Human Foods and Their Nutritive Value - Harry Snyder (e book reader pc TXT) 📗
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Fig. 50.-Bread from (1) Graham, (2) Entire Wheat, and (3) White Flour. The same amounts of flour were used in making all of the breads.
188. Variations in the Process of Bread Making.—Since flours differ so in chemical composition, and the yeast plant acts upon all the compounds of flour, it naturally follows that bread making is not a simple but a complex operation, resulting in a number of intricate chemical reactions, which it is necessary to control and many of which are only imperfectly understood. Bread of the best physical quality and commercial value is made of flour from fully matured, hard wheats, containing a low per cent of acid, no foreign ferment materials or their products, and at least 12½ per cent of proteids, of which the larger portion is in the form of gliadin. It is believed that a better quality of bread could be produced from many flours by slight changes or modifications in the process of bread making. It cannot be expected that the same process will give the best results alike with all types and kinds of flour. The kind of fermentation process that will produce the best bread from a given type of flour can be determined only by experimentation. Poor bread making is due as often to lack of skill on the part of the bread maker, and to poor yeast, as it is to poor quality of flour. Frequently the flour is blamed when the poor bread is due to other factors. Lack of control of the fermentation process, and the consequent development of acid and other organisms which check the activity of the alcoholic ferments, is a frequent cause of poor bread.
189. Digestibility of Bread.—Extensive experiments have been made by the Office of Experiment Stations of the United States Department of Agriculture, at the Minnesota and Maine Experiment Stations, to determine the digestibility and nutritive value of bread. Different kinds and types of wheat were milled so as to secure from each three flours: graham, entire wheat, and standard patent. The flours were made into bread, and the bread fed to workingmen, and its digestibility determined. The experiments taken as a whole show that bread is an exceedingly digestible food, nearly 98 per cent of the starch or carbohydrate nutrients and about 88 per cent of the gluten or proteid constituents being assimilated by the body. In the case of the graham and entire wheat flours, although they contained a larger total amount of protein, the nutrients were not as completely digested and absorbed by the body as were those of the white flour. The body secured a larger amount of nutrients from the white than from the other grades of flour, the digestibility of the three types being as follows: standard patent flour, protein 88.6 per cent and carbohydrates 97.7 per cent; entire wheat flour, protein 82 percent and carbohydrates 93.5 per cent; graham flour, protein 74.9 per cent and carbohydrates 89.2 per cent. The low digestibility of the protein of the graham and entire wheat flours is supposed to be due to the coarser granulation; the proteins, being embedded and surrounded with cellular tissue, escape the action of the digestive fluids. Microscopic examination of the feces showed that often entire starch grains were still inclosed in the woody coverings and consequently had failed to undergo digestion.[62], [64], [67], [86]
190. Use of Graham and Entire Wheat in the Dietary.—Entire wheat and graham flours should be included in the dietary of some persons, as they are often valuable because of their physiological action, the branny particles stimulating the process of digestion and encouraging peristaltic action. In the diet of the overfed, they are valuable for the smaller rather than the larger amount of nutrients they contain. Also they supply bulk and give the digestive tract needed exercise. For the laboring man, where it is necessary to obtain the largest amount of available nutrients, bread from white flour should be supplied; in the dietary of the sedentary, graham and entire wheat flours can, if found beneficial, be made to form an essential part. The kind of bread that it is best to use is largely a matter of personal choice founded upon experience.
"When we pass on to consider the relative nutritive values of white and whole-meal bread, we are on ground that has been the scene of many a controversy. It is often contended that whole-meal is preferable to white bread, because it is richer in proteid and mineral matter, and so makes a better balanced diet. But our examination of the chemical composition of whole-meal bread has shown that as regards proteid at least, this is not always true, and even were it the case, the lesser absorption of whole-meal bread, which we have seen to occur, would tend to annul the advantage.... On the whole, we may fairly regard the vexed question of whole-meal versus white bread as finally settled and settled in favor of the latter."[28]
"The higher percentage of nitrogen in bran than in fine flour has frequently led to the recommendation of the coarser breads as more nutritious than the finer. We have already seen that the more branny portions of the grain also contain a much larger percentage of mineral matter. And, further, it is in the bran that the largest proportion of fatty matter—the non-nitrogenous substance of higher respiratory capacity which the wheat contains—is found. It is, however, we think, very questionable whether upon such data alone a valid opinion can be formed of the comparative values of bread made from the finer or courser flours ground from one and the same grain. Again, it is an indisputable fact that branny particles when admitted into the flour in the degree of imperfect division in which our ordinary milling processes leave them very considerably increase the peristaltic action, and hence the alimentary canal is cleared much more rapidly of its contents. It is also well known that the poorer classes almost invariably prefer the whiter bread, and among some of those who work the hardest and who consequently soonest appreciate a difference in nutritive quality (navvies, for example) it is distinctly stated that their preference for the whiter bread is founded on the fact that the browner passes through them too rapidly; consequently, before their systems have extracted from it as much nutritious matter as it ought to yield them.... In fact, all experience tends to show that the state as well as the chemical composition of our food must be considered; in other words, that the digestibility and aptitude for assimilation are not less important qualities than its ultimate composition.
"But to suppose that whole-wheat meal as ordinarily prepared is, as has generally been assumed, weight for weight more nutritious than ordinary bread flour is an utter fallacy founded on theoretical text-book dicta, not only entirely unsupported by experience, but inconsistent with it. In fact, it is just the poorer fed and the harder working that should have the ordinary flour bread rather than the whole-meal bread as hitherto prepared, and it is the overfed and the sedentary that should have such whole-meal bread. Lastly, if the whole grain were finely ground, it is by no means certain that the percentage of really nutritive nitrogenous matters would be higher than in ordinary bread flour, and it is quite a question whether the excess of earthy phosphates would not then be injurious."—Lawes and Gilbert.[68]
"According to the chemical analysis of graham, entire wheat, and standard patent flours milled from the same lot of hard Scotch Fife spring wheat, the graham flour contained the highest and the patent flour the lowest percentage of total protein. But according to the results of digestion experiments with these flours the proportions of digestible or available protein and available energy in the patent flour were larger than in either the entire wheat or the graham flour. The lower digestibility of the protein of the latter is due to the fact that in both these flours a considerable portion of this constituent is contained in the coarser particles (bran), and so resists the action of the digestive juices and escapes digestion. Thus while there actually may be more protein in a given amount of graham or entire wheat flour than in the same weight of patent flour from the same wheat, the body obtains less of the protein and energy from the coarse flour than it does from the fine, because, although the including of the bran and germ increases the percentage of protein, it decreases its digestibility. By digestibility is meant the difference between the amounts of the several nutrients consumed and the amount excreted in the feces.
"The digestibility of first and second patent flours was not appreciably different from that of standard patent flour. The degree of digestibility of all these flours is high, due largely to their mechanical condition; that is, to the fact that they are finely ground."—Snyder.[62]
For a more extended discussion of the subject, the student is referred to Bulletins 67, 101, and 126, Office of Experiment Stations, United States Department of Agriculture.
191. Mineral Content of White Bread.—Average flour contains from 0.4 to 0.5 of 1 per cent of ash or mineral matter, the larger portion being lime and magnesia and phosphate of potassium. It is argued by some that graham and entire wheat flours should be used liberally because of their larger mineral content and their greater richness in phosphates. In a mixed dietary, however, in which bread forms an essential part, there is always an excess of phosphates, and there is nothing to be gained by increasing the amount, as it only requires additional work of the kidneys for its removal. Few experiments have been made to determine the phosphorus requirements of the human body, but these indicate that it is unnecessary to increase the phosphate content of a mixed diet. It is estimated that less than two grams per day of phosphates are required to meet all of the needs of the body, and in an average mixed ration there are present from three to five grams and more. A large portion of the phosphate compounds of white bread is present in organic combinations, as lecithin and nucleated proteids, which are the most available forms, and more valuable for purposes of nutrition than the mineral phosphates. In the case of graham and entire wheat flours, a proportionally smaller amount of the phosphates are digested and assimilated than from the finer grades of flour.
192. Comparative Digestibility of New and Old Bread.—With healthy persons there is no difference whatever in the completeness of digestibility of old and new bread; one appears to be as thoroughly absorbed as the other. In the case of some individuals with impaired digestion there may be a difference in the ease and comfort with which the two kinds of bread are digested, but this is due mainly to individuality and does not apply generally. The change which bread undergoes when it is kept for several days is largely a loss of moisture and development of a small amount of acid and other substances from the continued ferment action.
193. Different Kinds of Bread.—According to variations in method of preparation, there are different types and varieties of bread, as the "flat bread" of Scandinavian countries, unleavened bread, Vienna bread, salt rising bread, etc. Bread made with baking powder differs in no essential way from that made with yeast, except in the presence of the residue from the baking powder, discussed in Chapter XII. Biscuits, wheat cakes, crackers, and other food materials made principally from flour, have practically the same
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