Distribution of Phosphorus in the Starch Granule

Gelatinized root-starch granules possess a peripheral membrane containing phosphoric-acid ester. This has led some investigators to infer that the natural ungelatinized granule also possesses such a membrane. If this be so, it follows that the P₂O₅ percentage in the small granules of a given sample should be greater than in the large granules, provided the thickness of the hypothetical membrane of the two sorts of granules is about the same.

There are two pertinent observations in the literature. Fernbach 1 found the small-sized granules to contain decidedly more P₂O₅ than the large-sized ones in the same sample of potato starch. He concluded, however, that the granule consists of a nucleus rich in phosphorus upon which are superimposed layers of phosphorus-free material. Kavcic 2 determined the P₂O₅ content of starch from 4 potato varieties with granules of different average diameters. While his figures indicate a tendency for the P₂O₅ percentage to increase as the average granule size decreases, the differences are not as great as they should be considering that the mass of a solid sphere increases as the diameter increases far more than the surface area. The interpretation of such data as these of Kavcic is difficult without knowledge of the frequency distribution of granule sizes in each sample. Another difficulty is the possibility that the P₂O₅ content of starches varies with the variety or the conditions of growth independently of granule size. Mangels 3 has reported such variations for wheat, though he did not correlate them with granule size.

All these difficulties may be avoided by separating each starch sample to be analyzed into fractions of different granule size, and analyzing each fraction separately. The writer had a sample of cassava (tapioca, manioc) starch so separated in the metallurgical laboratory of the university by air flotation in an air classifier. E. Hines analyzed the several fractions by Embden's method for phosphorus.