40 PROVISIONAL METHODS FOR ANALYSIS OF FOODS. 



2. SELECTION OF SAMPLE. a 



When the cheese can be cut take a narrow wedge-shaped segment reaching 

 from the outer edge to the center of the cheese. Cut this into strips and pass 

 through a sausage-grinding machine three times. When the cheese can not be cut 

 take the sample with a cheese trier. If only one plug can be obtained take it per- 

 pendicular to the surface of the cheese at a point one-third of the distance from the 

 edge to the center and extending either entirely or only half way through it. When 

 possible draw three plugs one from the center, one from a point near the outer 

 edge, and one from a point halfway between the other two. For inspection purposes 

 reject the rind; but for investigations requiring the absolute amount of fat in the. 

 cheese include the rind in the sample. It is preferable to grind the plugs in a 

 sausage machine, but when this is not done they are cut very fine and carefully 

 mixed. 



3. SEPARATION OF FAT FOR EXAMINATION^ 



(a) FIRST METHOD. 



Cut about 300 grams of cheese into fragments the size of a pea. Treat with 700 cc 

 of potassium hydroxid (50 grams per liter) at 20 C. in a large wide-necked flask, 

 and promote the solution of casein by vigorous shaking. In from 5 to 10 minutes 

 the casein will be dissolved and the fat will come to the surface in lumps. Collect 

 the lumps of fat into as large a mass as possible by a gentle shaking to and fro. Pour 

 cold water into the flask until the fat is driven up into the neck and remove it by 

 means of a spoon. Wash the fat thus obtained with as little water as will remove the 

 residue of the lye which it may contain. Experience has shown that the fat is not 

 perceptibly attacked by the lye in this treatment. By this method the fat is practi- 

 cally all separated in a short time and is then easily prepared for chemical analysis 

 by filtering and drying as directed in the official method/ 



(b) SECOND METHOD. 



Grind the cheese by passing it through a meat-cutting machine. Transfer it to a 

 large flask and pour warm water upon it, using 1 cc for every gram of cheese. Shake 

 thoroughly and add sulphuric acid (sp. gr. 1.82 to 1.825) slowly and in small quan- 

 tities, shaking after each addition of acid. The total amount of acid used should be 

 the same as the amount of water used. Remove the fat, which separates after stand- 

 ing a few minutes, by means of a separatory funnel, wash it free from acid, filter, 

 and dry. 



4. DETERMINATION OF WATER. e 



Place from 2 to 5 grains of cheese in a weighed platinum dish containing a small 

 quantity of porous material such as ignited asbestos or sand, to absorb the fat which 

 may run out of the cheese. Heat in a water-jacketed bath for ten hours and weigh; 

 the loss in weight is considered as water. Or, if preferred, place the dish in a desic- 

 cator over concentrated sulphuric acid and dry to constant weight. Renew the acid 

 when the cheese has become nearly dry. 



5. DETERMINATION OF FAT. e 



Cover the perforations in the bottom of the extraction tube with dry asbestos, and 

 on this place a mixture containing equal parts of anhydrous copper sulphate and 



U. S. Dept. of Agr., Div. of Chem., Bui. 46 revised, p. 55. 



b See also Appendix, p. 152. 



C U. S. Dept. of Agr., Div. of Chem., Bui. 51. 



d U. S. Dept. of Agr., Div. of Chem., Bui. 46 revised, p. 44, 3 (a). 



U. S. Dept. of Agr., Div. of Chem.. Bui. Id revised, p. 56. 



