286 Prof. A. J. Ewart. Comparative Study of Oxidation by 



The Influence of Poisons on Browning. 



As is well known, apple and potato pulp if killed by dropping into boiling 

 water, remains colourless in the presence of oxygen for an indefinite length of 

 time, and this according to Bourquelot is due, in the case of the apple, to the 

 destruction of the oxidase responsible for browning. When portions of apple 

 pidp are immersed in very dilute sulphuric or tartaric acids, the pulp turns 

 brown, whereas in stronger solutions it remains colourless. This might be due 

 to the stronger acid inhibiting or destroying the oxidase ferment, or 

 preventing its formation if only present as a zymogen in the living cell. 



Pulp pounded in its own volume of 1-per-eent. H2SO4 remains colourless 

 and gives no guaiacum test and no distinct decomposition of H2O2. If pounded 

 and allowed to brown before adding the sulphuric acid, no oxidase reactions are 

 shown after, but active ones before the addition of the acid. Slices of fresh 

 pulp decompose H 2 2 actively and also turn guaiacum blue. If pulp pounded 

 with 10-per-cent. H2SO4 is neutralised with ammonia and tested, it gives no 

 oxidase reactions. Apparently, therefore, the sulphuric acid acts directly 

 by destroying the oxidase present in the living cells. 



Pieces of apple pulp immersed in poisonous metallic solutions develop a 

 brown colour on drying, and the same is shown whatever the concentration. 

 In 1 and 5-per-cent. solutions of lead nitrate, however, the browning is fainter 

 than usual and is mainly confined to the veins. Lead nitrate destroys 

 oxidase ferments and hence apparently the production of browning and the 

 presence of oxidase are not exactly parallel. The addition of dilute ammonia 

 rapidly turns the pulp a deeper brown, but the immediate addition of dilute 

 H2SO4 or HC1 restores the original pale colour. Hence the browning 

 produced by ammonia is not quite the same as the permanent brown produced 

 in slowly dying pulp cells. If the pulp is soaked in dilute ammonia for some 

 hours, acids will not, however, entirely remove the brown colour. 



When pieces of pulp are soaked in a poisonous solution, a certain time 

 elapses between the first penetration of poison and the death of each cell, and 

 this time-interval will be greater in the case of deeply seated cells than of 

 superficial ones. This is well shown when prepared potatoes are immersed in 

 5 or 10-per-cent. solutions of lead nitrate. Chromogen oxidation only takes 

 place towards the inner boundary of the diffusion zone. 



To eliminate the time factor the pulp was rapidly pounded in a mortar 

 with the poisonous solution and then tested with guaiacum and H 2 2 . French 

 crab apples were used. 



