XX PRESERVATION OF MILK 381 



In this connection Ayers and Johnson ^ have carried out 

 a number of very interesting experiments which deserve close 

 attention. They made careful comparison of the bacterial 

 content of commercially pasteurised and raw market milk, both 

 when fresh and on each successive day as long as the milk 

 was fit for consumption. By commercial pasteurisation they 

 mean milk heated from 60" C. to 65-6° C. in the "holder" 

 process, or up to 71'1" C. in the "flash" process. They 

 tested both types of pasteurisers. They divided the organ- 

 isms present in the milk samples into three groups — pepton- 

 ising, lactic acid, and alkali or inert bacteria. Under 

 peptonising bacteria were classed all forms which liquefied 

 gelatin. All bacteria that gave red colonies with litmus were 

 classed under the head of lactic acid bacteria. In the group 

 called alkali formers or ine^'t bacteria were classed all those 

 forms which do not noticeably produce acid or liquefy gelatin. 

 The total counts and percentages of the different groups were 

 as far as possible carefully worked out. 



The following were the main results obtained by these 

 workers. They found that the relative proportion of the groups 

 of peptonising, lactic acid, and alkali or inert bacteria was 

 approximately the same in efficiently pasteurised milk and 

 in clean raw milk. In both cases the alkali or inert forms 

 constituted the largest group, the lactic acid bacteria next, 

 while the peptonisers were in the minority. When both of 

 these milks — the efficiently pasteurised and clean raw milk 

 — were stored, the group relations changed ; but when the 

 changes which took place were compared, it was found that they 

 were the same in each. At the time of souring, the group pro- 

 portions changed, so that the lactic acid bacteria constituted 

 the largest group, with the alkaK or inert forms next in order, 

 while the peptonisers, as initially, were in the smallest propor- 

 tion. In both of these classes of milks the group of pepton- 

 isers sometimes increased slightly in its proportion to the 

 other two groups during the first few days, but then gradu- 

 ally decreased and always formed the smallest group. 



Even with inefficiently pasteurised milk and dirty raw 

 milk the peptonisers formed the smallest proportion of the total 



^ Bulletin No. 126, U.S. Department of Agriculture, Bureau of Animal In- 

 dustry, 1910. 



