BACTERIOLOGICAL ANALYSIS OF MILK. 



growth and it wants to be brought to the same grade of 

 acidity as the gelatin medium already prepared. When a 

 solution is neutral to litmus it is about 2.5% acid to phenol- 

 phthalein, i. e., it requires '25 c.c. normal NaOH per liter to 

 bring it to the neutral point of phenolphthalein. The gelatin 

 solution has been brought to 1.5%, and the litmus solution, 

 in order to be brought to the same grade of alkalinity, must 

 have added to it 10 c.c. (i. e., 1.0% by volume) normal 

 NaOH per liter (2.5 1.5=1.0). Normal NaOH is 

 therefore added to the litmus in proportion of 10 c.c. per 

 liter, after which the solution is sterilized in steam in a flask 

 closed with cotton in the same manner as the gelatin. It will 

 be evident that the gelatin and the litmus solutions may be 

 mixed in any proportion without changing the reaction. 



Milk Whey Culture Medium. For some purposes a cul- 

 ture medium made from milk is preferable to the one de- 

 scribed above. It is made as follows : 



To about two quarts of milk there is added enough rennet 

 to curdle it in half an hour. The curd is to be cut with a 

 knife and the whey strained out through cheese cloth. Place 

 the whey in a flask and sterilize one hour in an autoclave 

 at a pressure of twenty pounds. This sterilization kills the 

 spores, after which the whey is to be filtered through filter 

 paper. Then add 13% of gelatin, dissolve by moderate heat, 

 and neutralize to phenolphthalein by adding NaOH from the 

 burette as already described. Bring the reaction to 1.5% 

 acid as before, add the white of an egg, boil briskly and filter 

 through cotton as usual. The filtered solution is to be placed 

 in test-tubes, 8 c.c. in each, and sterilized. In other respects 

 this medium is to be used exactly as the peptone gelatin de- 

 scribed above. This milk medium gives a sharper differentia- 

 tion of the different kinds of bacteria colonies, but does not 

 give quite such high numbers when used to determine the 

 number of bacteria in milk. 



