Human Foods and Their Nutritive Value - Harry Snyder (e book reader pc TXT) 📗
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When the flour is of good quality, the dough well prepared, and the bread properly baked, the loaf has certain definite characteristics. It should be well raised and have a thin, flinty crust, which is not too dark in color nor too tough, but which cracks when broken; the crumb, as the interior of the loaf is called, should be porous, elastic, and of uniform texture, without large holes, and should have good flavor, odor, and color.
Meal or flour from any of the cereals may be used for unleavened bread, but leavened bread can be made only from those that contain gluten, a mixture of vegetable proteids which when moistened with water becomes viscid, and is tenacious enough to confine the gas produced in the dough. Most cereals, as barley, rice, oats, and corn, some of which are very frequently made into forms of unleavened bread, are deficient or wholly lacking in gluten, and hence cannot be used alone for making leavened bread. For the leavened bread, wheat and rye, which contain an abundance of gluten, are best fitted, wheat being in this country by far the more commonly used.
172. Changes during Bread Making.—In bread making complex physical, chemical, and biological changes occur. Each chemical compound of the flour undergoes some change during the process. The most important changes are as follows[64]:
1. Production of carbon dioxid gas, alcohol, and soluble carbohydrates as the result of ferment action.
2. Partial rupturing of the starch grains and formation of a small amount of soluble carbohydrates due to the action of heat.
3. Production of lactic and other organic acids.
4. Formation of volatile carbon compounds, other than alcohol and carbon dioxid.
5. Change in the solubility of the gluten proteins, due to the action of the organic acids and fermentation.
6. Changes in the solubility of the proteids due to the action of heat, as coagulation of the albumin and globulin.
7. Formation and liberation of a small amount of volatile, nitrogenous compounds, as ammonia and amids.
8. Partial oxidation of the fat.
173. Loss of Dry Matter during Bread Making.—As many of the compounds formed during bread making are gases resulting from fermentation action, and as these are volatile at the temperature of baking, appreciable losses necessarily take place. Experiments show about 2 per cent of loss of dry matter under ordinary conditions. These losses are not confined to the carbohydrates alone, but also extend to the proteids and other compounds. When 100 pounds of flour containing 10 per cent of water and 90 per cent of dry matter are made into bread, the bread contains about 88 pounds of dry matter. In exceptional cases, where there has been prolonged fermentation, the losses exceed 2 per cent[64].
Fig. 46.—Brewers' Yeast.
174. Action of Yeast.—Yeast is a monocellular plant requiring sugar and other food materials for its nourishment. Under favorable conditions it rapidly increases by budding, and as a result produces the well-known alcoholic fermentation. It requires mineral food, as do plants of a higher order, and oftentimes the fermentation process is checked for want of sufficient soluble mineral food. The yeast plant causes a number of chemical changes to take place, as conversion of starch to a soluble form and alcoholic fermentation.
C6H10O5 + H2O = C6H12O6.
C6H12O6 = 2 C2H5OH + 2 CO2.
Alcoholic fermentation cannot occur until the starch has been converted into dextrose sugar. The yeast plant is destroyed at a temperature of 131° F. It is most active from 70° to 90° F. At a low temperature it is less active, and when it freezes the cells are ruptured. A number of different kinds of fermentation are associated with the growth of the yeast plant, and there are many varieties of yeast, some of which are more active than others. For bread making an active yeast is desirable to prevent the formation of acid bodies. If the work proceeds quickly, the rising process is completed before the acid fermentation is far advanced. If fermentation is too prolonged, some of the products of the yeast plant impart an undesirable taste and odor to the bread, and hinder the development of the gluten and expansion of the loaf.
175. Compressed Yeast.—The yeast most commonly used in bread making is compressed yeast, a product of distilleries. The yeast floating on the surface of the wort is skimmed off and that remaining is allowed to settle to the bottom, and is obtained by running the wort into shallow tanks or settling trays. It is then washed with cold water, and the impurities are removed either by sieving through silk or wire sieves, or, during the washing, by fractional precipitation. The yeast is then pressed, cut into cakes, and wrapped in tinfoil. When fresh, it is of uniform creamy color, moist, and of a firm, even texture[18]. It should be kept cold, as it readily decomposes.
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 analysis, 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]
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.
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
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