In the former case a new cell is detached from the parent cell, while in the latter the new cells (spores) arc formed inside the parent which it leaves only upon complete maturity Several recent investigators are inclined to be lieve in the sexuality of yeast, and dose research with various yeasts, as Sacch. farinosus, Sacch. Boilit, etc., has been made.
Two types of yeasts have in the course of time become culture plants in the real sense of the word, through intentional continuous culti vation for certain definite purposes. These types are (1) that known as the bottom yeast in the manufacture of beer, and (2) that which is the cause of the fermentation of the top-fer mented beers and of the whisky mash. The former is so called because it settles in a thick, creamy layer upon the bottom• of the fermenting vat during the slow fermentation of lager beers in temperatures of 40° to 50° F. to C.), while the latter is termed top yeast because in the more rapid fermentation at 54° to 65° F. (12° to 18° C.) (ale, weissbeer, etc.), it is sep arated as a yellowish brown foam, which is forced to the surface by the escaping carbonic acid gas. It is the last-named species which furnishes almost exclusively the material for the manufacture of that product known as com pressed yeast.
As has been said, there are a number of inde pendent species of yeast, which vary not only according to form and size of the cell, but also according to the composition of the nutrient medium in which they live, hence according to chemical and physical properties. Yeast flour ishes luxuriantly only then when it finds not only qualitatively but also quantitatively, in the nutrient medium all the substances necessary for its nutrition. The brewer, distiller, etc., therefore endeavors to produce as vigorous a yeast as possible, in order to be enabled to con duct the fermentation in the manner best suiting his purposes. For such normal nourishment, it is essential that diverse substances are at its disposal.
Yeast thrives well when, besides water car bon in the form of sugar, nitrogen in albumi nous compounds or ammoniacal salts and vari ous mineral components, especially sulphate of magnesia and potassium phosphate, ace at its disposal. If the necessary nutriment is lacking the vitality of the yeast cell may rest, but the cell does not perish.
Various analytical data are at hand in regard to the composition of the yeast. Dumas found
that the yeast contains: The differences found between bottom and top yeasts have been very slight. Wagner ex perimented upon top and bottom yeasts and obtained the following averages: A yeast may have all the necessary organic substances in the proper form and amount, yet it will not flourish if the salts are lacking. The yeast in this case is the same as any other plant organism, which must perish if it is deprived of its mineral nourishment. Hence a knowl edge of the requirements of mineral substances forayeast is of utmost importance, and many examinations were made to secure data on this point. Mitscherlich found in the ash of The most essential substances which the yeast requires for its sustenance are, accord ingly, phosphate of potassium and phosphate of magnesia. The above data are from elementary analyses. Naegeli later instituted investigations which should show the amount of the various combinations, which are observed in a yeast. According to his observations the yeast, when treated with water until its soluble components are absorbed, gives up to the water 37 per cent of the total weight of the dried yeast. The bodies thus brought in solution consist of com pounds closely related to vegetable slime. The amount of moisture in a yeast is exceptionally large, for if it is completely dried it loses 83 per cent of its weight, so that in 100 parts of yeast only 17 parts are solid components. The mucous part of the yeast cell, the protoplasm, contains more water than the membrane which surrounds it, and consists chiefly of albumin oids, and about 2 per cent of peptone. Accord ingly the composition of the yeast with 8 per cent nitrogen is : Per cent According to E. C. Hansen of Copenhagen, yeast is divided into two groups, the real Sac charomycetes, which are capable of forming endospores, and the non-saccharomycetes, which never form spores. He subdivides the former into such which separate out sucrase and engen der alcoholic fermentation, that is, which vig orously ferment saccharose, dextrose and maltose, as brewer's yeast; and such which fer ment sacchraose, dextrose levulose but not maltose as, for example, Saccharomyces Lud wigii; Sacch. exiguus, and into such which do not separate out sucrose and do not excite alco holic fermentation, as Saccharomyces men:bra nerfaciens.