The action of isoamylase on the surface of starch granules

Abstract

There is an apparent conflict regarding the distribution of amylose and amylopectin within cereal-starch granules. When starch was damaged in a laboratory mill, amylopectin was preferentially extracted which cold water’**. The relatively mild damage which occurs during the milling of wheat’ resulted in almost pure amylopcctin being solubilised into cold water. On more severe damage, which results from the use of laboratory milling, starch granules were shown by scanning electron microscopy to crack and craze’ and, although amylopectin was the major component solubilised by cold water, significant amounts of amylose were also present in the cold-water extract2y3. However, experiments on the developing kernels of barley4 showed that, within the large (A-type) granules of barley starch, there was a relatively higher concentration of amylopectin in the core with increasing amounts of amylose towards the periphery. In order to reconcile these apparent differences, it was speculated3s5 that there was a small number of clusters of amylopectin that protruded from the otherwise relatively uniform “hairy-billiard-ball” surface described by Lineback6. The purpose of the present study was to examine the surface of the starch granule, using enzymic techniques. Undamaged starch granules were washed free of any coldwater soluble materials, then incubated with bacterial isoamylase, and the material solubilised was assayed for total carbohydrate and for interaction with iodine. The results (Table I) showed that a small proportion of carbohydrate material was released. On interaction with iodine, the blue value’ and the &,,,, were consistent with data for linear oligosaccharides from debranched amylopectins. The unit chains released by the action of isoamylase on wheat-starch granules were subjected to gel filtration chromatography on Sephadex GSOSF. In Fig. 1, a peak associated with fractions 78-82 did not stain with iodine and corresponded to a mixture of malto-oligosaccharides with an average d.p. of _ 15. The carbohydrate in fractions 48-70 interacted strongly with iodine and corresponded to a d.p. of about 4045. Amylose, if present, would have been excluded from the gel, would have appeared in