Serious consequences have resulted from ignorance of this, as shown in the practice of feeding invalids on arrowroot. The popular notion that anything which thickens to a jelly when cooked must be proportionally nutritious is very fallacious, and many a victim has died of starvation by the reliance of nurses on this theory, and consequently feeding an emaciated invalid on mere starch in the form of arrowroot, etc. The selling of a fancy variety at ten times its proper value has greatly aided this delusion, so many believing that whatever is dear must be good. I remember when oysters were retailed in London at fourpence per dozen. They were not then supposed to be exceptionally nutritious and prescribed to invalids, as they have been lately, since their price has risen to threepence each.
The change which takes place in the cookery of starch may, I think, be described as simple hydration, or union with water; not that definite chemical combination that may be expressed in terms of chemical equivalents, but a sort of hydration of which we have so many other examples, where something unites with water in any quantity, the union being accompanied with an evolution of some amount of heat. Striking illustrations of this are presented on placing a piece of hydrated soda or potash in water, or mixing sulphuric acid, already combined chemically with an equivalent of water, with more water. Here we have aqueous adhesion and considerable evolution of heat, without the definitive quantitative chemical combination demanded by atomic theories.
In the experiment above described for separating the starch from wheat-flour, the starch thus liberated sinks to the bottom of the water, and remains there undissolved. The same occurs if arrowroot be thrown into water. This insolubility is not entirely due to the intervention of the envelope of the granules, as may be shown by crushing the granules while dry, and then dropping them into water. Such a mixture of starch and cold water remains unchanged for a long time—Miller says "an indefinite time."
When heated to a little above 140° Fahrenheit, an absorption of water takes place through the enveloping membrane of the granule, the grains swell up, and the mixture becomes pasty or viscous. If this paste be largely diluted with water, the swollen granules still remain as separate bodies, and slowly sink, though a considerable exosmosis of the true starch has occurred, as shown by the thickening of the water. It appears that in their original state the enveloping membrane is much folded, the folds probably forming the curious marking of concentric rings, which constitutes the characteristic microscopic structure of starch-granules, and that, when cooked at the temperature named, the very delicate membrane becomes fully distended by the increased bulk of the hydrated and diluted starch.
A very little mechanical violence, mere stirring, now breaks up these distended granules, and we obtain the starch-paste so well