According to Vauquelin, French wheat flour contains about 10% of water, 11% of gluten, 71% of starch, 5% of sugar, and 3% of gum; and the water of the dough amounts to about 50%. The quantity of the bran in wheat ranges under 2%.
Pur. This article of food is very frequently adulterated both by the miller and the baker, as has been before alluded to in the article on bread. The principal physical characteristics of wheat flour of good quality are the following—it has a dull white colour, somewhat inclining to yellow;—it exhibits no trace of bran, even when pressed smooth with the hand,
or with a polished surface;—its cohesiveness is so great that, on being squeezed in the hand, the lump is some time before it loses its shape;—it has a homogeneous appearance, and does not lose more than from 6% to 12% by being carefully dried in a stove. The smaller the loss in this way the finer is the quality, other matters being equal, and the more economical in use.[313] (See below.)
[313] See also Bread, Adult, and Exam.
Tests. 1. Solution of ammonia turns pure wheat flour yellow; but if any other corn has been ground with it, pale brown; or if peas or beans have been ground with it, a still darker brown.
2. Solution of potassa, containing about 12% of caustic alkali, dissolves pure wheat-flour almost completely; but when it is adulterated with the flour of the leguminous seeds (beans, peas, &c.), the cellulosæ of these substances remains undissolved, and its hexagonal tissue is readily identified under the microscope. Mineral substances (chalk, plaster of Paris, bone dust, &c.) are also insoluble in this test, and appear as a heavy white sediment.
3. Boiling water poured on the sample causes the evolution of the peculiar odour of pea or bean flour when these substances are present. Bread made with such flour evolves a like odour on being toasted.
4. Pure hydrochloric acid poured on potato flour, or on wheat flour adulterated with it, develops a smell of rushes; it also dissolves starch, but changes the colour of pure wheat-flour to a deep violet.
5. Nitric acid turns wheat flour of an orange-yellow colour, but forms a stiff and tenacious jelly with potato fecula, the colour of which it does not alter.
6. A portion of the suspected sample submitted to dry distillation in a stoneware retort, and the distillate collected in a receiver containing a little water, the latter is found to remain perfectly neutral if the wheat flour is pure, but acquires a distinctly alkaline reaction when beans, pulse, or pea meal is present. (Rodrigues.)