Evidently something changed some part of the apple or grape, the sugar, (C6H12O6), into alcohol, 2(C2H6O), and carbon dioxide, 2(CO2). This chemical process is known as fermentation.
Apparatus to show effect of fermentation. N, molasses, water and yeast plants; C, bubbles of carbon dioxide.
Yeast causes Fermentation.—Let us now take a compressed yeast cake, shake up a small portion of it in a solution of molasses and water, and fill a fermentation tube with the mixture. Leave the tube in a warm place overnight. In the morning a gas will be found to have been collected in the closed end of the tube (see Figure on page [138]). The taste and odor of the liquid shows alcohol to be present, and the gas, if tested, is proven carbon dioxide. Evidently yeast causes fermentation.
What are Yeasts?—If now part of the liquid from the fermentation tube which contains the settlings be drawn off, a drop placed on a slide and a little weak iodine added and the mixture examined under the compound microscope, two kinds of structures will be found (see Figure below), starch grains which are stained deep blue, and other smaller ovoid structures of a brownish yellow color. The latter are yeast plants.
Yeast and starch grains. Notice that the starch grains around which are clustered yeast cells have been rounded off by the yeast plants. How do you account for this?
Size and Shape, Manner of Growth, etc.—The common compressed yeast cake contains millions of these tiny plants. In its simplest form a yeast plant is a single cell. The shape of such a plant is ovoid, each cell showing under the microscope the granular appearance of the protoplasm of which it is formed. Look for tiny clear areas in the cells; these are vacuoles, or spaces filled with fluid. The nucleus is hard to find in a yeast cell. Many of the cells seem to have others attached to them, sometimes there being several in a row. Yeast cells reproduce very rapidly by a process of budding, a part of the parent cell forming one or more smaller daughter cells which eventually become free from the parent.
Conditions favorable to growth of Yeast.—Experiment.—Label three pint fruit jars A, B, and C. Add one fourth of a compressed yeast cake to two cups of water containing two tablespoonfuls of molasses or sugar. Stir the mixture well and divide it into three equal parts and pour them into the jars. Place covers on the jars. Put jar A in the ice box on the ice, and jar B over the kitchen stove or near a radiator; pour the contents of jar C into a small pan and boil for a few minutes. Pour back into C, cover and place it next to B. After forty-eight hours, look to see if any bubbles have made their appearance in any of the jars. If the experiment has been successful, only jar B will show bubbles. After bubbles have begun to appear at the surface, the fluid in jar B will be found to have a sour taste and will smell unpleasantly. The gas which rises to the surface, if collected and tested, will be found to be carbon dioxide. The contents of jar B have fermented. Evidently, the growth of yeast will take place only under conditions of moderate warmth and moisture.
Carbohydrates necessary to Fermentation.—Sugar must be present in order for fermentation to take place. The wild yeasts cause fermentation of the apple or grape juice because they live on the skin of the apple or grape. Various peoples recognize this when they collect the juice of certain fruits and, exposing it to the air, allow it to ferment. Such is the saki or rice wine of the Japanese, the tuba or sap of the coconut palm of the Filipinos and the pulqué of the Mexicans.