188 COMPOSITION OF THE DIGESTIVE JUICES. [cH. VIII. 



chloride is enormous. It is therefore of the utmost 

 importance in all quantitative experiments on the action 

 of the enzyme to ensure that about o'i per cent, of the salt 

 is present. The beneficial effect of sodium chloride on 

 salivary digestion offers a simple teleological explanation 

 of the desire for salt when eating carbohydrate foods. 



The estimation of ptyalin has been attempted by a 

 variety of methods, none of which are very satisfactory. 

 The most important of these methods are : 



1. Roberts' A chromic Point method. A given amount 

 of enzyme is added to a measured amount of starch paste 

 at 40 C. Portions of the digest are treated with dilute 

 iodine at intervals and the time when the iodine fails to give 

 a colour is noted. 



2. Wohlgemuth's method. A fixed amount of soluble 

 starch is digested with varying amounts of the enzyme for 

 a stated time. Iodine is added to each, and the amount 

 of enzyme that just converts the whole of the starch to 

 dextrin is determined. 



3. Reduction methods. Starch is digested with the 

 enzyme and the amount of maltose formed in a given time 

 is determined. 



C. Lovatt Evans (Journal of Physiology, xliv., p. 220) 

 has criticised the various methods, and claims that the 

 only correct method is a reduction method carried out 

 under certain defined conditions of starch concentration, 

 temperature, etc. His main contention is that the amount 

 of maltose formed is only proportional to the amount of 

 enzyme added, provided that 



1 . Less than 30 per cent, of the starch has been 

 converted, at which stage a blue reaction is still 

 obtained with iodine. 



2. The digestion period is a short one (ten 

 minutes). 



3. The concentration of the starch is about 3 

 per cent. 



