60 PHYSIOLOGICAL CHEMISTRY 



This body should soon be transformed into erythro-dextrin which gives a red 

 color with iodine, and this in turn should pass into achrob -dextrin which gives no 

 color with iodine. This is called the achromic point. When this point is reached 

 test by Fehling's test to show the production of a reducing body. A positive 

 Fehling's test may be obtained while the solution still reacts red with iodine 

 inasmuch as some maltose is formed from the soluble starch coincidently with 

 the formation of the erythro-dextrin. How long did it take for a complete trans- 

 formation of the starch? For a graphic representation of the above changes see 

 page 56. 



13. Separation of the Products of Salivary Digestion. To 25 c.c. of starch 

 paste in a small beaker add i c.c. of saliva and stir thoroughly. At intervals of 

 one minute test a drop of the mixture by the iodine test. If the blue color per- 

 sists after five minutes add another i c.c. of saliva. When the mixture reacts 

 red with iodine, indicating that erythrodextrin has been formed, add 100 c.c. of 

 95 per cent alcohol. Allow to stand until the white precipitate has settled. 

 Filter, evaporate the filtrate to dryness, dissolve the residue in 5-10 c.c. of water 

 and try Fehling's test (page 26) and the phenylhydrazine reaction (see Glucose, 3, 

 page 22). On the dextrin precipitate try the iodine test (page 45). Also 

 hydrolyze the dextrin as given under Dextrin, 4, page 48. 



14. Digestion of Dry Starch. In a test-tube shake up a small amount of dry 

 starch with a little water. Add a few drops of saliva, mix well, and allow to 

 stand. After 10-20 minutes filter and test the filtrate by Fehling's test. What 

 is the result and why? 



Dry starch is very slowly digested by salivary amylase. 



15. Digestion of Inulin. To 5 c.c. of inulin solution in a test-tube add 10 drops 

 of saliva and place the tube in the incubator or water-bath at 4oC. After one- 

 half hour test the solution by Fehling's test. 1 Is any reducing substance present? 

 What do you conclude regarding the salivary digestion of inulin? 



16. Influence of Temperature. In each of four tubes place about 5 c.c. of 

 starch paste. Immerse one tube in cold water from the faucet, keep a second at 

 room temperature, and place a third in the incubator or the water-bath at 4OC. 

 (If the temperature of the bath or incubator is allowed to rise to 7oC. or over the 

 enzyme is destroyed and no digestion takes place.) Now add to the contents of 

 each of these three tubes two drops of saliva and shake well ; to the contents of 

 the fourth tube add two drops of boiled saliva. Test frequently by the iodine 

 test, using the test-tablet, and note in which tube the most rapid digestion occurs. 

 Explain the results. 



17. Influence of Dilution. 2 Take a series of six test-tubes each containing 

 9 c.c. of water. Add i c.c. of saliva to tube i and shake thoroughly. Remove 

 i c.c. of the solution from tube i to tube 2 and after mixing thoroughly remove 

 i c.c. from tube 2 to tube 3. Continue in this manner until you have 6 saliva 

 solutions of gradually decreasing strength. Now add starch paste in equal 

 amounts to each tube, mix very thoroughly, and place in the incubator or water- 



1 If the inulin solution gives a reduction before being acted upon by the saliva it will be 

 necessary to determine the extent of the original reduction by means of a "check" test (see 

 P- 47)- 



2 The technic of Wohlgemuth's method (see Chapter X) may be employed in this test 

 if so desired. 



