Abstract
Starch, as it occurs in nature, is not soluble in cold water, and hence one cannot determine the specific rotation of the untreated substance. The several soluble starches, which have formed the basis of numerous optical studies, represent natural starches altered either by heating or treatment with reagents, or by treatment with enzymes, or by some combination of the above procedures.
The reported values of the specific rotation of the various starches range from 216.0° to 182.0°. This wide range of results indicates that the determinations were made on starches hydrolyzed to a varying extent, or on untreated solutions so opalescent as to prevent accurate evaluation of the specific rotation.
It was shown by Alsberg, Griffing, and Field 1 that clear solutions could be prepared from purified starch by dry-grinding in a pebblemill, dispersion in cold water, and centrifugation. This technique is particularly effective with wheat and corn starches. These solutions are water-clear, dialyze through collodion, and give an intense blue with iodine. Such solutions are always very dilute. 2 , 3
The specific rotation of these solutions has been determined. The source of light employed was an electric lamp of the type described in Circular 44, U. S. Bureau of Standards, page 34. Determinations of the concentration of the starch solutions were made by the method of Munson and Walker.
The average of a number of determinations gave 196.0° for the specific rotation of the cold-water-soluble portion of ground corn starch, and 194.7° for wheat starch, with the light-source specified. This difference is not significant in view of the fact that it lies in the range of analytical error in determining the concentrations of such dilute solutions. These determinations afford no evidence that the cold-water-soluble fractions of ground corn and wheat starches differ in character.
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