MICROBIOLOGY OF ALCOHOL AND ALCOHOLIC PRODUCTS. 439 



spore formation. By this method a contamination of i : 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 kilogram 

 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 second- 

 ary 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 isinglas 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. 



DISEASES OF BEER. 



Beer may show defects due to imperfections in the raw material or in 

 the methods of manufacture. These are principally abnormal flavors 

 and lack of clearness. 



The diseases properly so called are due to wild yeasts or to bacteria. 

 The disease-producing yeasts may be derived from the starter, from the 

 vessels with which the beer comes in contact, or from the air. They 



