MICROBIOLOGY OF ALCOHOL AND ALCOHOLIC PRODUCTS. 435 



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

 must be stopped and the filtering surface removed. 



For wines containing little sediment the filter 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 if 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. There 

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

 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 fermenta- 

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

 materials containing sugar. The liquid before fermentation is called Wort. 



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 5. cerevisicB. In some spontaneously fermented beer, other yeasts, 

 Torulcc and bacteria take part, but in ordinary beers most of these are con- 

 sidered 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 Torulas); 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 considerable quanti- 

 ties of lactic and acetic acids. 



Typical Composition of Various Beers. 



