33 



LESSON IV 

 FOODS 



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



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

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

 (skimmed mUk). The specific gra\nty of skimmed milk is higher owing to 

 the removal of the lightest constituent — the cream. 



3. The reaction of fresh milk is neutral or sUghtly alkaline. 



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 proteid in milk, into casein. Tlie casein 

 entangles the fat globules. The hquid residue is termed ivliey. No curdling 

 is produced if the rennet solution is pre^-iously boiled, because heat kills 

 ferments. 



5. Take some mUk 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 calciimi 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 ; cm-dling 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 di-ops 

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

 the fat is formed. 



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

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

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

 magnesium) phosjjJiates by ammonia, which precipitates these phosphates. 

 Phosphates may also be detected by adding nitric acid and ammoniiun 

 niolj'bdate 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 mUk. 



9. Caseinogen, like globtihns, is precii^itated by saturating mUk with 

 sodium chloride or magnesium sulphat«, and by half saturation with 

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



heat. The precipitate produced by saturation with salt floats to the 

 '.face with the entangled fat, and the cley salted whey is seen below after 

 an hour or two. 



B. Flour. — Mix some wheat flour with a little water into a stiff dough. 

 "Wrap this up in a piece of mushn and knead it imder a tap or in a capsule of 

 water. The starch grains come through the holes in the muslin (identify by 

 iodine test), and an elastic sticky mass remains behind. This is a proteid 

 called gluten. Suspend a fragment of gluten in water ; add nitric acid and 

 boil ; it turns yellow ; cool and add ammonia ; it turns orange (xanthoproteic 

 reaction). Bo'U another fragment with Millon's reagent ; it turns a brick-red 

 colour. 



