XXII] DAIRY PRODUCTS 291 



16 ACETIC SERUM.-TENTATIVE. 



(a) Zeiss immersion refractometer reading. — To 100 cc. of milk at a temperature 

 of about 20°C. add 2 cc. of 25% acetic acid (sp. gr. 1.035) in a beaker and heat the 

 mixture, covered with a watch glass, in a water bath for 20 minutes at a tempera- 

 ture of 70°C. Place the beaker on ice water for 10 minutes and separate the curd 

 from the serum by filtering through a 12.5 cm. folded filter. Transfer about 35 cc. 

 of the serum to 1 of the beakers that accompanies the control-temperature bath 

 used in connection with the Zeiss immersion refractometer, and take the refrac- 

 tometer reading at exactly 20°C., using a thermometer graduated to tenths of a 

 degree. A reading below 39 indicates added water; between 39 and 40, the addition 

 of water is suspected. 



(b) Ash. — Transfer 25 cc. of the serum to a flat-bottomed platinum dish and 

 evaporate to dryness on a water bath. Then heat over a low flame (to avoid spat- 

 tering) until the contents are thoroughly charred, place the dish in an electric 

 muffle, preferably with pyrometer attached, and ignite to a white ash at a tem- 

 perature not greater than 500°C. (900°F.). Cool and weigh. Express the result 

 as grams per 100 cc. Results below 0.715 gram per 100 cc. indicate added water. 

 Multiply by the factor 1.021 (dilution of the acetic serum being 2%) to obtain the 

 result on the sour serum ash. 



1 7 SOUR SERUM.— TENTATIVE. 



(a) Zeiss immersion refractometer reading. — Allow the milk to sour spontane- 

 ously, filter and determine the immersion refractometer reading of the clear serum 

 at 20°C. A reading below 38.3 indicates added water. 



(b) ^s/i^— Determine the ash in 25 cc. of the serum, obtained in (a), as directed 

 in 16 (b). Results below 0.730 gram per 100 cc. indicate added water. 



18 ZEISS REFRACTOMETER READING OF COPPER SERUM.— TENTATIVE. 



To 1 volume of copper sulphate solution (72.5 grams of copper sulphate per 

 liter, adjusted if necessary to read 36 at 20°C. on the scale of the Zeiss immersion 

 refractometer, or, to a specific gravity of 1.0443 at ^^) add 4 volumes of milk. • 

 Shake well and filter. Determine the Zeiss refractometer reading of the clear 

 serum at 20°C. A reading below 36 indicates added water. 



GELATIN, 



19 Qualitative Test. — Tentative. 



To 10 cc. of the milk add an equal volume of acid mercuric nitrate solution (mer- 

 cury dissolved in twice its weight of nitric acid (sp. gr. 1.42) and this solution 

 diluted to 25 times its volume with water), shake the mixture, add 20 cc. of water, 

 shake again, allow to stand 5 minutes and filter. If much gelatin is present, the 

 filtrate will be opalescent and cannot be obtained quite clear. To a portion of the 

 filtrate contained in a test tube, add an equal volume of saturated aqueous picric 

 acid solution. A yellow precipitate will be produced in the presence of any con- 

 siderable amount of gelatin, while smaller amounts will be indicated by a cloudi- 

 ness. In the absence of gelatin the filtrate will remain perfectly clear. 



20 PRESERVATIVES.— TENTATIVE. 



Proceed as directed under X. To test for salicylic or benzoic acid acidify 100 

 cc. of the milk with 5 cc. of hydrochloric acid (1 to 3), shake until curdled, filter 

 and treat the clear filtrate as directed under X, 2, 3 or 8. 



