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]]