A Photomicrographic Study of Gelatinized

A rigid paste of starch and water is responsible to an important degree for the physical properties of baked products made with wheat flour. Starch constitutes about 55 per cent of the weight of the starch-water fraction of bread dough and about 35 to 40 per cent of that of muffin and cake batters. The ratio of starch to its own combined water is even higher than these figures indicate because not all the water present in a baked flour mixture is free to go to starch but is associated with proteins and other constituents; nor is all the starch in bread necessarity as completely hydrated as it was in the gels studied here. Even so, such amounts of starch are relatively great as can be seen from the fact that a cooked starch paste containing only 5 per cent starch will upon cooling set to a tender gel which retains the shape of a mold. The shapely mass produced by heating together starch and water will be called in this paper a gel, and the possibility that its internal structure may be unlike that of gels produced by particles known to be of coUoidol dimensions will be ignored. Starch gels, especially those of about 5 per cent concentration, have properties in common with gels of gelatin and pectin; they appear quite homogeneous, can be cut to leave a straight-edged surface, and may show syneresis upon standing. These characteristics all play a significant rôle in starchy food preparations. The heating of starch in the presence of water brings about changes which have been rather unanimously called '^gelatinization,'' though different workers have interpreted the term to refer to changes with respect, in turn, to microscopic appearance of the granules, translucency of suspensions, and viscosity of fairly dilute pastes. Changes in such factors occur over a rather wide range in temperature. When gelatinization has proceeded to a certain point, a suspension will often set to the gel form already mentioned as a desirable one in many food preparations. It was with the purpose of relating the factors of concentration, temperature of heating, and microscopic changes in the granules to the ability of a suspension to assume form as a gel that this work was undertaken. Records on hard and soft wheat starches were obtained in the form of photomicrographs showing the altered gran-