524 



MICROBIOLOGY OF SPECIAL INDUSTRIES 



through the filter and as soon as the rate of filtration becomes too slow, the operation 

 must be stopped and the filtering surface renewed. 



For wines containing little sediment, the iUter must be primed. This is accom- 

 plished by putting some finings in the wine first passed through the filter. The 

 priming is more effective and the output of the filter much increased L£ a little in- 

 fusorial earth is used with the gelatin. 



For the more perfect clearing of old wines some form of pulp filter is used. These 

 are various devices by which the wine is forced through a mass of cellulose or as- 

 bestos pulp and freed from all floating matter. Some of the best of these, carefully 

 used, remove nearly all of the bacteria present. 



BEER 



Beer is an alcoholic beverage made from certain cereal grains by 

 transformation of the starch to sugar, dilution with water, and fermen- 

 tation with yeast. There is usually an addition of hops and sometimes 

 of materials containing sugar. The hquid before fermentation is 

 called wort. 



Typical Composition of Various Beers 



Raw Materials and Microorganisms of Brewing 

 Grains Employed. — Barley, rice and maize are the grains most commonly used, 

 wheat, rye and oats but rarely. Cane and beet sugars and syrups sometimes form 

 part of the fermentable material. 



Yeasts of Beer. — The yeast used is usually one of the many forms 

 of S. cermsicB. In soriie spontaneously fermented beer, other yeasts, 

 Torula and bacteria take part, but in ordinary beers most of these are 

 considered as disease-producing organisms and injurious. 



Kinds of Beer. — The principal varieties of beer are: lager beers, fermented with 

 bottom yeasts; ales, fermented with top yeasts (and Torulm); porters, similar to ale 

 but dark in color owing to the use of caramelized malt; weisbiers, in which lactic 

 bacteria are abundant; and certain local types in which bacteria produce con- 

 siderable quantities of lactic and acetic acids. 



