While it is difficult to separate the various species of yeast cells, the phenomena of spore formation has led the way to accomplish it.
At a temperature of 54 degrees F., Sacchoromyces Cerevisial will show ascospore formation in 200 hours, while Sacchoromyces Pastorianus at the same temperature forms spores already in 77 hours. This difference in time of the maturing of the spore formation of the various species of yeast being known, is utilized in transferring the spore of any specific species upon culture plates of nutrient gelatine, upon which the spores develop into little colonies of yeast cells.
The healthiest and strongest appearing cell is then cut out with a sterilized platinum wire and transferred into a flask of sterilized malt wort, and the reproduction from a single cell of any given species is begun. In this manner pure yeast culture is accomplished.
In the fermenting vats growing yeasts are often contaminated by spores of undesirable species from the atmosphere, and result in producing conditions unfavorable for the purposes desired. In such cases we must resort to a pure yeast culture to re-establish the desired fermentation.
_Manufacture of Compressed Yeast._—Compressed yeast is the result of alcoholic fermentation of malt and grain worts. As it is of material interest to the baker to acquaint himself with a general knowledge regarding the manufacture of compressed yeast, a short but clear description is given below.
_Treating the Grain._—Malt is produced by soaking barley or other grains in water and spreading in thin layers on the floors of the malting rooms. Being moist and in consequence supplied with artificial heat, the grains begin to sprout. As the rootlets grow in size a product is being formed in the germ that has the power to convert starch into sugar. This product is called Diastase. This reaction is still clouded with a good deal of mystery, and it has as yet never been clearly defined.
We know this much, however, that some parts of the nitrogenous matter of grains are chemically changed into Diastase.
Practice teaches the maltster, by the size the rootlets attain, when the maximum diastasic strength of the malt has been reached.
The sprouting of the malt is now arrested by drying the malt in kilns at a temperature of 131 to 176 degrees F., which evaporates the moisture and kills further germination.