174 ORGANIC AGRICULTURAL CHEMISTRY 



The function of hydrochloric acid in the gastric juice is also 

 somewhat in doubt. It is probably not, as has been supposed, 

 to assist in the hydrolysis of the protein, though the action of 

 pepsin itself is best in acid solution. Its most important func- 

 tion is probably that of a germicide to prevent the action of 

 putrefying bacteria. It has been found that hydrochloric 

 acid of approximately the same strength as that found in the 

 gastric juice acts best as a germicide. We have stated that 

 free hydrochloric acid of much less strength, viz. 0.003 to 

 0.006 per cent, destroys the activity of ptyalin. When, how- 

 ever, the free hydrochloric acid reacts with protein it forms 

 protein salts, or, as it is then termed, combined hydrochloric acid. 

 This combined acid is not nearly so effective as a germicide 

 as the free acid. The food mass as it comes from the mouth 

 remains for some time below 0.003 P^^ ^^^t acidity as the acid 

 in the gastric juice first combines with the protein and only 

 gradually exerts an influence as free acid, thus destroying 

 ptyalin action. Eventually the whole food mass becomes 

 saturated with free acid of about 0.2 per cent and is rendered 

 germ-resistant while the action of ptyalin is destroyed. 



EXPERIMENT STUDY XXVin 



Gastric Digestion 



(i) Action of Pepsin {a) Preparation of Egg Albumen. Cut some 

 glass tubing of small diameter (2-3 mm.), in twelve-inch lengths. 

 Using a tube as a pipette, suck it one half full of fresh egg albumen. 

 Holding the finger over the mouth end, insert the portion containing 

 the albumen into a beaker full of water at 8o°-9o° C, keeping the 

 finger over the end until the albumen has coagulated. Heat the 

 water to 100° and allow the tube to remain in the water until the 

 albumen is thoroughly coagulated. Prepare several tubes in this 

 way. When the albumen is all hard remove the tubes from the 

 water and with a sharp file scratch and break the tube containing 

 coagulated albumen into one half or one fourth inch lengths. 

 These sections of tubing containing coagulated albumen are now 

 used as test pieces for studying the action of pepsin, {h) Prepare 



