_Gluten_ (albuminoids).--As previously stated, gluten is separated by kneading the flour and repeated washing with water. After the removal of the amylaceous and soluble ingredients, the residue is carefully dried and weighed. A far more accurate method is to make a combustion of a small portion of the flour with cupric oxide, and determine the quantity of nitrogen obtained, the percentage of which, multiplied by 6·33, gives the percentage of gluten.[51] The proportion of gluten in flour ranges from about 8 to 18 per cent. From 10 to 12 per cent, is deemed necessary in order to make good bread, and, in England, any deficiency in this constituent is remedied by the addition of bean or other flour, but in the United States this practice is seldom required. _Substances soluble in cold water._--About five grammes of the flour are digested with 250 c.c. of cold water, and the solution filtered, and evaporated to dryness. Good flour is stated to yield 4·7 per cent. of extract when treated in this manner, the soluble matters consisting of sugar, gum, dextrine, vegetable albumen, and potassium phosphate. The latter salt, which constitutes about 0·4 per cent. of the extract, should form the only mineral matter present. _The Ash._--The ash of flour is determined in the usual manner, by ignition in a platinum dish. It varies in amount from 0·3 to 0·8 per cent., and should never exceed a proportion of 1·5 per cent. When of good quality, wheaten flour is perfectly white, or has only a faint tinge of yellow. It should be free from bran, and must not show red, grey, or black specks, nor possess a disagreeable odour. It should also exhibit a neutral reaction and a decided cohesiveness, acquiring a peculiar soft and cushion-like condition when slightly compressed. Formerly, wheaten flour was mixed with various foreign meals, such as rye, corn, barley, peas, beans, rice, linseed, buckwheat, and potato starch; but at present this form of adulteration is probably but rarely resorted to, at least in the United States. The presence of mildew, darnel, ergot, and other parasites of the grain, constitutes an occasional contamination of flour. The most frequent admixture consists, however, in the addition of alum, which, although more extensively used in bread, is also employed in order to disguise the presence of damaged flour in mixtures, or to improve the appearance of an inferior grade; its addition to a damaged article serves to arrest the decomposition of the gluten, thereby preventing the flour from acquiring a dark colour, and disagreeable taste and odour.