Yeast, the froth which rises upon the surface of beer and other liquors during fermentation, consisting principally of microscopic globules of a fungoid plant. This plant is also found in that variety of yeast which is developed in sedimentary fermentation. (See Brewing.) The history of this plant begins with its discovery in beer by Leeuwenhoek in 1680 by microscopical examination. Fabroni in 1787 regarded yeast as a vegeto-animal substance residing in peculiar utricles in grapes as well as in corn, but does not seem to have attached great importance to the existence of the yeast globules discovered by Leeuwenhoek. Thenard in 1803 recognized a relation between yeast cells and fermentation, but most of the chemists of that day who investigated the subject of fermentation seem to have regarded the functions of yeast as having more of a chemical than a physiological nature. In 1825 Desmazieres found organisms in yeast which he regarded as animals. It was not till about 1837 that Cagniard de la Tour took up the microscopical observations of Leeuwenhoek, and, as has been said, "rediscovered the yeast plant." He declared that by its influence the equilibrium of the molecules of sugar was broken up, and measured the diameter of the cells, which he placed at 1/2500 an inch? and also observed that they developed by budding.

Schwann of Jena made independently, about the same time, similar discoveries. Their observations were confirmed by Quevenne, Mitscherlich, and Turpin; the last placed the organism in the genus torula of Persoon, and this classification has been recognized until very recently. Yeast has also received the name of mycoderma vini. The torula has a mycelium, and it is held that ferments never have. Meyen, considering yeast to be a fungus, created a new genus for it under the name of saccharomyces. Kützing and others placed it among algae, and in a separate genus called cryptococcus. Whether yeast is the cause or the effect, or simply an accompaniment of fermentation, has long been disputed, and it has not been positively decided that its presence is necessary for the commencement of the process of vinous fermentation; but the great weight of opinion leans toward the affirmative. (See Fermentation.) The most prominent advocate of this theory is Pasteur, who has made numerous elaborate experiments, not only to elucidate the nature of yeast, but to oppose the theory of spontaneous generation. Reess and others have divided the yeast genus of fungi into several species.

Of these, saccharomyces cerevisiai, or the yeast of beer, is again divided into two varieties, sedimentary or bottom and surface yeast; but the drawings of them have much resemblance, and in fact it is known that the one variety is readily convertible into the other by cultivation. The sedimentary yeast is developed at a considerably lower temperature than surface yeast, and with much less evolution of carbonic acid gas. This want of buoyant gas is the cause of its settling to the bottom, and there, in consequence of less exposure to the air, it becomes less active; but its activity can be readily restored by raising the temperature of the fermenting mass, in which case, after a few fermentations, the sedimentary is converted into surface yeast. The cells of saccharomyees cerevisioe are round or oval and from 00031 to 00035 of an inch in diameter. (See figs. 1 and 2.) The cell wall is an elastic membrane of colorless cellulose, with colorless protoplasm, which often contains small granules, and one or two vacuoles containing cellular juice.

When the cells are not undergoing development they are usually separate; but when the yeast is forming, its method of growth causes the cells to be joined to one another in pairs, groups, or chains, the latter being more particularly the case in the rapid development of surface yeast. During fermentation, or the development of yeast, there is an elevation of temperature, probably due to the combustion of oxygen, which may be obtained from the air or from the decomposition of sugar in the fermentable liquid. In fact the respiration or consumption of oxygen by the yeast cells bears some comparison to the respiration of animal pulmonary tissue. The multiplication of the cells of saccharomyees when in contact with an appropriate fermentable liquid is by budding, but under other circumstances, as has been shown by Keess and others it may multiply by means of spores. (See Schützenberger "On Fermentation," New York, 187G, pp. 49, 50.) S. ellipsoideus is Pasteur's ordinary alcoholic ferment of wine. The adult cells have an ellipsoidal form, being about 00024 in. in length by -000176 in. in breadth. (See fig. 3.) The multiplication by budding and by spores does not differ from that of S. cerevisioe.

S. exiguus, fig. 4, according to Reess, has cells of only 000098 in. in breadth by 000118 in. in length, and multiplies like the other varieties. S. conglomeratus, fig. 5, is rare, and is found mostly in the must of wine toward the end of fermentation. It has spheroidal cells 000236 in. in diameter, conglomerated together; the cells, springing from buds, do not become detached from the parent cell until they have attained the same size. S. apiculatus, fig. 6, is the most abundant alcoholic ferment, and is found on the surface of all kinds of fruit, especially on berries and stone fruits. It has been found in certain kinds of beer, as that of Belgium, which undergoes spontaneous fermentation, yeast not being added to the wort. According to Engel, this species does not belong to the genus saccharomyees, and he calls it carpozyma apiculata. The greater diameter of the cells is about 000236 in. S. Pastorianus, fig. 7, is a species which appears in the after fermentation of wines, especially of sweet wines, and those of other fruits than the grape. The cells are oval, pyriform, or clubshaped, and vary in dimensions from 000236 to 00078 of an inch. 8. Reesii, fig. 8, accompanies S. ellipsoideus in the must of red wines. It has elongated cylindrical cells.

S. my coderma (mycoderma vini) is shown in fig. 9. The mucor mucedo and M. racemosus, fig. 10, have the property, when placed in a solution of sugar and protected from access of oxygen, of transforming or dividing their mycelium into joints having the form of balls, which latter multiply by budding. "This fact," Schiitzenberger remarks, "which is indisputably proved, gives considerable support to the theories brought forward by some men of science as to the transformation of ferments, from one to another, according to the conditions under which they are placed." The classification of Reess as given above is not accepted on all hands, for the chief reason just given, that it has been observed that yeast fungi appear to have the property of changing into a variety of forms. Thus Dr. W. B. Carpenter ("The Microscope and its Revelations," London, 1874) says: "It would appear that yeast may be produced by sowing in a liquid favorable to its development the sporules of any of the ordinary moulds, such as penicillium glaucum, mucor, or aspergillm, provided the temperature be kept up to blood heat; and this even though the solution has been previously heated to 284° F., a temperature which must kill any germs it may itself contain." Prof. J. Oienkowski has made a series of experiments on the development of rnycoderma vini, in which he finds that the white pellicle which forms on the surface of various organic fluids, as urine, beer, milk, fruit juice, and cucumber juice, consists principally of two ingredients, rnycoderma vini and oidium lactis, the special ferment of milk. (See " Quarterly Journal of Microscopical Science," April, 1875.) - The chemical composition of yeast is remarkable from its large amount of nitrogen.

Careful analyses by Schlossberger give the following for the two varieties of beer yeast:

Saccharomyces cerevisisB   Yeast of sedimentary beer, budding, magnified 400 diameters.

Fig. 1. - Saccharomyces cerevisisB - Yeast of sedimentary beer, budding, magnified 400 diameters.

Saccharomyees cerevisiae   Yeast of surface beer, budding, magnified 400 diameters.

Fig. 2. - Saccharomyees cerevisiae - Yeast of surface beer, budding, magnified 400 diameters.

Saccharomyees ellipsoideus, in process of budding, magnified 600 diameters.

Fig. 3. - Saccharomyees ellipsoideus, in process of budding, magnified 600 diameters.

Saccharomyces exiguus, magnified 350 diameters.

Fig. 4. - Saccharomyces exiguus, magnified 350 diameters.

Saccharomyees con , glomeratus, magnified 600 diameters.

Fig. 5. - Saccharomyees con-, glomeratus, magnified 600 diameters.

Saccharomyces apiculatus, magnified 600 diameters.

Fig. 6. - Saccharomyces apiculatus, magnified 600 diameters.

Saccharomyces Pastorianus, alcoholic ferment of wine, magnified 400 diameters.

Fig. 7. - Saccharomyces Pastorianus, alcoholic ferment of wine, magnified 400 diameters.

Saccharomyces Reesii, ferment of red wine, magnified 850 diameters.

Fig. 8. - Saccharomyces Reesii, ferment of red wine, magnified 850 diameters.

Saccharomyces rnycoderma, magnified 250 diameters.

Fig. 9. - Saccharomyces rnycoderma, magnified 250 diameters.

Mucor racemosus, ferment in mass.

Fig. 10. - Mucor racemosus, ferment in mass.

CONSTITUENTS.

Surface yeast.

Bottom yeast.

Carbon

49.9

48.0

Hydrogen

6.6

6.5

Nitrogen

12.1

9.8

Oxygen

31.4

35.7

Ashes...................

2.5

3.5

An analysis by Mulder of the organic elements without the ashes gives a composition nearly allied to albumen: carbon, 53.3; hydrogen, 7.0; nitrogen, 16.0. There are probably, therefore, one or more albuminoid substances in the yeast cell, in which it resembles other vegetable cells. An analysis by Schlossberger, in which he treated the yeast with a weak solution of potash, did not give a result which so nearly agreed with albumen, but he obtained a residuum which when dissolved in acetic acid showed a composition allied to that of cellulose: carbon, 44.9; hydrogen, 6.7; nitrogen, 0.5; remaining ashes, 1.1. Mitscherlich says the ashes of beer yeast are thus composed:

CONSTITUENTS.

Surface yeast.

Bottom yeast.

Phosphoric acid.........

53.9

59.4

Potassa .................

38.8

28.3

Magnesia

60

8.1

Lime...................

1.0

4.3

Silica

traces

....

In this it is seen that the chief constituents are phosphoric acid and potash, and a calculation of the state in which all the elements are combined may be made as follows:

CONSTITUENTS.

Surface yeast.

Bottom yeast.

Phosphoric acid

41.8

39.5

Potassa

39.8

28.3

Magnesian phosphate....

16.8

22.6

Calcium phosphate

2.3

9.7

These analyses have a strong resemblance, particularly in albuminoid elements, to those obtained with mushrooms and other fungi. The elaborate experiments of Boussingault show that ordinary plants have the power to eliminate nitrogen from its saline compounds, the nitrates, and the question has arisen whether yeast has the same power. The experiments of Dubrunfaut lead to the affirmative, while those of Ad. Mayer give a negative indication. The experiments of Pasteur, in which he supplied the growing yeast with a solution of pure sugar, to which were added ammonium tartrate and the ashes of yeast (containing phosphates), go to show that the ammonium salt slowly yields its nitrogen, which is transformed into albuminoid matter, while the phosphates contained in the ashes furnish mineral matter to the. new plant. But according to the observations of M. Cloëz it is possible that ammoniacal salts are gradually transformed, before the nitrogen is appropriated, into nitrates; this idea agrees with the ordinary phenomena of nitrification, and it has been found that, although yeast may decompose ammoniacal compounds, its own more natural nitrogenous aliment is contained in the juices of plants.

M. Pasteur maintains the absolute dependence of the development of yeast upon the presence of alkaline phosphates; but the statement was disputed by Liebig, who contended that other conditions of M. Pasteur's experiments prevented development.