PRACTICAL EXERCISES 719 



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



(4) (a) Put 10 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 casein- 

 ogen 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. 8). 



(6) Test some of the filtrate (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) phos- 

 phates 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 - * m il k a( ld. 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 of 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. 8) 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 solu- 

 tion 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. 



6. 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. 8). 



(2) Shake up some finely-grated cheese in a dry test-tube with ether. 

 Take off the ether with a pipette, and evaporate on a water-bath till 

 only a few drops remain. With a glass rod put a drop of the ether on 

 a piece of filter-paper. A greasy spot is left, showing that fat is present. 



7. Flour. (i) Mix some wheat-flour v/ith a little water into a stiff 

 dough. Let it stand for a few minutes at body-temperature to facilitate 

 the formation of gluten. Wrap a piece in cheese-cloth, forming a kind 



