This section is from the book "A Book Of Recipes For The Cooking School", by Carrie Alberta Lyford. Also available from Amazon: A book of recipes for the cooking school.
Yeast bread or "light bread" is made by using yeast to produce the gas necessary for making the bread light.
Yeast is a kind of plant without leaves, stems, or blossoms, microscopic in size but larger than bacteria. It consists of a single egg-shaped cell which splits in the center to form new plants or sends out little protuberances from the ends which break off as little buds to form new plants. The yeast plant multiplies very rapidly, many hundreds of cells being produced in a few minutes.
Yeast plants are present in the air and found in the form of "bloom" on the surface of grapes, plums, and other fruits.
The yeast plant needs moisture and heat for growth. The most favorable temperature is from 65 degrees to 90 degrees F. Above 139 degrees the activities of yeast become arrested and at 212 degrees the plant is killed. A low temperature renders yeast inactive and at 32 degrees it may become ruptured.
For food the yeast plant requires some nitrogenous and sugary material both of which it finds in the bread dough, for it acts together with the ferments in wheat in changing the starch to sugar.
As yeast grows it breaks sugar up into alcohol and carbon-dioxide, a process known as alcoholic fermentation. When yeast has grown in dough long enough to produce alcohol and carbon-dioxide in large quantities it is necessary to check this action by kneading the dough and finally by baking it as another form of fermentation is likely to set in and make the dough sour by the production of an acid. This is known as acetic fermentation and must not be allowed to begin in bread as it changes the flavor. Lactic acid fermentation may take place in bread if the milk used is not first scalded.
Yeast produces a bread which is more wholesome and more porous than baking powder breads and which has a special flavor because some of the starch has been converted into sugar. In yeast breads there remain no harmful products because the alcohol and carbon-dioxide are both entirely given off during the baking.
Commercial yeast is grown in a mixture usually prepared of barley and water. The yeast plants are collected, thoroughly washed, combined with fine starch, and pressed into small cakes which are wrapped in tin foil. This is known as "compressed yeast." There is not enough moisture retained for the yeast to grow; if it is kept cold it can be kept fresh for several days. Compressed yeast should have a pleasant, alcoholic odor, be a light tan in color and not be marked by dark streaks or mould.
"Dry yeast" or "yeast foam" is combined with some starchy substance and is so free from moisture that it will keep in any temperature. It is cheaper than compressed yeast but longer time for rising is necessary if dry yeast is used.
Liquid yeasts are grown in a liquid solution prepared from flour, warm water, and hops. They are not usually handled commercially.
Leaven is a portion of dough kept to be used as a starter in the next rising. It must be kept cold. That is the old-fashioned method of starting bread.
Only yeast that is fresh and in good condition should be used in making bread. Use 1/2 cake compressed yeast for 1 quart liquid if the bread is set at night; 2 cakes if set in the morning. Soften 1 yeast cake in 1/4 cup lukewarm water. One cake of dry yeast is sufficient for one quart of liquid if it is to rise over night.