This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
Uses. Starch, either alone or in flour, of which it may compose as much as 75 per cent and averages 65 to 70 per cent, is used in cookery: for thickening, as in soups and sauces; to form molded gels, i.e., cornstarch puddings and similar desserts; and, because hydrated starch has adhesive qualities, as a cementing or binding material for holding pieces of food together in the sauce base of croquettes and in the framework of such baked products as muffins, biscuits, cakes, and breads. Hence its characteristics of special interest in food preparation are its gel-forming properties. Alsberg (Wheat Studies) has given an excellent review of the role of starch in bread making.
Sources of starch. Most of the starches and starchy foods used in food preparation are obtained from the cereals: rice, barley, rye, corn, and wheat. Starch is also obtained from tubers, potatoes and sweet potatoes being the principal sources. In addition tapioca, arrowroot, cassava, and some other starches are used to a limited extent.
Starch from different sources varies in size, shape, and properties. Some starches are more suitable for some purposes than others. Some of these differences are in thickening capacity, adhesiveness, ease and rate of hydration, and rate of hydrolysis by diastase. Samec states there are two possible explanations for these differences. (1) It may be supposed that starch is a homogeneous substance existing in many different colloidal conditions and (2) it may be assumed that starch is a heterogeneous system built up from a polysaccharide (or several kindred polysaccharides) together with inorganic or organic substances which accompany it. Under the first condition the properties would result because of different colloidal conditions and under the second condition the components with the starch would modify its properties.
Size and shape of starch granules. Sjostrom has published excellent photomicrographs of the natural and gelatinized starch granules. Butcher gives excellent descriptions, and Scott gives both descriptions and illustrations. Starch is laid down as discrete particles called granules. Many granules are compound, i.e., composed of aggregates of 2 to 25 or more granules. Some starch granules have a characteristic black spot called the hilum, which is an air space visible microscopically as a black spot. The hilum, even in varieties in which it is a definite characteristic, is not always present in all granules. In some starches distinct concentric markings or striations occur which are visible because certain portions of the granules contain less water and thus are denser than other portions. Naturally the size and to a certain extent the shape will vary with the growing conditions, so that sizes reported in the literature vary considerably.
Fig. 43. - A photomicrograph of flour with enough water so that it will stick to the slide. Note that the starch granules are both oval and round. Magnification approximately x 100.
Wheat granules are of two shapes and sizes. They are embedded in a more or less continuous matrix of gluten, a relatively larger number of the granules being free or not embedded in the gluten of soft-wheat than in hard-wheat flours. Larger granules average 25 to 35 microns in diameter and are oval and lenticular (bun shape, being flatter on one side). The small granules are circular and average about 3 to 5 microns. The hilum may show as a circular spot, and striations if present are faintly visible in large granules only.
Rice starch is composed of aggregates of granules, though the polygonal (angular) granules are all small, 3 to 8 microns. No striations are visible and the hilum is stellate and visible only at high magnification.
Cornstarch granules are, according to the variety, polygonal or round. The size averages 10 to 25 microns and few aggregates are found. The hilum is stellate and in the center, and sometimes shows as a crack. Stria-tions are very indistinct.
Potato starch granules vary in size, the large ones being egg or oyster-shaped, the small ones usually circular. The diameter is usually 60 to 100 microns but may vary from 15 to 100 microns. The hilum is an annular or circular spot at the narrow end of the granule. The striations are usually very distinct in large granules, somewhat ring-shaped with wider spacing at the broad end.
Constitution of starch. A perusal of the literature of starch chemistry shows no unanimity of opinion in regard to the naming of the starch components, their number, nor some of their properties. However, it should be remembered that starch, as it is found in the plant tissues, is only one of many other components. It is separated from these other components by physical means and often, when the chemist starts to work with it, is a modified starch, the modification occurring through physical or chemical treatment.
Starch may occur as free starch or combined with phosphoric acid, with phosphorus and protein, or with free fatty acids. Samec states silicic acid is always present in starch. Many of the properties and characteristics of starch vary with its constitution.
The starch molecule. There seems to be agreement that cellulose is composed of beta-glucopyranose units, whereas starch is composed of alpha-glucopyranose units linked into a continuous chain. The terminal units of such a chain differ, because of their position, from one another and from the other units in the chain. One terminal unit should have reducing powers, just as glucose has reducing power; but the greater the number of glucose units combined as in starch, the less the relative reducing power should be. There is no agreement as to the number of glucose units constituting starch. Haworth states that he believes the chemical unit of starch to be composed of about 30 glucose units. However, he adds that these chemical units must aggregate to form physical units of much larger size. Richardson, Higginbotham, and Farrow have reported that the average chain length of starch varies from 460 to 1470 glucose units. Still longer chains than these have been reported.