Foods and home making

What care must be taken to make yeast plants grow? You will remem- ber that in Experiment 27, lukewarm water was added to the yeast. Let us see what would happen if boiling water were added. Experiment 28: To learn the effect of boiling water on yeast. (a) Put a teaspoonful of the yeast mixture prepared in Experiment 27 into a test tube. Fill the tube with boiling water. Set the tube aside for a few minutes. Then examine it. Do you see any bubbles in the tube? ( b ) What do you think the boiling water did to the yeast plants? It is interesting also to find whether or not yeast plants will grow in very cold water. Experiment 29: To learn the effect of ice water on yeast. (a) Put a teaspoon of the yeast mixture prepared in Experiment 27 into a test tube. Fill the tube nearly full of ice water. Surround the Figure 128. Growing Yeast Plants, Magnified When a yeast plant or cell grows, a bud forms at one side of the cell. When the bud grows large enough, it separates from the original cell ; a new cell is thus formed. Sometimes more one bud grows on a cell. Yeast Breads 427 tube with cracked ice or, if the weather is cold, place it outdoors. Let it stand for at least 15 minutes. Do you see any bubbles in the tube? (b) Now surround the tube with warm water. Let it stand for at least 15 minutes. Do you see any bubbles in the tube? (c) Did the ice water kill the yeast plants? Give a reason for your answer. In the garden, plants must be treated with care. They must have water and the right kind of soil. They need heat, but not too intense heat. Scalding water poured over garden plants would kill them, as boiling water killed the yeast plants. When yeast is mixed, care must be taken to use water that is not too warm. It is well to use lukewarm water because the yeast plants will begin to grow much more quickly if watered with lukewarm rather than with cold water. While the yeast plants are growing in the dough, the dough should be kept warm. How can we tell whether yeast is fresh? Sometimes dark spots appear on a compressed yeast cake. Such a yeast cake should not be used. Good results cannot be obtained from it because some of the yeast plants are dead. It would be waste- ful to add such yeast to the other ingredients used in making bread. A good cake of compressed yeast is uniformly creamy in color. It feels moist. It can easily be broken into crumbs. Compressed yeast cakes are delivered to most markets every day or two. They are wrapped in tin foil. What may make yeast dough sour ? 1 Sometimes yeast dough has a sour odor. When it is baked, the bread has a sour taste. The souring of bread dough often happens when the dough stands too long before baking. We have learned that alcohol, as well as carbon dioxide, is formed as a result of the growth of the yeast. Now if the dough stands too long, a change will take place in the alcohol and an 1 Note to the Teacher: For a detailed explanation, see page 529 of Chemistry by Greer and Bennett, Allyn and Bacon. 428 Dinner acid will be formed. The acid that is formed is tne same as that present in vinegar. The acid gives bread a sour taste. In order to prevent this, the dough should be made to rise quickly.