626 MICROBIOLOGY OF ALCOHOLIC FERMENTATION 



tation, the degree of attenuation, and by microscopic examination. In 

 breweries where modern pure culture methods are not used, the yeast 

 present is always of several forms or types. 



In any case, after a certain number of transfers, the yeast deteriorates 

 and finally may become thoroughly infected with bacteria. The bac- 

 teria are revealed by microscopic examination. Where pure cultures 

 are used, contamination with foreign yeast is shown by a change in the 

 time of spore formation. By this method a contamination of 1:200 

 may be discovered. 



When the yeast becomes contaminated, a new start must be made 

 with yeast from another brewery, which is uncertain or by a starter of 

 pure yeast, which is the only reliable method. 



The new start with pure yeast may be made by employing a kilo- 

 gram of pure pressed yeast or a corresponding amount of liquid yeast 

 and gradually increasing it to the desired amount by repeated small 

 additions of sterile wort. This must be done with special precautions 

 against contamination. Many large breweries use large pure yeast 

 machines which produce directly sufficient yeast to start a fermenting 

 vat. 



AFTER TREATMENT. The violent fermentation requires from eight 

 to eighteen days according to the temperature. It takes place in open 

 vats or sometimes, in top fermentation, in barrels. When sufficiently 

 attenuated, the beer is drawn off into large casks where the slow 

 secondary fermentation takes place at a low temperature and the beer 

 clears by depositing yeast and other sediment. The time required for 

 the secondary fermentation is from six to ten weeks or, with certain 

 types of beer, from two to four months or longer. 



A certain amount of dissolved carbonic acid is necessary for the 

 quality and keeping of the beer. This is obtained by tightly bunging 

 the casks at a suitable stage of the secondary fermentation. 



The clarification of the beer is sometimes assisted by placing a quantity of chips 

 of beech or other tasteless wood in the casks. Top fermentation beers are often 

 fined by the use of isinglass or animal gelatin. Low fermentation beers are usually 

 filtered. 



The beer is then ready for delivery to the consumer and is placed in barrels with 

 precautions to retain the dissolved carbonic acid. 



The clear beer may be put directly into bottles with the same precautions. Bot- 

 tled beers which are to be kept for some time or which are to be shipped to a dis- 

 tance are pasteurized after bottling at 60 to 65. 



