THE RELATION OF MICROORGANISMS TO MILK. 323 



SECOND STAGE. PERIOD PROM END OF GERMICIDAL ACTION TO 

 TIME OF CURDLING. The period following immediately after the ger- 

 micidal action is characterized by the rapid development of the lactic 

 organisms. Under normal conditions this group develops much more 

 rapidly than any other type. Not only do they increase rapidly in 

 actual numbers but their percentage also rises rapidly. There may 

 be a continual increase in numbers in the other species, but their growth 

 is much less rapid than that of the Bact. lactis acidi type. As this period 

 advances certain of the miscellaneous types may cease to grow entirely. 

 During this time the gas-producing acid organisms of the B. coli and 

 Bact. lactis aerogenes type may develop more or less rapidly, but if the milk 

 is held at temperatures not much above 20, the Bact. lactis acidi type will 

 develop much more rapidly, so that by the time the milk becomes sour 

 and curdles, this type will constitute 99 per cent approximately of the total 

 number in the milk. From the standpoint of the milk consumer milk 

 ceases to be of value when the end of this period is reached, but there are 

 further developments which are of importance in certain lines of dairy 

 manufactures, notably cheese making. 



THIRD STAGE. PERIOD FROM TIME OF CURDLING UNTIL ACIDITY 

 is NEUTRALIZED. At the time milk curdles it contains enormous num- 

 bers of the lactic bacteria. The number usually runs into the millions 

 and may be even higher than one thousand million per c.c. By the 

 time the coagulation takes place the acidity of the milk is so high that 

 the growth of the lactic organisms is checked and from this time on 

 their number decreases with more or less rapidity. 



During the period following the curdling certain other types of 

 organisms which have existed in the milk during the earlier stages now 

 begin to grow. The organisms especially important in this stage are 

 Oidium lactis, certain species of molds, and yeasts. These organisms 

 are able to grow in a highly acid medium, and as a result of their devel- 

 opment the acid is decreased until the milk finally presents a neutral 

 or alkaline condition resulting from the decomposition of the proteins 

 in the milk. 



FOURTH STAGE. FINAL DECOMPOSITION CHANGES. The reduction 

 of the acidity affords favorable conditions for the growth of certain types 

 of organisms which have remained in the milk during the earlier stages 

 but have been practically dormant. In this fourth stage the conditions 

 are suitable for the growth of the liquefying and peptonizing bacteria 



