90 DAIRY LABORATORY GUIDE 



other disinfectant and apply to a board or fence post 

 in the same vicinity. On the following day secure 

 cijltures from the posts and determine by means of 

 plate cultures the difference in the number of the 

 bacteria obtained from the two sources. 



EXERCISE XXI 



Tests for Preservatives 



ECHO + H2O 



Add one drop of formaldehyde to J pint of milk 

 and shake thoroughly. Obtain, by means of a pipette, 

 17.6 c.c. or any convenient amount of the milk, and 

 place in Babcock test bottle. Add a similar amount 

 of sulphuric acid and allow to stand quietly for five 

 minutes. Note color hne at junction of acid and 

 milk, which should be a distinct purple or violet, 

 indicating the presence of formaldehyde. Repeat the 

 experiment using concentrated hydrochloric acid. 

 Also add a few drops of ferric chloride to aid the color 

 reaction. Formaldehyde is a colorless, volatile liquid 

 chemically intermediate between methyl alcohol and 

 formic acid. When consumed with milk it produces 

 various intestinal disturbances, and if present in 

 excessive amounts produces poisoning. 



Test for Boracic Acid. H3BO3 



Place I gram or any convenient amount of borax 



in a pint of milk and shake thoroughly. Obtain 



sample of milk by means of pipette and place in 



small porcelain dish. Evaporate about one-half to 



