I3 8 PHYSIOLOGICAL CHEMISTRY. 



Thus far it has not been found possible to isolate a soluble 

 enzyme from the cell ferment. One may be present, but at- 

 tempts to obtain it have failed. 



Besides this Bacterium aceti several other vinegar ferments 

 are known. Most of them float in the air, and when lodged 

 in a weak alcohol containing certain mineral substances pro- 

 duce a fermentation quickly. A dilute aqueous solution of 

 pure alcohol will not ferment in the same way; the presence 

 of various salts and organic matters in addition is necessary. 

 An experiment may be made to illustrate vinegar or acetic acid 

 fermentation. 



Ex. If available a fruit juice, freshly expressed and left in contact with 

 the skin, should be allowed to undergo alcoholic fermentation. Or, a 

 sugar solution, as described some pages back, may be allowed to ferment. 

 The weak alcoholic liquid obtained in the case of the fruit juice will next 

 turn sour from the production of acetic acid by the action of the germs 

 on the skin. In the case of the alcohol from the sugar it may be neces- 

 sary to add a little " mother of vinegar " from a vinegar factory to induce 

 the fermentation. Presence of the air is necessary to complete the change. 

 The acid strength of the product may be finally tested by means of a 

 standard alkali solution and phenol-phthalein. 



THE OXIDASE ENZYMES. 



We come now to a very brief consideration of an obscure 

 but interesting subject ^ about which ojur knowledge is of com- 

 paratively recent origin. In certain vegetable substances re- 

 actions occur which are ascribed to the presence of a class of 

 oxidizing enzymes called oxidases. These changes are illus- 

 trated by the blackening of an apple, potato or beet which is 

 cut and exposed to the air. The cut surfaces soon turn dark. 

 If the same substances are thoroughly heated before the cut- 

 ting the color change does not follow. Potato or apple pulp 

 speedily darkens in the air, but if previously cooked the nat- 

 ural light color persists. To account for these and many sim- 

 ilar reactions it has been assumed by many chemists that the 

 fruits or vegetables in question contain an oxygen-carrying 

 enzyme and at the same time some chemical substance on 

 which this can act with the production of color, the oxygen 



