Yeast is a microscopic plant of fungus growth, a collection of living one-celled organisms that partakes of the nature of plant life.
How does it Grow? With proper warmth, moisture and food, the walls of these little one-celled plants bulge on the side in an oval shape. This bulge soon separates from the parent cell and becomes an independent organism. Other cells form in the same way from the parent cell, and also from each new cell, and thus the yeast plant multiplies.
What of Its Care? The little yeast cells are tenacious of life, and can live under most adverse circumstances. They are killed by exposure to heat above the boiling point of water, but they endure cold much better, being able to continue life in a suspended form at two degrees below freezing. From 65 to 72 degrees. Fahrenheit is most favorable to the growth of yeast. The best collection of yeast cells massed together in a dormant state, is the ordinary yeast cake, either dry or compressed.
How long does it Live? Yeast cells may be kept alive and vigorous for many days if kept in a dry and cool place.
What is its Relation to Flour and Bread? The yeast is softened in water to separate the yeast cells that they may be easily distributed through the flour.
In the starch and gluten of the flour, they find their food. Sugar hastens their growth, while salt retards it.
The yeast cells, finding their favorite food, begin to grow, changing the starch of the flour into sugar, and the sugar into carbonic gas and alcohol. The gas, in its efforts to escape, expands the elastic gluten of the dough in which it is mixed, and lifting up the mixture, the bread is "raised."
By subjecting the dough to heat (baking), the alcohol and carbonic gas are driven out and the cell-walls are fixed, and thus sweet bread is produced.
For reasons stated above, your flour and utensils should always be warm.
The life of the yeast is constantly in jeopardy, while afire to flour. If properly kept in a dry place. Improves its quality.