Dilute some of gelatinized starch with water, pour some of it into a test tube and add a drop of tincture of iodine; the cold solution will at once turn a deep blue. Heat the liquid gently to boiling point; the blue coloring will gradually disappear; upon cooling the liquid it again turns blue.
The action of tincture of iodine at low temperature is so intensive that the smallest particles of starch contained in any substance can be detected by its use. By means of tincture of iodine starch adulteration in compressed yeasts may be detected.
Water poured on a lump of starch is rapidly absorbed. Starch is very hygroscopic, that is, absorbs moisture readily, and furthermore, retains the absorbed moisture tenaciously. Hence flour will absorb moisture from the atmosphere. A high percentage of moisture in flour is conducive of putrifying of the gluten and albumin of the flour, rendering the flour unfit for use.
Into a small pan, which has been lightly greased, to prevent sticking, heat while constantly stirring some starch flour. It will turn brown; that is, it has been converted into dextrin. Pure dextrin is soluble in water and is largely used as a substitute for gum arabic. The same conversion occurs in bread, as the starch in the exterior of the loaf is changed into dextrin by the high temperature of the oven and forms the crust of the loaf.
Constituents of starch. Starch is composed of the three elements—Carbon, Hydrogen and Oxygen. The composition of starch is the same as sugar. It contains the same proportions of hydrogen and oxygen as is contained in water, namely, in proportions of 2-1 (H2O). Therefore starch is a hydrate, and as these water-forming elements are combined with carbon it is called carbohydrate.
To this group of bodies, besides starch, dextrin, all kinds of sugar and cellulose are classified.
D. Albumen contents of flour. In a flask place 20 oz. of wheat flour and saturate with 100 oz. of cold water; shake up vigorously several times during one-half hour. Then let the flask stand for some time, after which pour the clear liquid obtained carefully into a filter paper arranged in a funnel.
This clear filtrate is placed in a beaker and heated in a water bath to the boiling point. The clear filtrate will become turbid flocculent. These flakes are caused by a substance which is soluble in cold water and which evaporates upon being heated. This substance is albumin. The albumin of flour is of the same composition as the white of an egg, and is classified under the albuminoids. Wheat flour contains 1.5 per cent. of albumin.
E. Extractive matter of flour. After concluding above experiment, filter the coagulated mixture. Place the clear filtrate in an evaporating dish and drive off the moisture by placing the dish in a steam bath. After some time the water will have been driven off, leaving a solid residue in the dish. This residue contains sugar, dextrin, a gum-like substance, and a small quantity of mineral salts, principally potassium phosphate.