Yeast. A large number of vegetable fungi are classified as yeasts, all of them small and microscopic organisms. The best known is that which is distinguished as Saccharomycetes cerevisiae. It forms small rounded or elliptic cells which multiply in saccharine and other solutions by a process of budding, a small cell forming on the side of the parent, growing, and finally becoming detached. Under the influence of the yeast saccharine liquids undergo fermentation (q.v.), by which they decompose with the formation of alcohol and carbonic acid, together with smaller quantities of other products. This fermentation is that on which the preparation of almost all our fermented and spirituous liquors depends, as also the use of yeast in bread making. During the fermentation the yeast cells multiply rapidly and rise to the top of the vat or containing vessel as a scum, which is skimmed off and known as yeast or barm. This is again used to induce fermentation in fresh liquors. The yeast cells multiply most rapidly, and hence induce fermentation most speedily, if the temperature is about 25° C. (or at 77° F.), and at about 40° C. almost entirely lose this power, being killed by high temperatures.