Studies on starches of high amylose-content : Part VII. Observations on the potentiometric iodine-titration of amylomaize starch

Abstract

The abnormal character of the potentiometric iodine-titration curves obtained at 20° for amylomaize starch are not a direct result of the high amylose-content, since the comparable starch from the wrinkled-seeded pea gives a normally-shaped iodine-titration curve. It has been found that the relative extent of binding of iodine by an amylomaize depends on the method of dispersion of the starch granules. The use of dimethyl sulphoxide gave the most satisfactory iodine-binding curve. Residual contaminating protein and fat in the starch, and the inherent instability of the dispersion, do not influence the titration. Potentiometric iodine-titration curves have been obtained for a series of amylomaizes having apparent amylose-contents varying from 57 to 75%. Although the relative order of iodine sorption was in agreement with the apparent amylose-content, the curves possessed no point of inflexion, and estimations of iodine affinity were not practicable. Short chain-length amylose has been prepared, and added to normal amylopectin and to mixtures of normal amylose and normal amylopectin. It has been found that the iodine-binding characteristics of such mixtures resemble those of the anomalous 36-unit amylopectin and amylomaize starch. The significance of this finding is discussed.