vii ALCOHOLIC FERMENTATION 75 



able to withstand starvation and drought for a long time ; 

 when placed under favourable circumstances they develop 

 into the ordinary form of Saccharomyces. So that repro- 

 duction by multiple fission appears to be, in the yeast-plant, 

 a last effort of the organism to withstand extinction. 



The physiology of nutrition of Saccharomyces has been 

 studied with great care by several men of science and 

 notably by Pasteur, and is in consequence better known than 

 that of any other low organism. For this reason it will be 

 advisable to consider it somewhat in detail. 



The first process in the manufacture of beer is the pre- 

 paration of a solution of malt called " sweet wort." Malt 

 is barley which has been allowed to germinate or sprout, i.e., 

 the young plant is allowed to grow to a certain extent from 

 the seed. During germination the starch which forms so 

 large a portion of the grain of barley is partly converted into 

 sugar : barley also contains soluble proteids and mineral 

 salts, so that when malt is infused in hot water the sweet- 

 wort formed may be looked upon as a solution of sugar, 

 proteid, and salts. 



Into this wort a quantity of yeast is placed. Very soon 

 the liquid begins to froth, the quantity of yeast increasing 

 enormously : this means of course that the yeast-cells are 

 budding actively, as can be readily made out by microscopic 

 examination. If while the frothing is going on a lighted 

 candle is lowered into the vat the flame will be immediately 

 extinguished : if an animal were placed in the same position 

 it would be suffocated. 



Chemical examination shows that the extinction of the 

 candle's flame or of the animal's life is caused by a rapid 

 evolution of carbon dioxide from the fermenting wort, the 

 frothing being due to the escape of the gas from the liquid. 



After a time the evolution of gas ceases, and the liquid 



