Fig. 47.—Wheat Starch Granules after Fermentation with Yeast, as in Bread Making.
If the gluten is of poor quality, or deficient in either gliadin or glutenin, the dough mass fails to properly expand because the gas is not all retained. The amount of gas formed is dependent upon temperature, rapidity of the ferment action, and quality of the yeast and flour. If the yeast is inactive, other forms of fermentation than the alcoholic may take place and, as a result, the dough does not expand. Poor yeast is a frequent cause of poor bread.
The temperature reached in bread making is not sufficient to destroy all the ferment bodies associated with the yeast, as, for example, bread sometimes becomes soft and stringy, due to fermentation changes after the bread has been baked and stored. Both bread and flour are subject to many bacterial diseases, and one of the objects of thorough cleaning of the wheat and removal of the bran and débris particles during the process of flour manufacture is to completely eliminate all ferment bodies mechanically associated with the exterior of the wheat kernel, which, if retained in the flour, would cause it readily to become unsound.
178. Production of Soluble Carbohydrates.—Flour contains naturally a small amount of soluble carbohydrates, which are readily acted upon by the alcoholic ferments. The yeast plant secretes soluble ferments, which act upon the starch, forming soluble carbohydrates, and the heat during baking brings about similar changes. In fact, soluble carbohydrates are both consumed and produced by ferment action during the bread-making process. Flour contains, on an average, 65 per cent of starch, and during bread making about 10 per cent is changed to soluble forms. Bread, on a dry matter basis, contains approximately 6 per cent of soluble carbohydrates, including dextrine, dextrose, and sucrose sugars.[[64]]
The physical changes which the starch grains undergo are also noticeable. Wheat starch has the structure shown in illustration No. 33. The starch grains are circular bodies, concave, with slight markings in the form of concentric rings. When the proteid matter of bread is extracted with alcohol and the starch grains are examined, it will, be seen that some of them are partially ruptured, like those in popped corn, while others have been slightly acted upon or eaten away by the organized ferments, the surface of the starch grains being pitted, as shown in the illustration. The joint action of heat and ferments on the starch grains changes them physically so they may more readily undergo digestion. The brown coating or crust formed upon the surface of bread is mainly dextrine, produced by the action of heat on the starch. Dextrine is a soluble carbohydrate, having the same general composition as starch, but differing from it in physical properties and ease of digestion.
179. Production of Acids in Bread Making.—Wheat bread made with yeast gives an acid reaction. The acid is produced from the carbohydrates by ferment action. Flour contains about one tenth of 1 per cent of acid; the dough contains from 0.3 to 0.5 per cent, while the baked bread contains from 0.14 to 0.3 per cent, but after two or three days slightly more acid is developed.[[64]] During the process of bread making, a small portion of the acid is volatilized, but the larger part enters into chemical combination with the gliadin, forming an acid proteid. When the alcoholic fermentation of bread making becomes less active, acid fermentations begin, and sour dough results. It is not definitely known what specific organic acids are developed in bread making. Lactic and butyric acids are known to be formed, and for purposes of calculation, the total acidity is expressed in terms of lactic acid.
The acidity is determined by weighing 20 grams of flour into a flask, adding 200 cubic centimeters of distilled water, shaking vigorously, and leaving the flour in contact with the water for an hour; 50 cubic centimeters of the filtered solution are then titrated with a tenth normal solution of potassium hydroxid. Phenolphthalein is used as the indicator. It cannot be said that all of the alkali is used for neutralizing the acid, as a portion enters into chemical combination with the proteids. If the method for determining the acid be varied, constant results are not secured. Unsound or musty flours usually show a high per cent of acidity.
