Human Foods and Their Nutritive Value Part 9

176. Dry Yeast is made by mixing starch or meal with fresh yeast until a stiff dough is formed. This is then dried, either in the sun or at a moderate temperature, and cut into cakes. By drying, many of the yeast cells are rendered temporarily inactive, and so it is a slower acting leaven than the compressed yeast. A dry yeast will keep indefinitely.

177. Production of Carbon Dioxid Gas and Alcohol.--Carbon dioxid and alcohol are produced in the largest amounts of any of the compounds formed during bread making. When the alcoholic ferments secreted by the yeast plant act upon the invert sugars and produce alcoholic fermentation, carbon dioxid is one of the products formed. Ordinarily about 1 per cent of carbon dioxid gas is generated and lost during bread making. About equal weights of carbon dioxid and alcohol are produced during the fermentation. In baking, the alcohol is vaporized and aids the carbon dioxid in expanding the dough and making the bread porous. If all of the moisture given off during bread making be collected it will be found that from a pound loaf of bread there are about 40 cubic centimeters of liquid; when this is submitted to chemical a.n.a.lysis, small amounts of alcohol are obtained. Alcoholic fermentation sometimes fails to take place readily, because there are not sufficient soluble carbohydrates to undergo inversion, or other food for the yeast plant.

Starch cannot be converted directly into alcohol and carbon dioxid gas; it must first be changed into dextrose sugars, and these undergo alcoholic fermentation. Bread gives no appreciable reaction for alcohol even when fresh.[64]

[Ill.u.s.tration: 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 ma.s.s 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 a.s.sociated 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 debris particles during the process of flour manufacture is to completely eliminate all ferment bodies mechanically a.s.sociated 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 ill.u.s.tration 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 ill.u.s.tration. 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 t.i.trated with a tenth normal solution of pota.s.sium 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.

[Ill.u.s.tration: FIG. 48.--APPARATUS USED IN STUDY OF LOSSES IN BREAD MAKING.]

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 quant.i.ty 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.

[Ill.u.s.tration: 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 ma.s.s 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 quant.i.ty 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 const.i.tuents, 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 alb.u.mins 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 a.s.similation.

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 cla.s.ses of compounds, nitrogenous and non-nitrogenous. a.n.a.lyses 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 a.n.a.lyses 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].

INFLUENCE OF ADDITION OF STARCH AND GLUTEN TO FLOUR

===================================================================== | SIZE OF LOAF | WEIGHT --------------------------------------------------------------------- Wheat flour, 14 ounces | 22-1/2 17-1/2 | 18.75 Wheat flour, 10% wheat starch | 23-1/2 17 | 18.25 Wheat flour, 12.2% wheat starch | 21-1/2 17 | 18.00 | | Wheat flour, 210 grams, about 8 ounces | 12-3/4 9 | 12.00 Wheat flour, 10% gluten added, 210 grams | 12-1/2 9 | 12.75 Wheat flour, 20% gluten added | 12 8-3/4 | 13.00 =====================================================================

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 b.u.t.ter (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:

COMPOSITION OF FLOUR, AND BREAD MADE FROM IT IN DIFFERENT WAYS

===================================================================== MATERIAL | WATER | PROTEIN | FAT| C.H.| ASH --------------------------------------------------------------------- | % | % | % | % | % Flour | 10.11 | 12.47 |0.86|76.09 |0.47 Bread from flour and water | 36.12 | 9.46 |0.40|53.70 |0.32 Bread from flour, water, and lard | 37.70 | 9.27 |1.02|51.70 |0.31 Bread from flour and skim milk | 36.02 | 10.57 |0.48|52.63 |0.30 =====================================================================

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 product.

[Ill.u.s.tration: 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 12-1/2 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 const.i.tuents being a.s.similated 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 coa.r.s.er 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 pa.s.s 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 coa.r.s.er 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 ca.n.a.l is cleared much more rapidly of its contents. It is also well known that the poorer cla.s.ses 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 pa.s.ses 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 apt.i.tude for a.s.similation are not less important qualities than its ultimate composition.

"But to suppose that whole-wheat meal as ordinarily prepared is, as has generally been a.s.sumed, 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. .h.i.therto 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 a.n.a.lysis 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 const.i.tuent is contained in the coa.r.s.er 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 coa.r.s.e 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 pota.s.sium. 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 a.s.similated 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.