PRACTICAL EXERCISES 611 



determine the specific gravity of the skimmed milk. It will be found 

 to have increased since the fat is of lower specific gravity than the 

 rest of the milk. Normal cow's milk has a specific gravity of 1,028 

 to 1,034, skimmed milk 1,033 to 1,037. 



(3) Test the reaction of the milk to litmus-paper. It is slightly 

 alkaline. 



(4) (a) Put TO c.c. of milk in a test-tube, and nearly fill it up with 

 water. Add strong acetic acid drop by drop. A precipitate of 

 caseinogen is thrown down which entangles the fat, and carries it 

 down mechanically along with it. Filter off the precipitate. Keep 

 the filtrate for (b). Wash the precipitate with water, scrape a 

 portion of it off the filter, and add to it some 2 per cent, sodium 

 carbonate solution . The caseinogen dissolves, while the fat remains in 

 suspension . The solution gives the colour reactions for proteins (p. 7) . 



(b) Test some of the filtrate by Trommer's test (p. 10) for lactose. 

 Add dilute sodium carbonate solution to another portion till it is 

 only slightly acid. Boil, and lactalbumin is coagulated. Remove 

 the lactalbumin by filtering, and test this filtrate for earthy (i.e., 

 calcium and magnesium) phosphates by adding a few drops of 

 ammonia, which precipitates them as a slight cloud. 



(c) To 5 c.c. of milk add an equal volume of saturated ammonium 

 sulphate solution. The caseinogen is precipitated, entangling the 

 fat. Filter off. The filtrate may be used to test for lactalbumin 

 by boiling. The addition of water to the precipitate of caseinogen 

 (and fat) on the filter causes the caseinogen to dissolve, as it is soluble 

 in weak salt solutions. Caseinogen can also be precipitated by 

 saturating milk with sodium chloride or magnesium sulphate. 



(5) To 5 c.c. of milk add a couple of drops of 20 per cent, sodium 

 or potassium hydroxide, and then a few c.c. of ether. Shake up. 

 The ether dissolves the fat, and the opacity of the milk diminishes. 

 Take off the ether with a pipette, evaporate away most of it on a 

 water-bath, and place a drop or two of the remainder on a filter- 

 paper. A greasy stain is left, showing the presence of the fat cf the 

 milk or butter. 



(6) Clotting of Milk. (a) To a few c.c. of milk in a test-tube add 

 a few drops of rennet. Place the tube in a bath at 40 C. In a short 

 time a clot or curd is formed, consisting of casein, which is derived 

 from the caseinogen. The fat is entangled in the clot. On standing 

 some time the clot contracts, and exudes the whey. Boil some of 

 the whey after slight acidulation with acetic acid ; the lactalbumin and 

 whey-protein are coagulated. Test another portion of whey for proteins 

 by one of the general protein tests (p. 7) e.g., the xanthoproteic. 



(b) Repeat (a) but use rennet which has been previously boiled. The 

 milk is not curdled, because the ferment has been inactivated by boiling. 



(c} To 10 c.c. of milk add 3 c.c. of i per cent, potassium' oxalate. 

 Divide the oxalated milk into three portions A, B, and C. To A 

 add a few drops of rennet, to B i c.c. of 2 per cent, calcium chloride 

 solution and a little rennet, and to C i c.c. of 2 per cent, calcium 

 chloride solution alone. Put the tubes at 40 C. Clotting will 

 occur in B, but not in A or C. 



5. Cheese. (i) Rub up some finely-grated cheese in a mortar 

 with 2 per cent, sodium carbonate solution. Filter. The filtrate 

 contains casein, which can be precipitated by adding dilute acetic 

 acid by drops to a portion of the filtrate. The precipitate is soluble 

 in excess of the acid. With another portion of the filtrate perform 

 some of the general protein tests (p, 7). ^ 



392 



