*"'Fermentation' may be defined as a generic term applied to that group of chemical changes which are consequent on and inseparable from the life and development of certain microscopic organisms."
There are many kinds of fermentation, a few of which are enumerated: Alcoholic fermentation is used in bread making. Its products are alcohol and carbon dioxide. Viscous or ropy fermentation produces a ropy condition, and is probably the cause of the ropy bread sometimes seen. A familiar example of lactic fermentation is seen in the souring of milk. Its most important product in that case is lactic acid. Acetous fermentation is seen in the change of fruit juice into vinegar; the sugar of the liquid being changed so that acetic acid is formed. In putrefactive fermentation, offensive products are formed. The bread maker is especially interested in alcoholic fermentation and the things which accelerate, retard, or injure its action in giving best results.
Yeast is regarded as a microscopic plant. It is generally believed that, in order that yeast be most healthy, it must have access to light and air, in order that it have the requisite amount of oxygen.
A noted English authority on bread making has made experiments to prove this theory. He says: *"The experiments prove very clearly that the agitation has resulted in the yeast being in every instance more vigorous in action. In case of the spontaneous ferment, there was a distinct, though slow, evolution of gas. The sample pitched with pressed yeast had more than twice the capacity for causing the evolution of gas than had those which were pitched with brewer's yeast. It is plain that agitation in some way increases the vigor of yeast." It is found that yeast grows better, also, when a large surface is exposed to the air.
Yeast grows most rapidly between the temperatures of 25 degrees C. to 35 degrees C, which equals 77 degrees F. to 95 degrees F. Yeast ceases to grow at a little less than 50 degrees F., but is not always killed even by freezing. Freezing, however, is apt to injure it. Although yeast grows rapidly at 95 degrees F. (a temperature at which the liquid merely feels warm to the hand, as the human temperature is about 90 degrees F.), the temperature employed in English breweries is said to be from 65 degrees F. to 72 degrees F.
Let us ascertain why the brewer does not hasten the process of fermentation. From numerous recent researches, there is evidence of a number of organisms which possess the power of producing lactic acid by the conversion of glucose. One or more of these is always present in commercial yeasts. Lactic fermentation proceeds most favorably at a temperature of about 35 degrees C. and is retarded and practically arrested at a temperature which still permits the growth and development of the yeast organism. The other bacterial and allied ferments are also affected in a similar manner by temperature. Hence 75 degrees F. to 77 degrees F., which allows the yeast to grow well, but retards the growth of foreign ferments, is far better than a higher temperature.