be acted upon by yeast if left in the air. The yeast grows and changes the sugar into alcohol and carbon dioxide (cf. §§ 175 and 204). This is the reason why grape juice, if left in the air, ferments and becomes wine. Place a teaspoon of molasses and Add of a cake of yeast. Pro Do this experiment (Fig. 119): a cup of water in a flask or bottle. vide a stopper with one hole in it. Through this hole run a glass tube FIG. 119. A sugar solution (molasses) being fermented by the action of yeast. The test tube contains limewater. and let the other end of the tube dip under the surface of limewater which is in a test tube or slender bottle. As you found out in § 37, carbon dioxide makes limewater look milky. Leave the fermenting solution in a warm (not hot) place for several days. Look often for any change in the limewater. Note the smell of the sugar solution. What has been formed in it? For what is the limewater a test? 202. How Does Yeast Act in Making Bread? The yeast we buy and use is grown in a yeast factory. The yeast plants are produced in great numbers in a food they thrive on, then are gathered together into yeast cakes and pressed. Dry yeast, which contains the plants in the spore stage, may be kept for weeks or even months, but it takes longer to begin its growth when it is put into the sponge. Compressed" yeast cakes are bought in a soft form, which contains the plants in the growing stage, but this form cannot be kept very long, especially in warm weather. The soft yeast has the advantage of acting much sooner than the dried. Rising bread should be kept at about 70°-90° F., as the plants do not grow if they are too cool and are easily killed (when growing) if too hot. The yeast in the bread produces alcohol and carbon dioxide, and the carbon dioxide and the alcohol vapor act just as the carbon dioxide does when formed from baking powder. The bubbles push the dough up and expand in the baking. During baking the yeast plants are killed and the alcohol is driven off. What a wonderful process "raising bread by yeast" is! 203. Exercises. 1. What are three good reasons for the cooking of food? 2. Ask your mother or a cook whether meat to be roasted should be put into a very hot oven or one of moderate temperature and why. 3. Why is some bread heavy, or soggy? Why is it not good to eat? How is "angel food " cake raised? 4. Tell why" toasted" bread is so healthful. Is a soft-boiled egg more digestible than a hard-boiled one? Why? 5. What is the difference between baking soda and baking powder? 6. Soda and sour milk are often used together to make dough rise. Why? What kind of substance does sour milk contain? How does this act with soda? 7. What nutrients are lost when food is cooked for a long time in water? 8. Find out what foods can be made out of skim milk. CHAPTER XXII THE PRESERVING OF FOODS 204. Why Do We Can and Preserve Foods? - You know what happens if most foods are left exposed to the air: they spoil, or "rot." The air about us is full of the spores of bacteria, yeast, and molds (cf. §§ 34, 175, 201), microscopic plants which include some of man's Fruiting produces the spores. best friends as well as some of his worst enemies. As they float about in the air, the plant spores fall upon our foods, which are their foods also (Fig. 120). When these tiny plants feed upon our FIG. 120. The mold that grows upon bread, and a fruiting body that food they break it up into new compounds, some of which go off as gases. In time the food is completely destroyed. The result of the action of these unseen destroyers, when we look at it in the right way, is really very beneficial to the earth, for they remove waste foods and dead plant and animal matter. But if we wish to guard food that we want to eat, we must either seal it away from the air, or we must treat it in such a way that destructive plants cannot grow in it. 205. How Does Drying Preserve Food? - Drying is one way of treating food so that it will be impossible for bacteria to attack it. Bacteria and other germs need moisture for growth, just as a human being does. Although a dried apple may be covered with spores, they will not start growing until there is enough moisture present. The method of keeping fruits and vegetables by drying them was practiced by our ancestors long ago. It is easy and requires very little equipment. Old attics used to be hung with dried corn and apples; many of the medicines were dried herbs. In the dry sections of the West " jerked Every prosperous farmer, up FIG. 121. - A smoke house for smoking fish. The smoke passes through a long pipe before it enters the house, in order that it may be cooled. to a generation ago, had a smoke house. Before the days of refrigerator cars and easy access to meat markets, the farmer killed his own meat, and as the meat could not all be eaten at once, some of it had to be preserved, or protected from the germs of the air. It was hung in the smoke from a wood fire until the smoke had not only given a delicious flavor to the meat, but also had rendered it safe. Smoke is a preservative because it contains a substance called creosote. 207. What are Salting and Pickling? Two common home preservatives are salt and vinegar; they season food as well as preserve it. Salt and vinegar make solutions in which the germs of decay cannot live. Saltpeter (which the chemist calls potassium nitrate) is also used. It makes the red color of meat more intense. You know how red "corned beef" is; saltpeter is used in preserving it. 208. How Does Sugar Preserve Food? Have you ever seen your mother put up preserves, or jelly, or candied fruits? She does not seal the jars or cans tightly, as she does in ordinary canning. Why then do not the germs and spores of the air fall into the fruit and cause it to ferment or mold? If you were to find out how much sugar is used in preserving" fruits, you would have the answer: the amount of sugar is much larger than in canning; so large that germs cannot live in the sugar solution. 209. What are the Principles of Canning? — The methods of preserving food by smoking and by the use of salt, vinegar, and sugar have been practiced for many centuries. A wonderful step in progress was made in the nineteenth century, when the method of preserving food by sealing it tightly in glass or tin cans came into general use. The principles of canning are : (1) to sterilize the food, that is, to kill any bacteria or spores which may be on it; (2) to seal it air-tight and prevent the entrance of any more bacteria or spores. Fruits and vegetables are attacked freely by germs from the air; in canning, the food is cooked until the |