TRANSFERENCE OF FOOD MATERIALS 121 



difference in the chlorides gives the amount of hydrochloric 

 acid. In the first incineration all the chlorine not combined 

 with fixed bases is driven off whilst in the second owing to the 

 excess of alkali all the chlorine is retained. The difference 

 is therefore due to hydrochloric acid free or combined with 

 organic bases such as proteins. 



The indicators used to show the presence of acid in gastric 

 contents are numerous, but the following are good examples : 



TABLE XXIII 



Indicator. log[H] Remarks. 



Phenol Blue . . 1-2-2-8 ) Does not change colour if only organic 



Methyl Violet ... 4 j acids are present. 



Gunzberg's Reagent . 4 i in 10,000 HC1, does not change 



colour if only organic acids are 

 present. 



Tropacolin oo . Acetic acid gives colour, but it dis- 



appears on warming. 



Congo Red . . . Reacts with organic acids. 



Phenolphthalein . . 8-9 Gives total acidity including carbonic 



acid to stage of bicarbonate. 



The methods used to show the rate of digestion of protein 

 are numerous. They consist in comparing the rate of digestion 

 of a protein by an enzyme with a control containing exactly 

 the same materials but with the enzyme destroyed (by heat) 

 before adding it to the other ingredients. The methods 

 depend on measuring either the rate of solution of an insoluble 

 protein or the rate of production of nitrogenous compounds 

 which are not precipitated by protein precipitants. 



Methods based on rate of solution of insoluble protein : 

 Rate of solution of fibrin.* 

 Depth of colour due to liberation of a dye from stained 



fibrin, f 

 Length of glass tube from which coagulated egg albumin J 



or gelatine have been dissolved. 

 Clearing of an opalescent suspension of insoluble protein 



(edestin, egg albumin). || 



Methods based on liberation of soluble nitrogenous compounds : 

 Removal of proteins by heat coagulation and estimation of 

 nitrogen remaining in the solution by Kjeldahl's 

 method. 



* A. Gruenhagen, Arch. f. d. ges. PhysioL, 1872, vol. 5, p. 203. 

 f P. Griitzner, Arch. f. d. ges. PhysioL, 1874, vol. 8, p. 452. H. E. 

 Roaf, Journ. PhysioL, 1919, vol. 53, proc. p. Ixvii. 

 J S. G. Mett, Arch. f. PhysioL, 1894, p. 58. 

 C. Fermi, Arch. f. Hygiene, 1890, vol. 10, p. i. 

 || E. Fuld and L. A. Levison, Biochem. ZeiL, 1907, vol. 6, p. 473, 



