180. Volatile Compounds produced during Bread Making.—In addition to carbon dioxid and alcohol, there is lost during bread making a small amount of carbon in other forms, as volatile acids and hydrocarbon products equivalent to about one tenth of one per cent of carbon dioxid. The aroma of freshly baked bread is due to these compounds. Both the odor and flavor of bread are caused in part by the volatile acids and hydrocarbons. The amount and kind of volatile products formed can be somewhat regulated through the fermentation process by the use of special flours and the addition of materials that produce specific fermentation changes and desirable aromatic compounds. Some of the ferment bodies left in flour from the imperfect removal of the dirt adhering to the exterior of the wheat kernels impart characteristic flavors to the bread. The so-called nutty flavor of some bread is due to the action of these ferment bodies and, when intensified, it becomes objectionable. Fungous growths in unsound flour and bread result in the liberation of volatile products, which impart a musty odor. Good odor and flavor are very desirable in both flour and bread.
181. Behavior of Wheat Proteids in Bread Making.—Gluten is an ingredient of the flour on which its bread-making properties largely depend. The important thing, however, is not entirely the quantity of gluten, but more particularly its character. Two flours containing the same amounts of carbohydrates and proteid compounds, when converted into bread by exactly the same process, may produce bread of entirely different physical characteristics because of differences in the nature of the gluten of the two samples. Gluten is composed of two bodies called gliadin and glutenin. The gliadin, a sort of plant gelatin, is the material which binds the flour particles together to form the dough, thus giving it tenacity and adhesiveness; and the glutenin is the material to which the gliadin adheres. If there is an excess of gliadin, the dough is soft and sticky, while if there is a deficiency, it lacks expansive power. Many flours containing a large amount of gluten and total proteid material and possessing a high nutritive value, do not yield bread of the best quality, because of an imperfect blending of the gliadin and glutenin. This question is of much importance in the milling of wheats, especially in the blending of the different types of wheat. An abnormally large amount of gluten does not yield a correspondingly large loaf.
Fig. 49.—Bread from Normal Flour (1); Gliadin Extracted Flour (2);
and from Flour after Extraction of Sugar and Soluble Proteids (3).
Experiments were made at the Minnesota Experiment Station to determine the relation between the nature of the gluten and the character of the bread. This was done by comparing bread from normal flour with that from other flour of the same lot, but having part or all of its gliadin extracted.[[64]] Dough made from the latter was not sticky, but felt like putty, and broke in the same way. The yeast caused the mass to expand a little when first placed in the oven; then the loaf broke apart at the top and decreased in size. When baked it was less than half the size of that from the same weight of normal flour, and decidedly inferior in other respects. The removal of part of the gliadin produced nearly the same effect as the extraction of the whole of it, and even when an equal quantity of normal flour was mixed with that from which part of the gliadin had been extracted, the bread was only slightly improved. In flour of the highest bread-making properties the two constituents, gliadin and glutenin, are present in such proportions as to form a well-balanced gluten.
The proteids of wheat flour are mainly in an insoluble form, although there are small amounts of albumins and globulins; these are coagulated by the action of heat during the bread-making process, and rendered insoluble. A portion of the acid that is developed unites with the gliadin and glutenin, forming acid proteids, which change the physical properties of the dough. Both gliadin and glutenin take important parts in bread making. The removal of gliadin from flour causes complete loss of bread-making properties. Ordinarily from 45 to 65 per cent of the total nitrogen of the flour is present in alcohol soluble or gliadin form. Proteids also undergo hydration during mixing, some water being chemically united with them, changing their physical properties. This hydration change is necessary for the full development of the physical properties of the gluten. The water and salt soluble proteids appear to take no important part in the bread-making process, as their removal in no way affects the size of the loaf or general character of the bread. Because of the action of the acids upon the gliadin, bread contains a larger amount of alcohol soluble nitrogen or gliadin than the flour from which the bread was made. It is believed that this action changes the molecular structure of the protein so that it is more readily separated into its component parts when it undergoes digestion and assimilation.
182. Production of Volatile Nitrogenous Compounds.—When fermentation is unnecessarily prolonged, an appreciable amount of nitrogen is volatilized in the form of ammonia and allied bodies, as amids. During the process of bread making, the yeast appears to act upon the protein, as well as upon the carbohydrates, and, as previously stated, losses of dry matter fall alike upon these two classes of compounds, nitrogenous and non-nitrogenous. Analyses of the flours and materials used in bread making, and of the bread, show that ordinarily about 1.5 per cent of the total nitrogen is liberated in the form of gas during the bread-making process, and analyses of the gases dispelled in baking show approximately the same per cent of nitrogen. When bread is dried, as in a drying oven, a small amount of volatile nitrogen appears to be given off,—probably as ammonium compounds formed during fermentation. The nitrogen lost in bread making under ordinary conditions is not sufficient to affect the nutritive value of the product. The losses of both nitrogen and carbon are more than offset by the increased solubility of the proteids and carbohydrates, the preliminary changes they have undergone making them more digestible and valuable for food purposes. The nitrogen volatilized in bread making appears to be mainly that present in the flour in amid forms or liberated as the result of fermentation processes. The more stable proteids undergo only limited changes in solubility and are not volatilized.
183. Oxidation of Fat.—Flour contains about 1.25 per cent of fat mechanically mixed with a small amount of yellow coloring matter. During the process of bread making the fat undergoes slight oxidation, accompanied by changes in both physical and chemical properties. The fat from bread, when no lard or shortening has been added, is darker in color, more viscous, less soluble in ether, and has a lower iodine number, than fat from flour. The change in solubility of the fat is not, however, such as to affect food value, because the fat is not volatilized, and is only changed by the addition of a small amount of oxygen from the air. When wheat fat and other vegetable and animal fats are exposed to the air, they undergo changes known as aging, similar to the slight oxidation changes in bread making.[[64]]
184. Influence of the Addition of Wheat Starch and Gluten to Flour.—Ten per cent or more of starch may be added to normal flour containing a well-balanced gluten, without decreasing the size of the loaf. When moist gluten was added to flour, thus increasing the total amount of gluten, the size of the loaf was not increased[[67]].