In beginning the bread lessons, it will be found that there are no new principles to teach. It will, however, be necessary to explain the new means of producing gas which is used in this particular mixture, namely, yeast.
From their lessons on the "Preservation of Food" and "Canning", the pupils are already acquainted with one class of microscopic plants. The little plants, in that case, were a source of great inconvenience to the housekeeper. Yeast may be introduced as another family of one-celled plants, but one which is most useful. Under good conditions these tiny plants will produce a large amount of carbon dioxide gas, provided they are given sufficient time. If, however, the gas be required quickly, soda and acid must be used. For this reason, plain flour mixtures, in which the carbon dioxide is quickly made, are called quick breads, to distinguish them from breads in which yeast is used. Examples of these are baking-powder biscuits, gems, corn-bread, etc.
The use of yeast is the simplest and cheapest way of obtaining carbonic acid gas, and mixtures so made remain moist longer than those in which baking-powder is used.
Throughout the introductory lesson, this fact must be kept prominently before the class, that yeast is a plant and, as such, requires plant conditions. The necessary conditions will be known from the lesson on "Bacteria", so that they have only to be reviewed. The pupils may be told that although they cannot see the plants, they can very plainly see the bubbles of gas which the plants give off when the latter are made active under favourable conditions.
LESSON I
OUTLINE OF LESSONS ON YEAST
1. Description of yeast:
Yeast is a one-celled plant which can be seen only with a microscope.
Under good conditions it becomes very active and multiplies rapidly by a process called _budding_. It is used by the housekeeper for the carbonic acid gas it gives off.
[Illustration: Yeast plants magnified]
2. Conditions necessary for the activity of yeast:
(1) Oxygen
(2) Water
(3) Food.--This must be sugar, or starch which it will change into sugar. Potato starch is more easily used by yeast than flour starch. It uses also some nitrogenous food and mineral matter.
(4) Heat.--The yeast plant thrives in a heat of about the same temperature as our bodies. A little extra heat will only make it grow faster; but excessive heat will kill it.
Freezing will not kill the plant, though cold makes yeast inactive.
3. Sources of yeast:
Yeast was first found as _wild yeast_ in the air, but now it may be obtained at grocery stores, in three forms:
(1) Liquid yeast.--The plants are put into a starchy liquid. This will keep only a few days, as the starch sours.
(2) Dry yeast.--The plants are put into a starchy paste and the mixture is dried. This form will keep for months, because it is perfectly dry but, for the same reason, it takes the plants a long time to become active when used.
(3) Compressed yeast.--The plants are put into cakes of a starchy mixture and left moist. They will keep only a few days. Good compressed yeast is a pale fawn colour, smells sweet, breaks clean, and crumbles easily.
4. Experiments with yeast:
Make a _yeast garden_ by using the plants obtained at the grocery store as follows:
Take half a cup of lukewarm water to give the plants moisture, a teaspoonful of sugar for immediate food, and the same of wheat starch (flour) for a reserve food. Beat the mixture to infold oxygen, and then put in one-quarter cake of yeast plants.
Divide the mixture among a number of test-tubes, so that each group of four pupils has three.
(1) Place one test-tube in warm water and heat to boiling.
(2) Place one test-tube in water which feels warm to the hand.
(3) Place one test-tube in cracked ice and freeze the mixture. Afterwards thaw, and place the same test-tube in lukewarm water.
Observe the results, and compare the amount of gas formed under the different conditions.
LESSON II
PRACTICAL BREAD-MAKING
Ingredients of plain bread:
1. Liquid.--(1) It wets the mixture and causes the ingredients to adhere.
(2) It furnishes steam for a lightening agent.
(3) It allows the gluten to become sticky and elastic.
(4) It furnishes moisture for yeast plants.
2. Yeast.--It gives off carbonic acid gas, which lightens the mixture.
3. Salt.--(1) It gives a flavour.
(2) It retards the growth of the yeast plant.
4. Flour.--(1) It thickens the mixture.
(2) It supplies food for the yeast plant.
(3) It supplies gluten for a framework for the mixture.
Amount of ingredients for one small loaf:
Liquid--1 cup or 1/2 pt.
Salt--1/2 tsp.
Flour--About three times the amount of liquid
Yeast--Amount depends on the time given the bread to rise, as follows:
12 hr. to rise 5 hr. to rise 3 hr. to rise 1/4 yeast cake 1/2 yeast cake 1 yeast cake
NOTE.--One cake of compressed yeast contains about the same number of yeast plants as one cake of dry yeast or one cup of liquid yeast.
Process in making bread:
(1) Mixing (stirring, beating, and kneading).-- (_a_) This mixes the ingredients. (_b_) It incorporates air to aid the yeast plant and to act as a lightening agent. (_c_) It makes the gluten elastic.