ioo METHODS OF STERILISATION. 



culture - bacteria, does not always hold good. The filtering cylin- 

 der, especially when used for the first time, retains varying amounts 

 of the individual constituents of the liquid passing through it, a 

 fact that was first recorded by FLUGGE and SIROTININ (I.) in 1888, 

 and more closely examined by ARLOING (I.) in 1892. We will, 

 in this place, merely refer to the oxidising influence of the air, 

 observed more particularly by Miquel in the separation of urase 

 from cultures of uric -bacteria. It is therefore advisable to perform 

 such filtering operations in an atmosphere of pure hydrogen. 



75. The Beer-Filters 



used in the brewery must also be briefly considered here. The 

 object of these appliances is to render the beer bright, i.e. per- 

 fectly clear and transparent, when drawn from the storage cask 

 and sent out to the purchaser. Under normal conditions this 

 clarification is effected in sufficient degree in the storage cask, and 

 recourse should therefore be had to the filter only in such cases 

 where, by reason of defective treatment or other unfavourable 

 circumstances, a turbid lager beer is to be made similar to a beer 

 of standard quality. Such was the practice in Bavaria until a 

 few years ago ; but since the great breweries in that country began 

 to $ater for the export trade, they have had to conform to the 

 tastes of their foreign customers, who judge the quality of beer 

 by the eye, and would, without having tasted it, set it down as 

 inferior if it were not perfectly bright. Therefore, in order to 

 render it acceptable to this large and continually increasing clientele, 

 the beer has to be passed through the filter. The South German 

 connoisseurs in beer, who judge their beverage by the flavour, 

 raised objections, and with reason, since filtration causes apart 

 from the exception aforesaid an uncalled-for depreciation of 

 quality. This applies primarily to the chemical composition, the 

 filter removing from the beer sundry mucoid substances, extremely 

 minute in quantity and of as yet undetermined composition, but 

 which, nevertheless, contribute to the fineness of the flavour, so 

 that an experienced palate can distinguish with certainty between 

 a filtered and unfiltered beer. This defect, regretted though it be 

 by connoisseurs, is, however, the lesser evil when compared with 

 the dangers, from a biological point of view, that are obviated by 

 filtration. 



Two main types of beer filters are in general use. The one 

 constructed by Enzinger consists chiefly of a number of chambers, 

 the walls of which are composed of perforated plates lined with 

 thick filter-paper, specially prepared for the purpose, and through 

 which the beer is forced by compressed air acting on the storage 

 cask. The second type of filter, recommended for brewery work 

 by Stockheim, contains as its acting ingredient purified (and there- 

 fore tasteless) cellulose of a felty nature. No objection can be 



