In the second place, lactic ferment is absolute poison for the undesirable bacteria, which may have developed, without injuring in any way the yeast cells proper, but rather has an influence for good toward them. Sulphuric acid is sometimes added to increase the acidity.
When the acidity reaches 2½ per cent. in the mash it is ready for further manipulation. Apparatuses to indicate the per cent. of acidity developed are used for the purpose of accuracy.
The acidulation of the mash having been satisfactorily completed, further operations are dependent upon the method selected to produce yeast. The older method is known as the “Vienna Process,” while the newer method is called “Aeration Process.”
The Older or Vienna Process.
Fermentation of the Mash.—At the completion of the acidulation of the mash it is at once cooled to 77 degrees F. This is accomplished by continuously agitating the mash by mechanical means with hollow plungers that are filled with ice or cold water, and which at the same time serves to aerate the mash.
The former method of cooling the mash in shallow vats, on account of infection and introduction by and of undesirable bacteria from the atmosphere into the mash, has been generally discarded.
Fermentation is now introduced by adding a certain quantity of compressed yeast, which must be free from starch adulteration.
In a short time a head begins to develop upon the surface of the mash, which gradually grows and rises to the top of the half-filled vats. The period of fermentation depends upon the temperature of the mash as well as the density of the mash.
The higher the density of the mash, the more vigorous the fermentation.
In general, the time consumed for proper fermentation is twelve to eighteen hours.