76 EXPERIMENTAL DAIRY BACTERIOLOGY 



when the media contains salt (NaCl) simulates closely that 

 of true digestion, but can be differentiated from it by flooding 

 the plates with dilute HC1 (1 per cent), when, in case the 

 change has been due to true digestion, the clear zone about 

 the culture will remain. If the clearing was caused by the 

 solvent effect of the acid, the addition of a larger quantity of 

 acid will cause a reprecipitation. 



The presence of gas is easily noted in the case of organisms 

 producing a curd. In other types where no curd is formed, 

 as, for example, in the case of lactose-fermenting yeasts, the 

 presence of gas may be indicated by gently shaking the tube 

 and noting the evolution of bubbles. 



Organisms of the same species vary greatly in the amount 

 of acid produced. The amount of acid can be determined in 

 tube cultures by adding to a tube of milk 5 cc. of distilled 

 water. Mix well, turn into a graduated cylinder, note the 

 volume, transfer the mixture to an evaporating dish or an 

 Erlenmeyer flask, add a few drops of the phenolphthalein 

 indicator, and titrate with N/20 NaOH. Express the results 

 as per cent of lactic acid. 



. . , cc. of N/20 x 0.45 

 Per cent of acid == - 



cc. of milk 



In studying the various organisms illustrating the different 

 kinds of fermentations four tubes of milk should always be 

 inoculated. Two of these should be incubated at 20 C. and 

 the remaining two at 37 C. 



Acid fermentation. The normal souring or acid fermenta- 

 tion in milk is usually caused by a number of different kinds 

 of bacteria growing together. In one sample of milk one type 

 may predominate ; in a second sample a different type may 

 be most active ; in a third still another form may be present 

 in greatest numbers. 



