Changes undergone by free amino‐acids during the manufacture of black tea

Abstract (1) An increase in total free amino‐acids during the withering stage of tea manufacture was confirmed, and was found to be dependent on storage conditions which promote the onset of senescence in plucked tea shoot tips (especially desiccation). The rate of increase was positively related to temperature up to the point where the tissues were killed, after which no further changes took place. Individually, all the major amino‐acids increase during withering except the most abundant amino‐acid, theanine, which shows an appreciable decrease. (2) During the fermentation stage of black tea manufacture the concentrations of leucine, isoleucine, serine, glutamic acid, glutamine, threonine and phenylalanine are appreciably reduced. Other free amino‐acids undergo little change in concentration. (3) During the drying stage of tea manufacture there is a small general decrease in free amino‐acid concentration. (4) There is an active metabolism of free amino‐acids in plucked tea shoot tips. Within 3 h of feeding glycine‐ 14 C, radioactivity could be detected in all the free amino‐acids and especially in serine which contained about ten times as much 14 C as did glycine after this period. Caffeine increased in concentration and in degree of labelling with time from feeding of glycine‐ 14 C which suggests that the amino‐acids are the precursors of this purine compound in tea shoot tips. (5) The effect of these changes on the organoleptic properties of black tea are discussed.