Effects of Molecular Weight on the Retrogradation of Amylose

Abstract

We have examined the effects of molecular weight on the morphological features of amylose precipitates, especially the particle size and size distribution, during rétrogradation of amylose using scanning electron microscopy. The amylose precipitates consisted of particles with a surprising uniformity in size, and the volume of each particle increased proportionally with the degree of rétrogradation. These results suggest that the rétrogradation of amylose proceeds via a nucleation process, which is completed within a short time after the onset of rétrogradation; this is then followed by growth of the nuclei. At a given rétrogradation percentage, the particle size is much larger for the particle made up of low-molecular-weight amy- lose(DP=172) than that of high-molecular-weight amylose (DP=2600), indicating that the number of nuclei formed in the initial stage is much smaller for the particle of low-molecular-weight amylose. In conjunction with the result on the rate of rétrogradation, it was found that the growth rate of nuclei is much higher for the low-molecular-weight amylose, implying the shorter amylose chain is easily incorporated into the particle.