Ccxxxvii. the Fermentation Process in Tea Manufacture

THE fermentation of the tea leaf is a stage in the manufacturing process of tea which has received but little attention in the past. Various qualitative studies have been made at the several Tea Research Institutes but no thorough investigation has been made apart from that of Oparin et al. [1935]. Results in this field have a considerable commercial use and they are also ofinterest to the biochemist, not only as an illustration ofthe growing importance of enzyme chemistry in industry but also as a detailed study of the chemical changes taking place in damaged plant tissue. Before dealing with the fermentation process itself it, may be of advantage to outline the various stages in tea manufacture. After plucking, the leaf is spread out in thin layers to undergo withering. During this process which usually takes about 18 hr. the leaf loses water and becomes flaccid. Unless this condition of flaccidity is produced the leaf is broken into flakes, instead of being twisted in the subsequent rolling process, and the resultant dry tea loses market value on account of its appearance. Not only is the withering process a physical preparation of the leaf for the further stages of manufacture but also some chemical changes may take place which partly determine the final quality of the manufactured tea. The nature of these changes however is still obscure. The next stage in manufacture is rolling. The primary object of this process is to rupture the leaf-cells and to expose the expressed juices to the air. At the same time the tea receives its characteristic twist. As soon as the leaf tissue is in any way damaged fermentation starts, so that it is impossible to separate the rolling process from the next stage which is that of fermentation. After a period of about 12 hr. rolling the leaf is spread thinly on a fermenting floor, usually of polished concrete, where the oxidation process which commenced in the rollers is continued for perhaps another 2 hr., after which the tea is fired to stop fermentation and to dry the leaf. During fermentation oxidative changes take place which largely determine the flavour and colour of the infused liquor. The amount of tannin extractable by hot water is considerably reduced, and some of the colourless tannin is transformed into red and brown products which arise from the original tannin, partly by oxidation and possibly partly by condensation. The precise mechanism ofthese changes is not yet properly understood.