12 MICRO-ORGANISMS AND FERMENTATION. 



however, requires frequent cleansing, and must also be 

 frequently sterilised by ignition, it having been proved 

 that the bacteria are able in time to grow through the 

 pores 1 . 



Liquids and solid nutritive substrata are in most cases 

 sterilised by heat. The way in which this must be done, 

 as well as the duration of the heating process, are dependent 

 on the nature of the substratum in question. Direct boiling 

 on the sand-bath may be employed for the purpose of steril- 

 ising, for example, brewery-wort in Pasteur-flasks, otherwise 

 the water-bath may be used. An excellent means of sterili- 

 sation is afforded by steam either at 100 C. or under pressure 

 (110 120 C.) by means of Papirfs digestor (autoclave). 

 During cooling, care must be taken that only absolutely pure 

 air comes in contact with the sterilised substance, the air 

 entering the vessel being filtered through cotton-wool or 

 passed through tubes bent several times, if it is drawn in 

 slowly and with no great force 2 . 



1 In breweries the filtration of beer has been resorted to during the 

 last few years, the filtering media commonly used being paper, cellulose, 

 asbestos, etc. By such filtration brewers sometimes succeed, it is true, in 

 freeing a beer originally sound from deposits of various kinds, and in 

 rendering it bright ; but, on the other hand, the fact must be emphasised, 

 that an indiscriminate employment of this method may occasion great 

 dangers, as has been directly proved by the experiments made by Thausiny, 

 Wichmann, Reinke, and others. If, namely, the filters are not sufficiently 

 effective, it may happen that only the yeast cells are retained, but not the 

 bacteria, which are then enabled to act with much greater energy upon 

 the liquid. Another great danger lies in the fact that a filter, owing to 

 deficient cleansing, may become a seat for the development of different 

 kinds of germs, contaminating all the beer passing through it. If a single 

 cask of a store-room has become infected, and the filter is not effectually 

 sterilised after the filtration of this beer, the disease will be communicated 

 to all the other beer. 



2 In the so-called Pasteurisation of beer, a merely relative sterilisation 

 is all that is generally aimed at ; that is to say, by a cautious treatment 

 of the beer at elevated temperatures, it is sought to check the yeast cells 

 to such a degree that they are capable only to a very limited extent of 

 multiplying and producing fermentation. It is only for transportation 

 to a great distance that it is attempted to kill all living germs contained 



