I. Blend a yeast cake with a cup of water in which a teaspoon of molasses has been mixed. Divide into four portions.
1. Freeze the first and then let it stand at room temperature.
2. Boil the second and let it cool to room temperature.
3. Heat the third to lukewarm.
In turn, fill a test tube with part of each of these three portions and invert in the remaining liquid, taking care not to admit bubbles of air into the tubes. Use cups or tumblers, if no beakers are at hand. While inverting, the test tubes may be covered with the thumb or a piece of stiff paper. Keep at room temperature, and note result after an hour or two.
4. Chill the fourth portion, fill a test tube with the mixture and invert as above, and place in an ice-box, or out of doors if it is cold weather. Compare the result with the others after the same length of time.
II. Examine under a microscope yeast from an actively fermenting liquid, and make a drawing of a "plant."
III. Observe the action of yeast with the following food materials and account for the results. Use 1/8 yeast cake and 1/4 c. liquid in each case. Invert in test tubes as before.
2. Water with 1/2 tsp. flour.
4. Water in which potatoes have been cooked.
5. Water and 1/2 tsp. of starch.
6. Water and 1/2 tsp. of sugar.
B. Make Sugar Cookies or Gingersnaps.
2 tbsp. fat 1/4 c. sugar 1 1/2 tbsp. egg 1 tbsp. sour milk
1/2 ssp. soda 1/4 tsp. baking powder 1/2 c. flour and amount necessary to roll
A few drops of vanilla
1/4 c. brown sugar 1/4 c. molasses 3 tbsp. fat 1/2 tbsp. ginger
1/4 tbsp. cinnamon 1 tbsp. lemon juice A little grated lemon rind 1 c. flour
1/8 tsp. soda
Roll as thin as possible.
The compressed and dried yeast cakes sold in stores are usually made from yeast obtained from distilleries. Here the yeast is grown by sowing it in "wort", as it is called, a mixture of crushed grain or mashed potatoes, warm water, and sprouted barley. Under these conditions the yeast plants multiply rapidly. The yeast which collects as a scum on top of the wort is preferred for bread yeast, although the yeast which settles out at the bottom is sometimes used. The yeast is freed from impurities as far as possible, pressed to remove the water, and then cut into cakes which are wrapped in tin foil. Starch may be mixed with the yeast before the pressing process takes place. A small percentage of starch helps to keep the yeast, especially in warm weather, as well as making it easier to mix with the flour in bread-making. As the amount of starch used varied from five to fifty per cent, the government recently has ruled that if starch is used in compressed yeast cakes, its presence must be stated on the label.
Compressed yeast cakes must be fresh to be in good condition, and this can be told readily on examination. The cakes should be creamy white, not dark; uniform in color, not streaked; a firm, even texture, not slimy; and there should be no disagreeable odor.
1. Yeast from a Dried Yeast Cake.
From Conn's " Bacteria, Yeasts, and Molds In the Home.".
2. The Same Yeast after a few Hours' Growth.
Compressed yeast in good condition can usually be purchased in places of any size. In remote districts, however, it cannot always be obtained while it is sufficiently fresh for use. Such communities may use dried yeast cakes which are made from the same yeast as the compressed yeast cakes. After the yeast is mixed with starch or meal, it is partly dried, either in the sun or at a low temperature under pressure. Under these conditions some of the yeast cells die, while others pass into a resting stage. Thus the dried yeast is not so active as compressed yeast and it takes longer to start fermentation. Time must be allowed for dried yeast to become actively growing and multiplying.
Some housekeepers "make yeast" at home by using commercial yeast to start the growth. This is usually accomplished by adding a little dried yeast, or yeast from a previous growth, to potatoes which have been grated, mixed with water, and boiled and cooled. A small amount of sugar is also added.
Brewers sell yeast in a liquid form, but usually to bakeries rather than for use in private families, for such yeast must be used at once and is more trouble to transport than are yeast cakes.
There are many different varieties of yeast, and the one sold for bread-making has really been selected as best for making fermented liquors, rather than because it is known to be the best for bread-making. Wild yeasts are sometimes used in making the so-called salt-rising bread. Here, milk is mixed with a little flour and salt and set aside to ferment. The action is apparently not always due to the same cause. Sometimes wild yeasts are present, and at other times the action is entirely due to bacteria. Bread made in this way is much less uniform than that made with cultivated yeast.
It is interesting to know that although fermentation has been recognized and practised from very early times, nothing definite was known about the cause until Pasteur worked out the problem in the middle of the nineteenth century. The account of how he went to work to prove that yeast-cells really produced the changes and that they did not originate spontaneously, as was believed previously, is a fascinating story of scientific research.1
1"life of Louis Pasteur," by Radot.
Conn. "Bacteria, Yeasts and Molds in the Home," section on Yeasts.
1. Compare the recipe for sugar cookies with the muffin recipe. What is the effect of the increase of fat and the addition of sugar? Why can cookies be rolled out without danger of toughening them ?
2. Compare the recipe given for sugar cookies with one containing more fat, as, for example, Miss Farmer's recipe for rich cookies. What difference will you expect in the two kinds of cookies ? Calculate the difference in cost.
3. Show what different flavors could be added to your recipes to make different varieties of cookies. Tell in each case how and when the new ingredient would be added.
4. What fats could be used in making sugar cookies? gingersnaps ?