EXPERIMENT STATION BULLETINS. 



293 



XXII. THE INFLUENCE OF LACTIC ACID AS FOUND IN WHET OBTAINED FROM 

 MILK OF DIFERENT DEGREES OF ACIDITY. 



It is necessary after studying the influence of carbon dioxide upon the growth of 

 micio-organisnis in milk and ascertaining its probable influence and to what extent 

 its influence may be exerted, to consider the action of lactic acid upon germ life as it 

 is produced during the fermentation of milk. Moreover no factor which apparently 

 lias a direct bearing upon the germ-content of milk can be eliminated without a possible 

 detriment to the conclusions about to be drawn later, consequently it is evident that 

 this item has a very pertinent connection. It is well established that anaerobic 

 conditions sometimes alter the acidity of cultures, primarily as the result of the 

 functional activity of the cell influenced by the exclusion of oxygen. The important 

 bearing of this fact will be brought out more prominently as we continue our studies. 



We have sought to obtain the actual fermentation products without materially 

 interfering with the food qualities of the milk. It is well known that as soon as 

 the acidity increases slightly, the milk is more likely to curdle during the process of 

 sterilization ; in other words, heat favors the curdling of milk in the presence of traces 

 of acid while the same milk in the cold shows no formation of curd. It is evident that 

 in order to secure the fermentation products, the curd must be removed and some other 

 nitrogenous substance added to take its place. The milk was allowed to sour for 

 variable lengths of time after which it was heated. The curd was now removed and 

 one per cent of peptone added. The media was then clarified and the acidity deter- 

 mined when they were sterilized in the ordinary manner. The media thus prepared and 

 having different acidities were inoculated with the various micro-organisms. In this 

 way the influence of the fermentation products could be studied. 



The table below indicates by a ( + ) plus sign the micro-organisms having grown 

 and the ( — ) minus sign the micro-organisms having failed to grow. 



TABLE XX. 



THE INFLUENCE OF LACTIC ACID AS FOUND IN WHEY OBTAINED FROM MILK OF 



DIFFERENT DEGREES OF ACIDITY. 



