FERMENTATION 143 tion of bubbles of carbonic acid gas between the branches, and rises to the top of the liquid, and therefore is called top yeast (Oberhefe). It is the kind used in the fermentation of the wort of ale and strong spirituous liquors. It would appear, however, that the mode of propagation FIG. 1. Growing 1 yeast cells and most minute germs, magnified 2,000 diameters (Beale). FIG. 2. Yeast cells, grow- ing during 48 hours, magnified 250 diame- ters (Beale). does not instantly change in either variety upon a change of temperature. Thus top yeast placed in a fermentable liquid having a tem- perature of 45 is said not to pass into bottom yeast at once, as though a habit had been ac- quired; and conversely, bottom yeast will not de- velop as top yeast under a certain length of time. Yeast globules do not in- crease in number in pure sugar ^solutions, but the older globules waste away while the new buds grow at the expense of their FIG. 8. Growing yeast contents. To effect the %$ *FXk*% fermentation of 100 parts nified 1,300 diameters of sugar requires about (Beale). one part of yeast, weighed when dry. When the proportion of sugar is greater the excess remains unaltered, the cells will be ruptured, and the solution will be found to contain a certain quantity of lactate and acetate of am- monia, and other ammoniacal salts. When, however, instead of a pure sugar solution, a saccharine vegetable infusion, as sweet wort, is employed as the fermentable liquid, the yeast cells rapidly increase at the expense of the azotized matters which are present and which are essential as their nutriment. During the fermentation of beer they often increase to eight or ten times their original quantity. The fol- lowing table, according to Mitscherlich, gives the composition of yeast in its active and in its exhausted state, the amount of ash being de- ducted : CONSTITUENTS. Active cells. Spent cells. Carbon 47'0 47' 6 Hydrogen 6*6 7-2 Nitrogen 10-0 5-0 Oxygen 35' 8 Sulphur 0-6 The inorganic matter represented by the ash amounts to about 7*5 per cent, of the dried 317 VOL. vii. 10 yeast, and is composed, according to Mulder, entirely of phosphates of potash, soda, lime, and magnesia. 3. Acetic Fermentation. Liebig regarded the conversion of alcohol into acetic acid rather as a process of eremacausis, or slow oxidation, by which hydrogen was removed and oxygen substituted; but as the process is facili- tated by ferments, particularly by the my coder- ma aceti, it is generally regarded as a species of fermentation. Alcohol is readily oxidized by the influence of finely divided platinum into acetic acid, and also by binoxide of manganese and bichromate of potash. It is supposed that the reaction includes two stages : first the for- mation of aldehyde by the abstraction of two equivalents of hydrogen, water being at the same time formed ; and subsequently the ad- dition of one equivalent of oxygen, as repre- sented by the following equations : C 2 H 6 Alcohol. O = C 2 H 4 O 4- Aldehyde. H 2 O Water. C 2 H 4 .+ O = C 2 H 4 2 Aldehyde. Acetic acid. If the supply of oxygen be insufficient, much of the aldehyde remains unconverted into acetic acid, and on account of its great volatility may pass away in vapor. Pure diluted alcohol does not absorb oxygen from the air, but requires the presence of some inducing body which shall modify the atomic character of the oxy- gen, and also perhaps of that of the alcohol, so that the affinity of the constituent hydrogen and the atmospheric oxygen shall be increased. 4. Lactic Fermentation. When milk is left to stand for a time, the lactic sugar (CiaH^Oia) which it contains decomposes into lactic acid. The transformation is exceedingly simple, con- sisting merely in the splitting up of the mole- cules of sugar into a less complex arrangement, Oi 2 H 2 4Oi2 becoming 203HO 3 , or lactic acid. Oaseine while passing into a state of decay was formerly supposed to be the ferment which in- duced the process ; but according to Hallier and others, it consists of minute organisms which are developed from spores of penicillium crustaceum. (See figs. *4, 5, 6, 7.) The pro- cess is usually accompanied or immediately fol- lowed by the coagulation of the milk, an ac- tion which is generally ascribed to the ab- straction of the alkaline constituents of the caseine, which are supposed to hold it in so- lution; but it is asserted by some observers that coagulation of new milk by rennet often commences before any lactic acid makes its appearance. Another mode of producing lac- teous fermentation is by the employment of glucose. When a solution of glucose is mixed with new sour cheese, or with milk and chalk, and exposed to a temperature of 75 or 80 F. for some weeks, with frequent stirring, the sugar is converted into lactic acid, which when chalk is used combines with the base, forming lactate of lime. The chalk is used for the pur- pose of combining with the acid, the accumu-
