A. Class Experiments. The Principle of Leavening.

1. Tie a piece of rubber sheeting over the top of a test tube, cool the tube, then heat it slightly. Notice the effects on the rubber. What effect has heat and cold upon the volume of the air in the tube?

2. How is the gas held in the dough?

Mix one teaspoon of flour with an equal amount of water. Repeat, using cornstarch instead of flour. Notice the difference in the result. To explain: mix 1/4 c. flour with water (a teaspoon at a time) to make a very stiff dough. Wash, by kneading it gently in a bowl of cold water until the part left, the gluten, no longer gives a blue color with iodine. (What has been washed out?) a. Reserve a pinch of the gluten, divide the rest into two balls. Bake one in a hot oven, the other in a slow oven. Explain difference in results.

b. With the piece reserved, determine if gluten is protein.

B. Apple Fritters.

Prepare a pop-over batter, using 1/4 c. liquid. Pare and core an apple and cut crossways into slices. Dip a slice into the batter. Is it thick enough to make a cover for the apple? Add enough flour to make a "cover-batter." Record proportions one would need for a cup of liquid. Fry in deep fat and serve with syrup


The term leavening means "making lighter." Bread is leavened and, instead of being a solid, heavy mass, is spongy, light, and porous. The process is supposed to improve not only the flavor but the digestibility of the mass.

The principle of all leavening is the expansion by heat of some gas which is thoroughly mixed through the batter or dough. In cooking, there are three agents which are commonly used in leavening doughs. The first of these is water vapor or steam. This, as in the pop-overs, is manufactured from the water present in the batter by the heat of the oven. Then further heat expands the steam still more. At the same time the heat hardens the expanded gluten, so that after a while no further stretching is possible. This explains why muffins and cake rise in the oven only at the beginning of the baking.

The second agent is air. This is mixed in a batter in two ways, - it may be entangled in the batter itself by beating rapidly, or it may be beaten into egg and then folded into the batter. Even snow may be folded in like egg and introduce some air into the mixture. When the batter full of tiny bubbles of air is heated, this air expands and, stretching the gluten by which it is held, it makes larger holes, thus leavening all the mass and making it rise.

Carbon dioxide is the third agent. This may be forced into the dough, a process, however, which is never used at home and rarely elsewhere. Carbon dioxide is, instead, manufactured in the dough itself. When yeast is put into bread, one is really starting a plant to grow. The plant feeds mainly on sugar. If cane sugar is present, it turns it first into glucose and fructose sugars and then breaks them up into carbon dioxide and alcohol. The heat of the oven acts on carbon dioxide exactly as it does on water vapor or steam, expanding it into larger bubbles. As alcohol is more easily turned into vapor than is water, it becomes a gas and, expanding, helps in the leavening process.

The other method of introducing carbon dioxide into doughs and batters will be shown in the next lesson.


1. What effect has heat on gluten?

2. What other proteins are hardened by heat?

3. After pop-overs are thoroughly baked, as they cool, what becomes of the steam? Why are they better eaten hot? Compare them to baked potatoes in this respect.

4. Why does an insufficiently cooked pop-over fall, when it is taken out of the oven?

5. Calculate the cost of pop-overs. How do they compare with the cost of the bread that they would replace in a meal?