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Bread Making: Yeast

Yeast.

The use of yeast as a leavening agent is thousands of years old. A microscopic examination of some bread made over 4400 years ago in Egypt showed the presence of dead yeast cells. Single Yeast Cells.

Structure of Yeast.

Budding Yeast Cells.

Yeast Spores.

Yeasts are tiny plants so small that when separated, they cannot be seen except by the aid of a microscope. Each one of these plants consists of a single round or oval cell with a transparent wall. This cell is filled with liquid and granular cell substance called protoplasm.

Under proper conditions a bud will grow to the size of the original cell then separate and in turn send out buds. Again the cells may remain attached to each other until a chain of four or five cells is formed.

The conditions necessary to the growth of the yeast plant are air, warmth, water, some form of sugar and some food from which it can obtain nitrogen. Starch will be changed into sugar for the use of the yeast plant if there is no usable form of sugar present.

If the yeast plant has to go for a long time without food and water, it will sometimes divide into spores (a kind of seed). The outside wall will then break down and these spores will float around in the air until they are given the right conditions when they will commence to grow the same as the original yeast cell. These spores can stand more unfavorable conditions without being harmed than the yeast cell. Method by Which Yeast Raises Dough.

As the yeast plant grows it changes the sugar into alcohol and into a gas called carbon dioxide gas. When these bubbles of gas are given off inside the bread dough, they are caught and held by the rubber-like gluten in the flour and in their efforts to escape push the dough up and cause the bread to rise and become light and porous. This process of changing sugar into alcohol and carbon dioxide gas is called fermentation. Effect of Heat on the Yeast Plant.

If any plant is to grow well it must receive the right amount of heat as well as the proper soil. Bearing in mind the fact that the faster the yeast plant grows the more bubbles of gas it will give off, the results of the following experiments will show at what temperature the yeast plant thrives best.

Experiments.

Put 1 t. molasses into each of three glasses. Soften a cake of compressed yeast with 2 T. of water. Put one-third of the softened yeast into each glass.