EXPERIMENT 77: CONDITIONS FOR GROWTH OF THE YEAST PLANT.—(a) Mix 1 tablespoonful of flour, 1 tablespoonful of sugar, 3/4 cake compressed yeast, and 5 tablespoonfuls of cold water. Put 1 tablespoonful of the mixture in a test tube and mark the tube "a." Fill the tube nearly full of lukewarm water and stand in a warm place for 15 minutes. Examine, noting especially the appearance at the top of the test tube. What kind of substance (gas, liquid, or solid) has been formed by the growth of the yeast plants?

(b) Put 1 teaspoonful of the yeast mixture in a test tube, and fill nearly full of boiling water. Label it "b" and after 15 minutes examine. Is there any change in the contents of the tube? What has happened to the yeast plants?

(c) Put 1 teaspoonful of the yeast mixture in a test tube, fill nearly full of cold water, and label it "c." Surround it with cracked ice or, if the weather is cold, place it out of doors. After 15 minutes examine. Is there any change in the contents of the tube? Why do not the yeast plants grow?

(d) Surround the tube marked "c" with lukewarm water and stand in a warm place. After 15 minutes examine. Are the yeast plants growing? Does freezing kill yeast plants?

(e) Mix 1/8 cake yeast with a little lukewarm water. Stand in a warm place and after 15 minutes examine. Will yeast grow in water alone?

(f) Mix 1/8 cake yeast, 1 tablespoonful of sugar, and a little lukewarm water. Set aside in a warm place so that the yeast plants may grow. Then examine under the microscope. Are there any budding yeast cells? Make a drawing of the plants as they appear under the microscope.

Draw at least two practical conclusions from these experiments as to the use and care of yeast for bread making

[Illustration: From Farmers' Bulletin 398 FIGURE 86—GROWING YEAST
PLANTS]

PROPERTIES OF YEAST.—Yeast consists of a mass of microscopic plants (see Figure 86). When placed under proper conditions these plants grow rapidly, and in so doing they separate the sugar that exists in flour into carbon dioxide and alcohol. The carbon dioxide lightens dough. The alcohol passes off as vapor in baking.

Plants need moisture, warmth, light, and the food that is furnished by the air and soil for their growth. Yeast plants require all of these except light. They are not green plants, hence they need no light. Moisture is obtained from the water, milk, or other liquids used in bread dough. Yeast thrives at a temperature of 90 degrees F. It is killed by a temperature above 130 degrees F. Hence the yeast is mixed with lukewarm water. The other liquids that are added to it are of the same temperature. Also, the dough is placed in a warm place while it is rising, or while the yeast plants are growing. The food required for the rapid growth of the yeast is obtained from the protein and carbohydrates in the flour.

Compressed yeast cakes which are wrapped in tinfoil and received fresh at the market every day or two are the most satisfactory to use. This yeast must be fresh for successful bread making. It is fresh when it is of a light color, is free from dark streaks, and is crumbly in texture.

Stiff Dough.—Approximately four parts of flour to one of moisture are used for stiff doughs. When sufficient flour has been added to stiff dough, it should not cling to the sides of the mixing bowl. This is an indication to the pupil of the proper stiffness of the dough. The test applies, however, only when there is no coating of flour over the dough. One should remember that the softest dough will not "stick," if covered generously with flour.

General Suggestions for Bread Making.—Use wheat bread flour, or a combination of wheat bread flour with whole wheat, or graham flour, or with flour or meal made from other grains, in making bread. Flour should be kept in a dry place. It is well to warm flour for bread before using.

If milk is used, scald or boil it to prevent it from souring. Water should be boiled and then cooled (see Why Foods Spoil).

With 1 pint of liquid 1/2 to 1 cake of yeast should be used. When it is desired to mix and bake bread in a few hours, a greater quantity of yeast may be used. If the yeast is fresh, most satisfactory results are secured when this is done. The use of much yeast, however, adds to the cost of bread. The less quantity of yeast (1/2 cake) is used when the dough is allowed to rise overnight. Mix 1 yeast cake in 1 cupful of lukewarm water before adding the rest of the liquid.

It is desirable to use sufficient yeast and to subject it to desirable conditions so that the dough will rise quickly. If the rising process occupies much time, certain kinds of bacteria which may be present in the yeast or other materials may act upon the alcohol present in the risen dough and convert it into acid. This produces sour dough and consequently bread of sour taste and odor.

Although it is customary to allow bread to rise twice, tasty bread may be secured by one rising. Bread raised only once, however, is usually of uneven grain, because the carbon dioxide bubbles formed during rising are uneven in size or are unevenly distributed. By kneading bread, the larger bubbles are broken or distributed more evenly through the dough. Since considerable gas is pressed out by kneading, it is necessary to allow the dough to rise a second time. It is well to make the dough into small loaves, and place them in small pans, so that the bread will be baked through.

Loaves of bread should bake at least 1 hour at a temperature varying from 375 degrees F. to 400 degrees F. During the first 20 minutes they should rise but slightly and just begin to brown; during the second 20 minutes they should continue to brown; during the last 20 minutes they should shrink from the sides of the pan, while still continuing to brown.

To soften the crust, rub it with a bit of butter or substitute a few minutes before taking from the oven and again after removing from the oven. After baking, place the loaves of bread on a bread cooler, or arrange them in such a way that the air may reach them on all sides. When cool, place in a covered tin box.

BREAD (2 loaves)

2 cupfuls hot water or milk and water 2 teaspoonfuls salt 2 teaspoonfuls sugar 1/2 tablespoonful fat 1/2 to 1 cake compressed yeast 1/2 cupful lukewarm water Bread flour (7 to 8 cupfuls)

Boil the water or milk and water. Pour it into a bowl and add the salt, sugar, and fat. Stir until the salt and sugar are dissolved, and the fat is melted. Mix the yeast with lukewarm water. When the first mixture is cooled to lukewarm temperature, add the yeast mixture to it. Then add flour enough to make it of the proper consistency (see Stiff Dough), using a knife for mixing. Turn out on a floured board, and knead until soft and elastic. Return the dough to the bowl, moisten, cover, and let rise until doubled in bulk. Then divide it into loaves, or shape into biscuits. Cover and allow the loaves or biscuit to rise in the pan in which they are to be baked until they are doubled in bulk. Bake the biscuits 30 minutes in a hot oven and the bread about 60 minutes in a moderate oven (see Oven Thermometers and Temperatures).

QUESTIONS

Why should the flour for bread be warmed before using?

What should be the temperature of all materials mixed with yeast? Why?

What should be the difference in the temperature of the oven for loaf bread and for biscuits? Explain.

Why should bread be stored in a covered tin box?

At what temperatures should biscuits and loaf bread bake? Why are these foods baked at different temperatures?

For how long a time should biscuits and bread bake? Explain the difference in the length of time of baking each.