of starch and debranched starch. Starch was solubilized by heating in dimethyl sulfoxide (90%) and debranched with isoamylase in buffer at pH 3.8. Separation of the reaction products of starches using a two-column HPSEC system gave a high molecular weight peak corresponding to that of linear amylose and two or three lower molecular weight peaks corresponding to those of linear maltooligosaccharides from the branched amylopectin fraction. For the range of genotype corn starches monitored in this study, chain lengths of the lower molecular weight linear fractions ranged from number-average degree of polymerization (DP[n]) 16 for waxy sugary 2 to DP[n] 88 for amylose extender waxy. The same chromatographic conditions for separating debranched starch fractions also separated starches from dent corn and amylomaize V into two distinct peak fractions corresponding to amylopectin and amylose. This separation was unexpected in view of the molecular weight exclusion limits for the two columns, which were significantly lower than the molecular weight of these fractions. The separation thus allowed for the direct comparison of the elution behaviors of starch and debranched starch using the same chromatographic conditions. In this way, it was shown that the amylose fractions in the dent and amylomaize V corn starches were mainly linear because their elution times were unchanged after debranching. The linear amylose contents of these starches, as determined by this HPSEC procedure, were lower than those determined by the iodine colorimetric procedure. This indicates some contribution of long linear chains of amylopectin to the amylose values determined colorimetrically. As expected, the linear amylose fraction was absent in four waxy genotype corn starches that contained only branched amylopectin fractions with a broad range of chain length distribution.