Observations on the isolation and subfractionation of amylose: The presence in amylose of a natural barrier to β‐amylolysis

Samples of amyloses isolated from potato, wheat, and iris germanica starch by conventional procedures have been subfractionated by the addition of ethanol and other precipitants to a dispersion in dimethylsulphoxide. Fractions having a very wide range in limiting viscosity number [η], were obtained, but more significantly the samples differed in their β‐amylolysis limits, [β]. On ultracentrifugation all samples were apparently homogeneous in dilute alkali, but most of them with [β] < 95 were heterogeneous in saline, and showed a minor, faster‐moving component. Evidence is presented that this component may contain the barrier to β‐amylolysis. For amyloses isolated from potatoes at varying stages of maturity, an increase in [η] is associated with a decrease in [β]. The significance of this and other factors controlling the apparent molecular size and the [β]‐value are discussed. Lightscattering measurements confirm the molecular weight and wide molecular weight distribution of natural amylose. Fractionation of starch pretreated with dimethylsulphoxide has been critically examined.