The yeast-plant, to which various names have been given, as torula cerevisiae, saccharomyces, consists of ovoid cells, which multiply by budding. The buds may remain attached, forming torula-chains, but when they attain the size of the parent cell they fall off and begin to multiply anew. When placed in saccharine solutions the plant, during the process of growth, decomposes the sugar and forms alcohol and carbonic acid.

In this process oxygen is usually absorbed from the air in considerable quantities, but fermentation can occur in saccharine solutions even when oxygen is excluded, though under such conditions the torula grows slowly. When plenty of oxygen is present, and the layer of fluid shallow, the torula grows luxuriantly, but there is very little fermentative change; while, on the other hand, when free oxygen is excluded the torula grows slowly, but there is marked fermentation.

Another plant nearly allied to yeast is the mycoclerma vini, the ferment which changes alcohol into acetic acid. The myco-derma is not regarded by Naegeli as a species distinct from torula, and it is considered by Grawitz to be the same as the fungus found in the aphthous patches which occur about the mouth and throat of children suffering from thrush, although this fungus is usually said to be an oidium.

To test the action of drugs on alcoholic fermentation, equal quantities of a solution of grape sugar with yeast are introduced into two test-tubes, and to one of them a little of the substance to be tried is added. These are then inverted over mercury and kept in a warm place for several days. The amount of gas developed is then measured, and the power of the drug to prevent fermentation is estimated by the diminution in the amount of carbonic acid produced, as compared with the standard.