BACTERIA IN FOODS 209 



pasteurised, for the simple reason that in the latter some 

 living germs have been unaffected. 



Sterilisation may of course be carried out in a variety of 

 modifications of the two chief ways above named. When 

 the process is to be completed in one event an autoclave is 

 used, in order to obtain increased pressure and a higher 

 temperature. Milk so treated is physically changed in 

 greater degree than in the slower process. The slow or 

 intermittent method is, of course, based on Tyndall's dis- 

 covery that actively growing bacteria are more easily killed 

 than their spores. The first sterilisation kills the bacteria, 

 but leaves their spores. By the time of the second appli- 

 cation the spores have developed into bacteria, which in 

 turn are killed before they can sporulate. 



The methods of pasteurisation are continually being 

 modified and improved, especially in Germany and America. 

 Most of the variations in apparatus may be classed under 

 two headings. There are, first, those in which a sheet of 

 milk is allowed to flow over a surface heated by steam or 

 hot water. This may be a flat, corrugated surface or a 

 revolving cylinder. The milk is then passed into coolers. 

 Secondly, milk is pasteurised by being placed in reservoirs 

 surrounded by an external shell containing hot water or 

 steam. Dr. A. L. Russell ' has described one apparatus 

 consisting of a pasteuriser, a water-cooler, and an ice-cooler. 

 The pasteuriser is heated by hot water in the outside case- 

 ment. To equalise rapidly the temperature of the water 

 and milk a series of agitators must be used. These are 

 suspended on movable rods, and hang verticall}?- in the milk 

 and -"/^gter chambers. By this ingenious arrangement the 

 heat is diffused rapidly throughout the whole mass, and as 

 the temperature of the milk reaches the proper point the 

 steam is shut off, and the heat of the whole body of water 

 and milk will remain constant for the proper length of time. 



"^ Report from Wisconsin Agricultural Experiment Station, 1896. 



