44 



PRACTICAL EXERCISES 



RELATING TO THE 



FOOD STUFFS AND ANIMAL LIQUIDS. 



I. Starch, Dextrin, Dextrose, Fat. 



! Starch, is insoluble in cold water. 



2. It dissolves imperfectly in hot water ; the liquid so obtained is opales- 

 cent. 



3. It gives a blue colour with iodine, which vanishes when the liquid is 

 heated, but returns on cooling, if the heating has not been prolonged. 



4- Dextrin is soluble in water. 



5. The solution gives a red brown colour with iodine, which vanishes on 

 heating. 



6. Dextrose (Grape-sugar) is crystalline and very soluble in water. 



7. It reduces many metallic oxides. 



8. The copper test. To a small quantity of ten per cent, solution of 

 cupric sulphate add about 5 c.c. of the liquid to be tested ; then solution of 

 caustic potash drop by drop until the solution is clear, and heat gradually. If 

 dextrose is present, the blue colour vanishes and a yellow precipitate appears 

 of cuprous hydrate, or a red precipitate of cuprous oxide. 



9. Conversion of starch into dextrose. Boil about 50 c.c. of starch solution 

 in a flask with a drop of 25 per cent, sulphuric acid for five minutes. The 

 liquid becomes limpid. It contains in addition to dextrose much unconverted 

 soluble starch. 



10. Fat. Lard is insoluble in water. By boiling with potash it yields a 

 solution of soap. 



11. Decompose the solution by adding a few drops of dilute sulphuric acid. 

 On heating, a layer of fatty acid collects on the surface. 



12. Microscopical preparations. Starch grains ; their disinte- 

 gration by hot water ; action of iodine on them. Crystalline forms of fatty 

 acids. 



II. Milk, Flour, Bread. 



I. Milk has (in London) usually an acid reaction, and a specific gravity of 

 from 1025 to 1030. After removal of the cream, the specific gravity is higher. 



