Both are very much alike in appearance, both incite alcoholic fermentation, but develop in a similar wort a number of widely different by-products, the analyses of which have thus far baffled the resources of the chemist. The action of these two species is readily recognized by the flavor and taste imparted to the fermented medium.
As the bouquet imparted to wine is attributed to the wine yeast cell (Sacchoromyces Ellipsoideus), characteristic of the grape juice, so the baker recognizes by the flavor of his baked product that the proper species of yeast has been employed, irrespective of the flavor which may have been obtained by other materials used in the baking.
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.