As fermentation proceeds, the density of the mash becomes less, while the yeast cells increase, and at the same time the temperature of the mash raises.
Fig. 8.
Yeast Cells Fully Developed.
The mash in this process contains the whole of the grains, and for this reason the head, which contains the yeast cells, and which is skimmed off as it rises, must be strained; it is subsequently washed and then pressed. In contrast to this method the newer or “Aeration Process” for the production of yeast presents entirely different phases.
Production of Yeast by Aeration Process.
This method was invented in Sweden about ten years ago, and is in use in many yeast factories to-day. A decided greater percentage of yeast yield is accomplished by the “Aeration Process.”
After the saccharification of the mash is completed, the extract called “wort” is strained to remove the husks and bran of the grains. Large vats containing a double bottom are used for this purpose, the inner or upper bottom being perforated. Spigots are attached to the bottom of the vats to draw off the “wort.” At first the extract appears opaque and is again returned to the mash. This pouring-back process is continued until the “wort” finally flows perfectly clear from the spigots. The extractive matter still adhering to the husks and bran of the grains is washed out or “sparged” with hot water.
Another way employed for recovering the clear “wort” is by means of the filter press. The percolation method, however, is preferable, as the extraction of the essential properties is more complete.
Fermentation is produced in the “wort” by adding small quantities of compressed yeast also, or by the use of pitching yeast. During the fermenting period a continuous stream of atmospheric air is forced through the “wort” by the aid of air pumps. In order to eliminate atmospheric dust and bacteria, the air before entering the “wort” filtered through cotton, and sterilized by passing it through a solution of salicylic acid. It is also necessary to distribute the air to all parts of the “wort” equally, by means of perfected tubes, which are attached to the main air pipe, branching out in various directions at the bottom of the fermenting vats, with the perforations facing downward. At the beginning the air current is very moderate, and is increased in accordance and in proportion of the yeast growth. At the final stages of fermentation the air current is again moderated. This forcing in of air, or rather oxygen, in the “wort” stimulates in an exceedingly large measure the propagation of yeast, but care is exercised in this respect, however, for if the air pressure be too strong a large per cent. of alcohol, a very important by-product, will be lost. The characteristic feature of this method, distinguishing it from the Vienna Process, is the continued aeration during fermentation, hence called “Aeration Process.”