NORMAL AND ABNORMAL MICROFLORA OF MILK 67 



by the lactic acid formed by other organisms ; as a group, they 

 are extremely sensitive to acid. There are, however, varieties of 

 Bacillus mycoides which can become accustomed to fairly large 

 amounts of acid and can even be induced to produce acid them- 

 selves. 



As the aerobic organisms grow best near the surface of the milk, 

 and the anaerobic near the bottom, the pseudo lactic acid bacteria 

 will be more plentifully represented in the cream, and the true 

 lactic acid bacteria will predominate near the bottom of the 

 vessel l . Hence the spontaneous curdling of milk by souring 

 always starts from the bottom. While several of the pseudo 

 lactic acid bacteria produce Isevo lactic acid, the common lactic 

 acid streptococci produce the dextro acid only ; the result of the 

 combined action of the two groups is a mixture of inactive and 

 dextro acids. After some time, these organisms are supplanted 

 by lactic acid forming rods, not only at higher temperatures 

 (thermobacteria), but also after keeping at lower temperatures 

 (strepto- and betabacteria) ; as many of these rods form inactive 

 acid, the so-called fermentation acid will consist chiefly of this 

 variety. 



In the spontaneous souring of milk at the ordinary temperature, 

 three stages may be distinguished. At the outset the original 

 flora of the milk develops rapidly ; as this is poor in true lactic 

 acid bacteria, the milk will only acquire a slightly unpleasant 

 smell and taste which may be described as stale. Only by degrees 

 do the streptococci gain the upper hand, whereupon the milk 

 becomes coagulated, and the acid which is formed masks the first 

 taints. Finally comes the third stage in which the lactic acid 

 rods predominate and in which the concentration of the acid may 

 rise from 0-6 per cent, to well over 1 per cent. Before this happens 

 however, a plentiful development of yeasts and moulds (especially 

 Torulae and Oidium lactis) will have taken place, and these 

 consume the acid or neutralise it by producing ammonia or other 

 basic products of protein hydrolysis ; in the latter case their 

 action is similar to that of chalk excepting that they destroy most 

 of the acid instead of conserving it. Through the joint action of 

 all these organisms the milk sugar will be fermented and the lactic 

 acid destroyed, so that the way is prepared for the putrefactive 

 organisms. As a rule, however, many weeks must elapse before 

 this state of affairs is brought about. The shallower the layer of 

 milk, the quicker will the acid disappear, provided, of course, 

 that the milk does not dry up in the meantime. 



1 When whipped cream, i.e., cream containing numerous air bubbles, is 

 kept at the ordinary temperature, coli and aerogenes bacteria will obtain 

 predominance in it. 



6 a 



