266 MILK 



is rarely completely used up by the lactic acid bacteria. By pro- 

 tein decomposition ammonia is formed and the acid is gradually 

 neutralized. The amount of acid is further decreased by molds, 

 since some are able to utilize it for food. 



Gradually the acid has completely disappeared, and proteolytic 

 bacteria, having survived chiefly as spores, begin to multiply, 

 producing protein cleavage products, and an ill-smelling fluid 

 results. This is the fifth phase. 



It should be emphasized that these periods are not distinctly 

 defined and that frequent deviations may occur. After part of 

 the acid has been neutralized in the fourth phase, it is quite 

 probable that some lactic acid bacteria again multiply and the 

 acid again increases. There may be a temporary reduction of 

 acid, followed by a temporary increase, and this process may be 

 repeated, so that the acidity goes back and forth several times. 

 However, in the course of time the milk-sugar becomes nearly 

 exhausted and, as this forms the chief food for lactic acid bac- 

 teria, acid formation becomes difficult. Variations from the 

 general scheme outlined may also be the result of differences in 

 the numeric strength of the groups of bacteria present. 



Furthermore, the temperature at which the milk is kept 

 naturally influences the results. Bacteria of the Bacillus coli 

 group, as stated before, multiply at 30 to 37 C. more rapidly 

 than at room temperature, while Streptococcus lacticus seems to 

 grow at room temperature at least as well as at higher tempera- 

 ture. At room temperature, therefore, we have a relatively 

 greater multiplication of Streptococcus lacticus and less growth 

 of the Bacillus coli group, so that the desirable products of the 

 streptococcus are more prevalent at the lower temperature. At 

 30 to 37 C., on the other hand, the products of the Bacillus coli 

 group may be prevalent, so that gas and undesirable flavors 

 develop. 



When the temperature is above 37 C. and up to 50 C. lacto- 

 bacilli enter the field, and by the large amount of acid produced 

 may suppress molds and yeasts. Under these conditions pro- 

 teolytic bacteria cannot grow, and the milk remains in a state of 

 intense acidity. 



Sometimes the number of lactic acid bacteria in fresh milk 

 is exceedingly low, and peptonizing bacteria may obtain an early 

 foothold. In such cases the milk putrefies before enough acid is 

 formed to check the putrefactive bacteria. Rennet-forming bac- 

 teria also sometimes are present in great numbers and the milk 

 coagulates without souring. This condition is known as sweet 

 curdling. 



It is evident from what has been said that there are many 



