34 Alfrc<l J. luvorl : 



The influence of lixiJ . 



Slices of pulp dropped into l)oiling water until lieated througli 

 remain colourless in the presence of oxygen, but the pulp gives 

 strong and the colourless liquid faint reactions for tannic acid 

 FeClj . KCN, etc.). After boiling filtered apple sap with dilute 

 tannic acid, the liquid turned as black with Fe CI3 as before boil- 

 ing. A section of pulp bro^wned to lialf its deijtli by soakinir in 

 dilute tartaric acid and then Ixtiled, did not develop any 

 further brown colouration on exposure to air, but both the pale and 

 the brown parts gave tannic acid reactions (Fe CI.5 KCN). Boiled 

 colourless pulp soaked in dilute sodium hydrate or ammonia and 

 exposed to air slowly turns reddish brown by oxidation right 

 through. Hence the absence of browning in boiled pulp i.s not due 

 to any decomposition or complete removal of the tannic acid in 

 the pulp cells. 



If the pulp is slowly heated up to the lethal temperature in air 

 it turns brown, and the same occurs in water, although tlie brown- 

 ing here is less pronounced, and the water accumulates small 

 amounts of tannic acid. 



Before we can understand these results it is necessary to consider 

 the influence of acids on browning, since although solutions of 

 tannic acid rapidly oxidise when alkali is added, they are not 

 appreciably oxidised when directly exj^osed to air for a week or 

 more either when ])ure oi' in tlie presence of glucose, cane sugar, 

 or citric, tartaric <>r malic acids, or only acquire a very faint 

 yellowish tinge on very prolonged exposure to air, which necessi- 

 tates the use of sterilised spore-free solutions. 



The influence of acids on the hroirnlnii of the pulp relJx. 



When tlie pulp is immersed in any mineral or organii.' iuid 

 beyond a cei-tain strength, l)ut not strong enougli to disi-ojour the 

 cell wall, tlie pulp cells do nc)t turn l)rowti. but remain tolouricss. 

 althougli they are killed. At lower dilutif>ns still snthcii'iit to kill 

 the pulp cells, l)rowiiing takes place. 



