66 DAIRY BACTERIOLOGY 



hours at the temperatures which are most general in actual 

 practice. 



Number of bacteria 

 per co. 



Immediately after milking . . . . 1,480 



After standing eighteen hours at 9 C. . . 2,100 



12 C. .. 5,600 



15 C. ._ 156,000 



18 C. . . 550,000 



21 C. . . 6,750,000 



Naturally, the cleaner the milk the greater will be the benefit 

 derived from cooling. As already pointed out, it is not advisable 

 to keep milk longer than twenty-four hours, even if cooled to 

 C. ; if it is to be kept longer it must be frozen in order to avoid 

 the risk of the development of fluorescent and other water bacteria 

 which produce an unpleasant taste ; at somewhat higher tem- 

 peratures certain toxic proteus bacteria will also develop. For 

 these reasons cooled milk or cream which has stood for any 

 length of time are to be regarded w r ith suspicion, even if apparently 

 unchanged. As a rule milk does not become coagulated when 

 kept at temperatures below 10 C., but above this temperature 

 coagulation takes place in the course of a few days owing to the 

 action of rennet and acid forming bacteria. At 20 C. milk 

 quickly becomes coagulated, obviously owing to the formation 

 of acid ; at this temperature the streptococci, especially Sc. lactis, 

 develop so freely that after a time they will come to con- 

 stitute about 90 per cent, of the bacterial flora. The presence of 

 large amounts of lactic acid inhibits the growth of other milk 

 bacteria, for which reason the sour milk thus produced is harmless 

 as an article of food. As already mentioned, at higher tempera- 

 tures the streptococci give place to rod bacteria which produce 

 higher concentrations of lactic acid. As regards the gas forming 

 lactic acid bacteria, a few of these grow even below 10 C., but as 

 typical intestinal bacteria they have as a rule a high optimum 

 temperature and will most readily obtain predominance at 

 38 to 40 C. ; this temperature is somewhat high for the strepto- 

 cocci, and the slowly growing lactic acid rod bacteria will only 

 come to exercise an inhibitory effect on the other forms at a later 

 stage. For similar reasons the temperature mentioned is also the 

 most favourable for the development of the anaerobic sporing 

 bacteria which form butyric acid. On the other hand, the aerobic 

 sporing hay and potato bacilli, which often constitute the main 

 flora of pasteurised milk, are practically inert in raw milk ; their 

 spores do not germinate at the ordinary temperature, and at 

 higher temperatures the growth of their vegetative cells is inhibited 



