LESSON VI 

 FOODS 



A. Milk. 1. Examine a drop of milk with the microscope. 



2. Note the specific gravity of fresh milk with the lactometer ; compare 

 this with the specific gravity of milk from which the cream has been removed 

 (skimmed milk). The specific gravity of skimmed milk is higher owing to 

 the removal of the lightest constituent the cream. 



3. The reaction of fresh milk is neutral or slightly alkaline to litmus. 



4. Warm some milk in a test-tube to the temperature of the body, and 

 add a few drops of rennet. After standing, a curd is formed from the con- 

 version of caseinogen, the chief protein in milk, into casein. The casein 

 entangles the fat globules. The liquid residue is termed whey. No curdling 

 is produced if the rennet solution is previously - boiled, because heat kilfs 

 ferments. 



5. Take some milk to which 0*2 per cent, of potassium oxalate has been 

 added ; warm to 40 C. and add rennet. No curdling takes place because 

 the oxalate has precipitated the calcium salts which are necessary in the 

 coagulation process. 



Take a second specimen of oxalated milk and add a few drops of 2-per- 

 cent, solution of calcium chloride, and then rennet ; curdling or coagulation 

 takes place if the mixture is kept warm in the usual way. 



6. To another portion of warm milk diluted with water add a few drops 

 of 20-per-cent. acetic acid. A lumpy precipitate of caseinogen entangling 

 the fat is formed. 



7. Filter off this precipitate, and in the filtrate test for lactose or milk 

 sugar by Trommer's test (see Lesson II.) ; for lact-albumin by boiling, or by 

 Millon's reagent (see Lesson IV.) ; and for earthy (that is, calcium and 

 magnesium) phosphates by ammonia, which precipitates these phosphates. 

 Phosphates may also be detected by adding nitric acid and ammonium 

 molybdate and boiling ; a yellow crystalline precipitate is formed. 



8. Fat (butter) may be extracted from the precipitate by shaking it with 

 ether ; on evaporation of the ethereal extract the fat is left behind, forming 

 a greasy stain on paper. The presence of fat may also be demonstrated by 

 the black colour produced by the addition of osmic acid to the milk. 



9. Shake up a little milk with twice its volume of ether ; the opacity of 

 the milk remains nearly as great as before. Repeat this, but first add to the 

 milk a few drops of caustic potash before adding the ether and then shake. 

 The milk which lies beneath the ethereal solution of fat becomes trans- 

 lucent. As a matter of fact ether dissolves the fat without the addition of 

 alkali, and the opacity of milk is therefore not due to the fat globules alone, 

 but largely to their caseinogen envelope. The clearing which takes place 

 when ether and alkali are added is due to an action of the reagents on the 

 caseinogen. 



10. Caseinogen, like globulins, is precipitated by saturating milk with 

 sodium chloride or magnesium sulphate, and by half saturation with 

 ammonium sulphate, but differs from the globulins in not being coagulated 



