DIGESTION IN THE MOUTH. 



291 



several members of the starch group C 13 H 20 10 , and the assumption that 

 the molecule of soluble starch has the formula of 10(C 12 H 20 10 ) greatly 

 facilitates the comprehension of the progressive hydrolysis of starch by 

 diastase (Brown and Heron). 



According- to this view, the composite molecule of soluble starch is 

 resolved through the action of diastase into two molecules of achroo- 

 dextrin and eight molecules of maltose by the following succession of 

 steps : 



One molecule of soluble starch --10(C 12 H 20 O 10 ) -f 8(H 2 O) = 



1. Erythrodextrin, 



2. 



3. Achroodextrin, 



4. 



5.- 



6. 



7. 



8. 



a 9(C 12 H 20 O 10 ) -f (C 12 H 22 O ai ) maltose, 



ft 8(C 12 H 20 10 )+2(C 12 H 22 11 ) 



a 7(C 12 H 20 10 )+3(C 12 H 22 :1 ) 



n a.f(* w o \ i ^/n XT (\ \ 



The final result is thus represented by the equation : 



Soluble Starch. 



Water. 



Maltose. 



Achroodextrin. 



Through the action of the diastatic ferment, therefore, the large 

 molecule of gelatinous starch is first separated into its component mole- 

 cules of soluble starch, and if we assume that this composite molecule 

 of soluble starch is composed of an aggregation of ten groups of the 

 radical C 12 H 20 O 10 , then progressively each one of these radicals assumes 

 one atom of water and becomes a molecule of maltose, the remainder of 

 the starch molecule, at the withdrawal of each radical, constituting one 

 molecule of the intermediary dextrin series. The dextrin molecule thus 

 becomes smaller and smaller, that is, contains fewer and fewer component 

 radicals, the higher dextrins giving a red with iodine, while the lower 

 dextrins give no reaction with iodine (Roberts). 



In order that starch should be converted by saliva into sugar, it 

 is necessary that the fluid be kept at the temperature of about 39 or 

 40 C. A temperature elevated above this will prevent conversion by 

 destroying the ferment, while a lower temperature will retard it, and the 

 temperature of freezing will prevent it completely, although the power 

 is not lost and may be regained when the temperature is again elevated. 

 This transformation is produced in a neutral or feebly alkaline medium, 

 and also, though to a much less degree, in a weak acid medium. An 

 excess of alkali, or even a slight degree of acidity (half of 1 per cent, of 

 hydrochloric acid), will prevent it completely. This is a point worthy 

 of note, since it indicates that the degree of acidity present in the 

 gastric juice during active digestion is sufficient to interrupt the action 



