312 PHYSIOLOGICAL CHEMISTRY 



f. Boil a few c.c. of saliva thoroughly for three or four 

 minutes, cool, and add 1 c.c. to 5 c.c. starch mucilage. 



Put all six tubes in a beaker of water warmed to 40 for 

 15 minutes. Test each solution for sugar with Benedict's solu- 

 tion and for starch with iodine. A positive Benedict indicates 

 that a portion of the starch has been broken down into a reduc- 

 ing sugar (maltose and isomaltose). If the iodine test is still 

 blue, some starch remains. If it is reddish, the solution con- 

 tains dextrine. If iodine gives no color, the material has all 

 passed to the aehroodextrin stage or beyond. 



Record the results of your observations and draw conclu- 

 sions as to the conditions under which ptyalin will digest starch. 



5. Effect of cooling. Place a test tube containing saliva in 

 a freezing mixture for a few minutes. Cool 5 c.c. of starch 

 mucilage in a separate test tube. Add 1 c.c. of the saliva to 5 

 c.c. starch mucilage and leave in a freezing mixture for 15 

 minutes. In a portion, test for sugar with Benedict's solution. 

 If the solutions have been well cooled, little or no digestion will 

 have taken place, at least much less than at body temperature. 



The most favorable reaction for ptyalin digestion is a very 

 weak acidity (Hydrogen-ion concentration N X 10' 5 - 3 ). A 

 hydrogen-ion concentration of N X 10' 4 is sufficient acid to 

 stop its action. It will act in a weakly alkaline solution such 

 as the saliva, however. The acidity of the contents of tube 

 number 5 above should be sufficient to destroy ptyalin. 



6. On Cane Sugar. Add saliva to a few c.c. of cane sugar 

 solution. Digest at 40 for 15 minutes and test with Benedict's 

 solution. Does saliva contain an invertase? 



On Proteins and Fats. Proteins and fats are not digested 

 by saliva, since this secretion contains no proteolytic or lipolytic 

 enzyme. The erepsin mentioned above is believed to be unim- 

 portant, and acts not on proteins, but on peptids. 



7. Progress of Digestion ~by Ptyalin. To 50 c.c. of starch 

 mucilage add 10 c.c. of saliva. Digest at 40 and at intervals of 

 a minute or two, remove a few drops and test with iodine. From 



