SOME RECENT WORK ON PLANT OXIDASES 125 



roasted immediately after picking, before any appreciable oxida- 

 tion takes place. In black tea, on the other hand, an oxidation 

 of the tannin is effected by an oxidase of the laccase class, 

 resulting in the production of a soluble brown substance to 

 which the colour of the infusion is due. The pungency, on the 

 contrary, is dependent on the amount of unoxidised tannin, 

 while the flavour is caused mainly by an essential oil. By 

 carefully regulating the different processes of withering, rolling, 

 and oxidation the qualities of the tea may be altered within 

 limits. All the operations involved must be carried out with 

 the utmost cleanliness to avoid bacterial contamination as far 

 as possible, as such gives rise to sourness, rendering the tea 

 unfit for consumption. 



In the procuring of cocoa beans, too, fermentation processes 

 are involved which loosen the seeds in the fruit. In these 

 stages yeasts and acetic acid producing bacteria are active. 

 Oxidases are also at work both during the fermentation and 

 subsequent drying, as shown by Loew, 1 the change from the 

 violet colour of the fresh bean to a deep brown being due to 

 their agency. 



The curing of tobacco again involves a fermentation. The 

 leaves after a preliminary withering are " sweated " in moderate 

 sized heaps, and fermented in very large heaps containing many 

 tons. It was at one time thought that this was a bacterial 

 process, but owing to the work of Loew 2 and other American 

 chemists it has been shown to be mainly one of respiration of 

 starch, sugars, and tannin, brought about by oxidases in con- 

 junction with hydrolytic enzymes. 



Another instance of the importance of oxidases in commerce 

 is that of the blemish known as " sap stain " in lumber, by 

 which considerable portions of the wood of certain trees when 

 exposed to the air by sawing into planks or beams were dis- 

 coloured, and so depreciated in value considerably. It has been 

 demonstrated by Bailey 3 that this is brought about by an 

 oxidase. 



The preparation of ensilage has been investigated by Russell 4 



1 Loew, Ann. Report, Porto Rico Agric. Expt. Station, 1907, quoted from 

 Fowler loc. cit. 



2 Loew, loc. cit. Quoted from Fowler, loc. cit. 



3 Bailey, Bot. Gaz. 19 10, 60, 142. 



4 Russell,/. Agric. Science, 1908, vol. ii. pt. 4. 



