Human Foods and Their Nutritive Value, Harry Snyder [free e books to read TXT] 📗
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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].
So long as the quality of the gluten is not destroyed, the addition of a small amount of either starch or gluten to flour does not affect the size of the loaf, but removal of the gluten affects the moisture content and physical properties of the bread. The addition of starch to flour has the same effect upon the bread as the use of low gluten flour,—lessening the capacity of the flour to absorb water and producing a dryer bread of poorer quality.
185. Composition of Bread.—The composition of bread depends primarily upon that of the flour from which it was made. If milk and butter (or lard) are used in making the dough, as is commonly the case, their nutrients are, of course, added to those of the flour; but when only water and flour are used, the nutrients of the bread are simply those of the flour. In either case the amount of nutrients in the bread is smaller than in the same weight of flour, because a considerable part of the water or milk used in making the dough is present in the bread after baking; that is, a pound of bread contains less of any of the nutrients than a pound of the flour from which the bread was made, because the proportion of water in the bread is greater. The following table shows how the composition of flour compares with that of bread, the different kinds of bread all having been made from the flour with which they are compared:
Thus it may be seen that the proportion of water is larger and of each nutrient smaller in bread than in flour, and that the nutrients of the flour are increased by those in the materials added in making the bread.
It is apparent that two breads of the same lot of flour may differ, according to the method used in making, and also that two loaves of bread made by exactly the same process but from different lots of flour, even when of the same grade or brand, do not necessarily have the same composition, because of possible variation in the flours. In bread made from flour of low gluten content, the per cent of protein is correspondingly low.
186. Use of Skim Milk and Lard in Bread Making.—When flours low in gluten are used, skim milk may be employed advantageously in making the bread, to increase the protein content. Tests show that such bread contains about 1 per cent more protein than that made with water. Ordinarily there is no gain from a nutritive point of view in adding an excessive amount of lard or other shortening, as it tends to widen the nutritive ratio.
187. Influence of Warm and Cold Flours on Bread Making.—When flour is stored in a cold closet or storeroom, it is not in condition to produce a good quality of bread until it has been warmed to a temperature of about 70° F. Cold flour checks the fermentation process, and is occasionally the cause of poor bread. On the other hand, when flour is too warm (98° F.) the influence upon fermentation is unfavorable. Heating of flour does not affect the bread-making value, provided the flour is not heated above 158° F. and is subsequently cooled to a temperature of 70° F. Wheat flour contains naturally a number of ferment substances, some of which are destroyed by the action of heat. The natural ferments, or enzymes, of flour appear to take a part in bread making, imparting characteristic odors and flavors to the
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