The effect of grinding upon starch and starch pastes.

Abstract

Several investigators have studied the effect of grinding upon raw starch. Schleiden reported that grinding starch with twice its weight of water gave the mass a salve-like consistency. Kraemer found that grinding with sand and Sponsler that grinding dry in a pebble mill rendered part of the starch soluble in cold water. These observations were verified by the writers. Grinding potato starch for 74 hours with flints in a pebble mill and wheat starch for 122 hours injures all the grains, as may be noted with the microscope. About 60 per cent of each of the starches was then soluble in cold water. The solution could be filtered quite clear with paper pulp, was not viscous, and gave a characteristic blue iodine reaction. Apparently the starch grain is protected by a membrane or membrane network impermeable to the soluble substance. When the membrane structure is injured, the contents may be leached out with the greatest ease. Ground starches suspended in water and heated do not gelatinize, and no paste can be obtained. The viscosity of suspensions of ground starch determined between 20° and 95° by means of a Stormer viscosimeter was relatively little more than that of water. Stiff starch pastes made by boiling 5 per cent of untreated wheat starch in water were ground cold in a pebble mill from 13 to 16 hours and their viscosity tested before and after grinding over a range of temperature from 50° to 90°. After grinding the viscosity was reduced to about one-fourth of its initial value. Beijerinck ground boiled potato starch with sand, thus rupturing the membranes. He found that this treatment rendered the starch in part soluble, 60 per cent passing into solution. This agrees very well with the observation above reported in regard to the solubility in cold water of ground unheated starch. The 'solubility of raw starch in cold water when ground indicates that the observation of Beijerinck does not depend upon depolymerization by heat and water but that soluble starch as such is present in considerable amount in the natural untreated starch grain.