STERILISATION. 21 



merit of this kind it is possible to eliminate all spores and to 

 kill all vegetative cells. This obviously depends, however, 

 upon the regular germination of the spores. The treatment 

 does not absolutely guarantee sterility, and before either 

 liquids or gelatines are used they must be kept under obser- 

 vation for a considerable time. In many cases filtration is 

 to be preferred to discontinuous sterilisation. The liquids 

 in daily use that are prepared with the help of micro-organisms, 

 beer, wine, vinegar, etc., always contain a residue of these 

 micro-organisms in a more or less vigorous condition. It is 

 desirable, by heating them, to arrest the fermentation. The 

 safest course is to sterilise the liquids, but as the temperatures 

 required to effect sterilisation usually produce great changes in 

 the liquids, it is necessary to limit the treatment to a temperature 

 that will suffice to weaken the micro-organisms, so that under 

 normal conditions they are extremely unlikely to propagate or 

 to bring about further fermentation (" pasteurisation "). It 

 is difficult to determine the best method when the nature 

 of the liquid will not admit of a high temperature being used, 

 while the result must depend upon the character and the 

 activity of the different micro-organisms present, as well as 

 upon the chemical composition of the liquid. It is, therefore, 

 impossible to establish any general rule. It is essential in 

 each case to determine experimentally both the temperature 

 and the time of treatment, after forming a judgment as to 

 what micro-organisms are present in the liquid. In the case of 

 beer different temperatures are used heating from 50 to 60 C. 

 for two hours, or from 65 to 70 C. for half an hour or more 

 and for wine, two hours' heating at 45 to 50 C. (C. Schulze}. 

 A slow cooling down after pasteurisation has often been 

 experimentally proved to give better results than rapid cooling. 

 The determination of the right temperature is obviously 

 rendered more difficult if the liquid harbours different species 

 of yeast, and still more so if at the same time the development 

 of bacteria has taken place, especially those species that form 

 spores. It has been proved that when the heating exceeds 

 certain limits, the flavour either of beer or wine quickly deteri- 

 orates, which is probably due in the first place to the decom- 

 position of albuminoids. 



