256 BACTERIOLOGICAL ANALYSIS OF MILK. 



to the material in the evaporating dish until it turns to a 

 very faint pink color. This indicates the neutral point. 

 Treat two other samples in the same way, and if the amount 

 of the sodium hydroxid added is the same in each case 

 it indicates the amount necessary to neutralize 5 c.c. of 

 the culture medium. The average of the three tests should 

 be taken. By calculation, determine the amount necessary 

 to neutralize the whole liter, and add sufficient amount 

 to the whole for neutralization. Instead, however, of 

 adding to the mixture the one tenth normal NaOH, 

 add a normal solution, which is ten times as strong, 

 and of which, therefore, only one tenth as much should 

 be added as would be required of the one tenth nor- 

 mal. The whole of the medium is thus to be neutralized. It 

 is well to test the accuracy of the neutralization by adding 

 a few drops of phenolphthalein to a little of the neutralized 

 medium. This should give the faint pinkish tinge; if it 

 does not, it means that the neutralization has not been prop- 

 erly affected. 



After neutralization boil for five minutes and restore to 

 the original weight, after which the reaction should be tested 

 again and corrected if necessary. 



The material thus neutralized is too strongly alkaline for 

 the proper growth of bacteria and must be rendered less 

 alkaline by adding HC1. The amount to be added should be 

 such as to bring the reaction to 1.5% acid. To produce this 

 acidity add to the neutralized medium 15 c.c. of normal HC1 

 for each liter. The acidity thus obtained is found to be that 

 at which common milk bacteria grow most readily. 



After adding HC1 in proportion of 15 c.c. to each liter 

 of solution, pour into the mixture slowly the white of an egg 

 mixed in a little water. Boil vigorously for a few minutes to 

 coagulate the albumen, and then filter through absorbent 

 cotton or through filter paper moistened with hot water. 

 The material filters rather more easily through absorbent 



