Obtaining the Yeast.—Fermentation of the “mash” or the “worts” proceeds at a lively rate. In observing the “head” or froth, during the “Vienna Process,” which at first is transparent, gradually assumes a milky or of more opaque appearance, caused by enormous increasing growth of yeast cells, filling up the froth bubbles. When the cells are fully developed the fermentation may be considered finished. Practice assumes, although the assumption is not always reliable, that this stage has been reached when the “head” or froth begins to recede. The only sure method to determine proper maturity of the yeast cell is by microscopic observation.
Placing some of the froth under the object glass of the microscope, the yeast cells most appear well developed and isolated from each other. It should be the exception rather than the rule that budding cells still be visible.
Not until assured that the proper time has been reached should the skimming of the upper portion of the froth be begun. This portion of the “head” contains the so-called “pitching yeast,” and is used largely in starting new propagation.
Large galvanized perforated spoons with long handles are used to skim off the froth. Repeated observations of the froth during the skimming are made, to ascertain the condition of the yeast cells.
The yeasty froth is immediately mixed with ice cold water to arrest further fermentation. This also serves to increase the keeping properties of the yeast.
The water containing the skimmed-off matter is now run through strainers of varying sized meshes, the coarser retaining the husks and bran, while the finer meshes prevent the gummy matter adhering to the yeast cells from passing.
The strained yeast cells are caught up in vessels containing water, where they precipitate in a compact layer, and is then ready to be washed.
In order to watch the settling of the yeast, these vessels are constructed with windows so as to give the operator a perfect vision of the settling.
This operation of washing the yeast in new water and allowing it to settle is repeated several times, at which time nearly all of the impurities have been removed and excellent keeping properties have been attained.