

MILK FERMENTATIONS 71 



four days at 37 C., together with an uninoeulated tube to 

 serve as a control. 



Test for nitrites. Dissolve 8 grams of the purest sulpha- 

 nilic acid in 1000 cc. of 30 per cent acetic acid (sp. gr. 1.041). 

 Dissolve 5 grams of a-amidonaphthylamine in 1000 cc. of 30 

 per cent acetic acid ; filter through a plug of washed absorbent 

 cotton. At the end of the period of incubation in the nitrate 

 broth remove 3 cc. of the culture to a clean test tube and 

 add 2 drops each of the naphthylamine and sulphanilic acid 

 solutions. The development of a red color indicates the pres- 

 ence of nitrites, the intensity of the shade indicating in a 

 general way the amount of nitrites present. The control tube 

 should be treated in the same manner as the culture. 



Test for ammonia. Nessler solution. Dissolve 50 grams 

 of potassium iodide in a minimum quantity of cold water. 

 Add a saturated solution of mercuric chloride until a slight 

 but permanent precipitate persists. Add 400 cc. of a 50 per 

 cent solution of potassium hydrate made by dissolving the 

 potassium hydrate and allowing it to clarify before using. 

 Dilute to 1 liter and allow the mixture to settle. One half 

 of the remaining portion of the culture used for the nitrite 

 test is removed to a test tube and a couple of drops of the 

 Nessler solution added. The presence of ammonia is indi- 

 cated by a yellow color, or by a yellow precipitate when the 

 ammonia is present in considerable quantities. 



The presence of ammonia in broth culture may be de- 

 tected by moistening a strip of filter paper with the Nessler 

 solution and placing the same in the neck of the tube, a yel- 

 low to reddish-brown color indicating the presence of ammonia. 



Exercise. From the cultures furnished, inoculate tubes of nitrate 

 solution. Determine, as described, the ability of the organisms to 

 produce nitrites and ammonia from the nitrate present. 



