14 MICRO-ORGANISMS AND FERMENTATION. 



would thus form comparatively poor nutritive media for the 

 alcoholic yeast. According to the nature of the substance and 

 the germs existing in it, this method requires different tempera- 

 tures both for sterilising purposes and for the germination of 

 the spores ; and, as the result is uncertain, the substances thus 

 heated should always be left for observation for a considerable 

 length of time before use, to make sure that they are really 

 sterile. 1 



(c) Sterilisation of the air is best attained, as stated 

 above, by means of cotton-wool filters : sulphuric acid, brine, 

 cloth filters, etc., are less efficient. In laboratories, where 

 work must often be performed in germ-free air, a glass 

 chamber is employed, the front of which can be raised suffi- 

 ciently for the operator to introduce his hands. Some time 

 before using the chamber, the whole of its inner surface 

 must be washed, and the chamber then closed. The dust 

 particles and germs suspended in the air will then settle to 

 the moist bottom and remain there. 



4. DISINFECTION. 



Another method of killing disturbing germs is by the use 

 of disinfectants, which act as poisons on the micro-organisms. 

 Not a few of these substances have found application in 

 practice. The limit for the employment of such poisonous 

 substances must be determined for each individual case. As 

 manipulations with such poisons may be deleterious to the 



1 Sterilisation is also attempted in practice for the purpose of introducing 

 brewers' wort in a sterile condition into the fermenting -tuns by means of a closed 

 cooling and aerating apparatus. It is true that the wort cannot keep absolutely 

 free from germs when the fermentation takes place in open tuns, but a great 

 deal can be effected in this direction by acquiring a thorough comprehension of 

 the matter. The expert brewer will always take care that the air in the 

 fermenting room is kept as free from germs as possible, by keeping the sur- 

 face of all appliances clean, and also that the tuns as well as all utensils 

 that are immersed in the fermenting liquid, e.g., thermometers, sample 

 glasses, etc., are always perfectly clean. As a matter of course, these pre- 

 cautionary measures could not acquire any real practical importance until, 

 through Hansen's reform, absolutely pure yeast had been introduced into the 

 fermenting room. 



