For purposes of study, starch may be prepared from the ground meal of cereals, potatoes, etc., by kneading the meal in a bag or sieve of fine-meshed muslin or silk, under a slow stream of water. The starch granules, being microscopic in size, readily pass through the cloth with the water, and may be caught in any suitable container. The starch is then allowed to settle to the bottom, the water poured off and the starch collected and dried.
Starch is insoluble in water; but if boiled in water, the granules burst and a slimy opalescent mass, known as "starch paste," is obtained. This is undoubtedly a colloidal suspension of the starch in water. By various processes, such as boiling with very dilute acids, treatment with acetone, etc., starch is converted into "soluble starch" which dissolves in water to a clear solution. Soluble starch is precipitated out of solution by alcohol, or by lead subacetate solution.
Air-dried starch contains from 15 to 20 per cent of water; but this can be completely removed, without altering the starch in any way, by heating for some time at 100° C.
The starch granules from different sources vary considerably in size and shape, and can generally be identified by observation under the microscope.
The most characteristic reaction of starch is the blue color which it gives with iodine. The reaction is most marked with starch paste or soluble starch, but even dry starch granules are colored blue when moistened with a solution of iodine in water containing potassium iodide, or with tincture of iodine.
When hydrolyzed, either by boiling with dilute acids or under the influence of enzymes, starch undergoes a series of decompositions, yielding first dextrins, then maltose, and finally glucose. These transformations can be traced by the iodine color reaction, as starch will show its characteristic blue, dextrins purple or rose-red, and maltose and glucose no color with iodine.
=Dextrins= may occur in plants as transition products in the transformation of starch into sugars, or _vice versa_. Most commonly, however, they are artificial products resulting from the partial hydrolysis of starch in the laboratory or factory. They are amorphous substances, which are readily soluble in water, forming sticky solutions which are often used as adhesives ("library paste" is a common example of a very concentrated preparation of this kind). They are precipitated from solution by alcohol, but not by lead subacetate (distinction from starch). They are strongly dextrorotatory (specific rotatory power +192° to +196°); are not fermented by yeast alone, but readily undergo hydrolysis to glucose which does ferment. There are several different modifications, or forms, of dextrins, depending upon the extent to which the simplification of the starch molecule by hydrolysis is carried. Three fairly definite forms are generally recognized, as follows: _amylo-dextrin_, or soluble starch, slightly soluble in cold water, readily so in hot water, giving a blue color with iodine; _erythro-dextrin_ easily soluble in water, neutral taste, red color with iodine; and _achroo-dextrin_, easily soluble in water, sweetish taste, no color with iodine.