MICROBES, MOULDS, AND YEAST. 213 



proteid (that is, it contains Carbon, Hydrogen, Oxyofen, 

 Nitrogen, Sulphur, and Phosphorus). It may be 

 extracted from the yeast cells by crushing these in 

 diatom earth, and by filtration and pressure. When 

 extracted in this manner the ferment can by itself 

 produce a vigorous alcohol fermentation in a sugar 

 solution. The ferment is a very delicate and sensitive 

 substance, and is almost as easily killed by poisons as 

 the living yeast cells. Those races of yeasts now 

 employed in breweries and distilleries have been culti- 

 vated from a very early period. Egyptian wine seems 

 to have been manufactured on very much the same 

 principles as those now used. The present yeasts in 

 cultivation are very numerous, although all are merely 

 races of probably one species, Saccharoniyces cerevisiae. 

 Each brewery or distillery, as a rule, possesses a mixture 

 of yeasts which differs from that found anywhere else. 

 Each brewery has also to guard against the insidious 

 attempts of wild yeasts to enter and destroy the more 

 useful cultured forms. Wild yeasts occur on the 

 outsides of ripe fruits and appear to spend some part 

 of the winter on the soil below fruit trees. They have 

 also been found on bees and other- insects which visit 

 flowers and upon the green flies (^Aphides) which secrete 

 sugar. It is possible by Hansen's methods to isolate a 

 particular race of yeast-plants, and even to form a 

 colony capable of doing the whole work of a brewery 

 from a single cell. This operation is necessary when 

 brewery yeast becomes contaminated by wild forms. 

 The process is difficult, but the principle is quite simple. 

 A solution is made in which there is about one yeast cell 

 in every drop of the liquid (generally brewer's wort). 

 Forty sterilised flasks of the wort are then inoculated, 

 each with one drop. There will be in most of these 



