BACTERIA IN FOODS 211 



dicating a smaller number of spore-bearing germs than that 

 which contains a high percentage of acid. 



Lastly, while the heating process is of course the essential 

 feature of efficient pasteurisation, it must not be forgotten 

 that rapid and thorough cooling is almost equally import- 

 ant. As we have seen, pasteurisation differs from complete 

 sterilisation in that it leaves behind a certain number of 

 microbes or their spores. Cooling inhibits the germination 

 and growth of this organismal residue. If after the heating 

 process the milk is cooled and kept in a refrigerator, it will 

 probably keep sweet from three to six days, and may do so 

 for three weeks. 



Before leaving this subject we may glance for a moment 

 at the bacterial results of pasteurisation and sterilisation. 

 The chief two of these are the enhanced keeping quality 

 and the removal of disease-producrng germs. The former 

 is due in part to the latter, and also to the removal of the 

 lactic acid and other fermentative bacteria. As a general 

 rule these bacteria do not produce spores, and hence they 

 are easily annihilated by pasteurisation. True, a number 

 of indifferent bacteria are untouched, and also some of the 

 peptonising species. The cooling itself contributes to the 

 increased keeping power of the milk, especially in transit to 

 the consumer. 



Pasteurised milks have the following three economical and 

 commercial advantages over sterilised milks, namely, they 

 are more digestible, the flavour is not altered, and the fat 

 and lact-albumen are unchanged. Professor Hunter Stew- 

 art, of Edinburgh, about two years ago, compiled from a 

 number of experiments the following instructive and com- 

 prehensive table (page 212). 



It will be admitted that this table exhibits much in favour 

 of pasteurisation ; yet the crucial test must ever be the effect 

 upon pathogenic bacteria. Fliigge has conducted a series 

 of experiments upon the destruction of bacteria in milk, and 



