MILK. 701 



there can be no doubt that the continued use of milk containing these 

 preservatives is detrimental to health, especially in the case of human 

 nurslings. For this reason many countries, States, and cities prohibit 

 legally the use of preservatives. 



Tests for preservatives in milk. 



Formaldehyde. Float a mixture of 10 c.c. of milk and 10 c.c. of water in a 

 test-tube, on concentrated sulphuric acid made pale yellow by addition of 

 ferric sulphate. A blue to violet color at the line of junction shows the pres- 

 ence of formaldehyde. Pure milk gives a greenish color. 



Salicylic acid. Acidify 25 c.c. of milk with acetic acid, boil, and filter. Ex- 

 tract the nitrate with an equal volume of ether. Shake the ether extract with 

 a dilute (straw-colored) solution of ferric chloride, On separating, the aqueous 

 solution shows a reddish-violet color when salicylic acid is present. 



Benzole acid. Proceed as in the foregoing test, but shake the filtrate with 

 an equal volume of solution of hydrogen dioxide before extracting with ether. 

 By this treatment benzoic acid is converted into salicylic acid, which is then 

 tested for by ferric chloride. 



Boric acid and borax. A few drops of the filtrate obtained as in the prece- 

 ding test are mixed with a drop of strong hydrochloric acid and a drop of satu- 

 rated alcoholic solution of turmeric. The mixture is evaporated to dryness on 

 a water-bath, and a drop of ammonia added to the residue when cold. A dull- 

 green stain shows the presence of boric acid or borax. 



In addition to the test for chemical preservatives, commercial milk 

 is now examined by bacteriological methods, the number and, if pos^ 

 sible, the character of the organisms being determined. By extreme 

 care in the production of milk it is possible to keep the count lower 

 than 30,000 per cubic centimeter ("certified milk"). A cheaper 

 method of producing good milk is by heating the milk for a certain 

 time to a temperature below the boiling-point, which will kill all the 

 pathogenic and many of the non-pathogenic bacteria. This method 

 is commonly called " pasteurization." It has the disadvantage that 

 the digestibility of the milk is lessened, an important point in infant 

 feeding. 



Experiment 94. (Analysis of milk?) As the proportion of fat varies at dif- 

 ferent periods of the milking, it is necessary to secure a sample from the well- 

 mixed yield of milk. a. Determine the specific gravity of milk, cream, and 

 skimmed milk by means of the lactometer (a urinometer answers the purpose). 



b. Fat. Determine the total butter fat by using Babcock's method, which is as 

 follows : Place 10 c.c. of milk into a small, specially constructed bottle provided 

 with a long, slender, graduated neck ; add 2 c.c. of a mixture of ainyl alcohol 

 37, methyl alcohol 13, and hydrochloric acid 50 parts; then fill the bottle grad- 

 ually with sulphuric acid. Place the bottle in a centrifugal machine and 

 rapidly revolve for three minutes, when the fat is forced to the top of the 

 mixture. Add enough warm water to float the separated fat into the neck, 

 when the exact percentage can be read on the scale. A special form of bottle, 



