Book of American Baking / A Practical Guide Covering Various Branches of the Baking Industry, Including Cakes, Buns, and Pastry, Bread Making, Pie Baking, Etc.

by Various

This is caused by a combination of the dissolved lime and the gas. The latter, which we have developed, is Carbon Dioxide (CO₂).

By the above tests we find that Carbon Dioxide is heavier than the atmosphere, and thus remains in the bottle undisturbed by atmospheric pressure, for a time, as it combines very slowly with the air. By creating a draught, the combining with the air is facilitated.

Top and Bottom Yeast.

Remove with a glass rod, during active fermentation, a little of the froth from the saccharine infusion and wash it off with a few drops of clean water, and place a small drop of the solution upon the object glass of the microscope. At 300 to 500 magnified strength we notice that the froth consists of small round or oval bubbles, which occasionally, though seldom, are elongated. They appear singly or in groups, and often look like strings of pearls. These bubbles are yeast cells. Figure 5 shows yeast cells after five hours of propagation. Each cell has a thin covering of fibrin or cellulose; while the interior contains a soft granular albuminous substance, called Plasma, or Protoplasma.

During vigorous fermentation at a temperature of 68 to 80 degrees F., the majority of the bubbles are forced to the surface of the fluid by the action of the escaping Carbon Dioxide, and at the final stages of fermentation gradually precipitate. At a temperature of 36 to 45 degrees F. the fermentation is slower, the generating Carbon Dioxide is less active in escaping and without sufficient force to bring the bubbles to the surface. The yeast cells to a great extent grow and settle on the bottom of the generating vessel.

These characteristics designate top and bottom yeast.

Fig. 5.

Both of these yeasts are of the same species, and either can be converted into the other by the changing of the temperature during propagation. They are recognizable by a slight difference in size. Owing to the more favorable conditions during growth top yeast is somewhat better developed than bottom yeast. Compressed yeast used in bread baking is top yeast.