K. DOMESTIC AND HOUSEHOLD ECONOMY. 439 



which beer or must undergo are due to the action of minute 

 microscopic organisms, vibriones, bacteria, etc., whose germs 

 are carried in the air, are contained in the materials used, or 

 are found adhering to the utensils employed in the brewery. 

 In the second place, under all the methods of brewing com- 

 monly employed, every must, every yeast, and every beer 

 contain these germs. Without their presence beer can only 

 undergo alcoholic fermentation, and possibly some minor 

 changes which may in certain cases improve its quality, and 

 in other cases produce nothing worse than vapidity. The 

 thing to be done, therefore, is to prepare a must free from 

 objectionable germs, and to ferment it by means of a yeast 

 similarly pure. Pasteur proceeds as follows : 



The must, prepared by the ordinary methods, is heated 

 very hot in order to destroy all germs contained in it. It is 

 then inclosed in a vat provided with a tight cover, whose in- 

 terior communicates with the outer air only through two 

 vertical tubes. At one of these tubes a current of carbonic- 

 acid gas is allowed to enter, its excess being discharged by 

 the other. In this vat, thus sheltered from all germs which 

 might otherwise sret in from the air, the must is allowed to 

 cool. The next step is to add the yeast; and the main 

 difficulty is to obtain this pure. This is done in a way 

 which Pasteur does not describe in detail, but which de- 

 pends upon the fact that the yeast plant multiplies more rap- 

 idly in the presence of atmospheric oxygen than in carbonic 

 acid, while almost the contrary is true of the objectionable 

 germs. A little of this pure yeast once obtained, it can be 

 preserved indefinitely, and permitted to multiply in apparatus 

 so constructed as to shelter it from the germs floating in the 

 air. The yeast, free from these germs, being added, without 

 contact of the air, to the must likewise free from germs, alco- 

 holic fermentation sets in. A beer is thus obtained which, 

 when finished, no longer offers a favorable medium for the 

 development of the above-mentioned microscopic organisms. 

 It can be preserved, as is usual, in barrels or bottles, and for 

 an indefinite time, even without the use of ice. Even high 

 temperatures do not affect it unfavorably. It undergoes 

 only those changes which are commonly produced by age, 

 and which are positive improvements rather than the re- 

 verse. The employment of ice-houses is thus avoided, the 



