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