FERMENTS 243 



part of the starch we eat is digested, that is changed 

 into a sugar, while the food is in the intestine and 

 under the action of the pancreatic juice. The chief use 

 of the saliva is mechanical rather than chemical, inasmuch 

 as it moistens the food and thereby assists mastication 

 and makes deglutition, or the swallowing of food, easy. 



8. Soluble Ferments or Enzjrmes.— The peculiar 

 substance, ptyalin, to which the chemical action of saliva 

 on starch is due, belongs to a class of substances known 

 as soluble ferments or enzymes. The word ferment was 

 originally applied to a living organism such as yeast which, 

 as in the case of brewing, while converting the sugar in 

 the wort into alcohol causes at the same time, on account 

 of the simultaneous production of carbonic acid gas, a 

 boiling up or frothing of the liquor ; hence the name 

 ferment (fervere = to boil up). 



But it is known now that such organised ferments can 

 be made to yield extracts which may be filtered so as to 

 be quite free from organisms and still be able to produce 

 the same changes as did the cells from which they are 

 prepared. Hence the name of soluble ferment or enzyme 

 (^ii/-tj; = j'east) was given to the substance in solution which 

 can bring about the same changes as the parent cell. 



Very little is known of the chemical nature of enzymes, 

 but they are strongly characterised by certain facts as to 

 the conditions under which their action takes place. 

 Thus : (i) Very minute quantities will effect a change in 

 a mass of the substance on which they are working which 

 is enormously large compared with the minute mass of 

 the enzyme, (ii) Their action depends closelj' on tempera- 

 ture. At 0' C. (32'- F.) they cease to act ; as the tempera- 

 ture rises they become increasingly active, and are most 

 active at about 40' C. (104' F.). At higher temperatures 

 they become less active and lose their powers permanently 

 if once heated to 100 C. (212' F.) as by boiling : they are 

 then said to be " killed." (iii) Their action in many cases 

 depends on the reaction; whether acid or alkaline or 



B 2 



