YEAST. The botanist and microbiologist apply the term yeast to a group of plants many of which exhibit a marked ability to change sugar into alcohol and carbon dioxide. The characteristics of the group, which includes hundreds of species, are quite restricted. These, together with discussion of taxonomy, are given under FUNGI and FERMENTATION. To the layman, un trained in botany, the term yeast suggests the cakes of pressed yeast available in almost every hamlet. The ability of certain yeasts to form carbon dioxide from sugar has caused some of them to be used for leavening bread. For centuries, other species have been used in the making of wine, alcohol, beer, etc.
Yeasts probably have as early origin as the bacteria. Griiss examined some fossil remains of Devonian plants and obtained striking evidence of the existence of budding fungi in this early age. This same investigator on examination of the sediment from a beer jar in Theban tomb of the XIth dynasty (2000 B.c.) iso lated a yeast which was named Saccharomyces Winlocki. Examin ation of "beer bread" found among the offerings in other tombs also yielded the same yeast. It is now known that yeasts are widely distributed in nature and that those species concerned in fermentation pass the winter in the soil. They are disseminated by bees, dust and other agents in the spring.
The use of yeast in such fermentations as that of bread has made it convenient to have a constant supply of fresh active yeast. While in former days the by-product of certain fermentation in dustries was used, pressed (or compressed) yeast is now available to those who desire it. To this end the organism is grown in suit able media and the crop harvested when a sufficient • crop of cells has appeared. The medium, according to an older method, con
sists of wort prepared from grains mashed in water. The mash prepared from grains is inoculated with lactic acid bacteria to "sour" it; the acid prevents putrefaction and also serves as food for the yeasts. The clear wort is passed into fermenters where it receives the seed yeast. The temperature is kept constant and rapid growth takes place. The yeast cells are then separated from the fluid in which they have grown by filter presses. They are mixed with starch and pressed into large cakes. These are sent to distributing centres, where they are cut and wrapped in the small size package commonly used in the home. In more recent times, yeast has been cultivated in mineral salt-sugar solu tions instead of the wort described above. The cells• are also in corporated in corn meal which is pressed into cakes.
Besides the application of yeasts in fermentology, they have been widely heralded as therapeutic agents. Their application to the cure of disease goes• back to very early times. Many of the statements on the use of yeasts in this manner rest upon uncon trolled experiments, if indeed they may be called experiments. We are told that the monks used yeast for curing plague and that Hippocrates advised its use in leucorrhea. Since 1917 great in terest was aroused by a publication of Hawk, et al. who reported beneficial results in furunculosis, acne vulgaris, constipation and certain other gastrointestinal and cutaneous diseases.