The need to understand the chemistry of tea manufacture was underscored in the early 1950s when the Tea Research Institute of East Africa was established[i]. The history of early work is presented in the following sections.
The tea research problems were outlined by Eden in 1954[ii] and he speculated on the morphological basis of tea quality[iii]. Thespeculations formed the basis of further experiments. However, greatest effort has been put into investigations of the chemical aspects of tea quality.
Chemical and biochemical aspects of tea quality
Chemistry of fresh leaves
It was recognised that it is necessary to quantify the total polyphenol content and polyphenol oxidase activity in the leaf as this could be related to liquor properties ofresultant black teas[iv]. The yield of dry enzyme powder from 10 g of fresh leaf was 1.3 g to 1.4 g (13419). Total oxidisable matter had little seasonal variation while enzyme activity fluctuated more with time of the year[v]. The accuracy of the measurements of the parameters was however less certain. During the Second Tea Research Institute of East Africa conference, a lecture was also given onthe chemistry of the tea leaf in relation to manufacture[vi]. However early techniques for processing tea were summarised by Hainsworth in 1969[vii].
Parameters of processed tea quality
Attempts were made to associate the chemical parameters of tea to taste[viii]. These studies revealed that the stimulating action of tea was due to caffeine and the strength and quality of the copper colourdue to the oxidation of the catechins to theaflavin and thearubigin. Both theaflavin and thearubigin form complexes with caffeine and protein which together with the aromatic compounds define the character of tea.
Quality is normally subjectively described by the tea trade. Producers define quality tea as that whose sale leads to higher income. Tea traders define quality tea asthat which gives maximum profit, while consumers define quality tea as one with overall taste and class. Research prompted by the requirement of the tea trade has been conducted to define chemical indicators of quality. Black tea can be assessed by the eye (look), tongue (taste) and nose (smell). These factors can be due to different quality parameters. The black teas produced in most Africancountries are termed as "plain" i.e. lacking in aroma. These teas are normally priced for their taste and colour parameters, factors which are due to theaflavin, thearubigin and caffeine levels[ix]. However, aroma can be an important Kenyan tea quality parameter[x]. Kenyan teas were noted to have very high theaflavin levels[xi] compared to Central African tea (234889) and the early quest for thechemical basis to Kenyan black tea quality was mainly centred on the study of these. Work in Malawi[xii] had shown that for Central African black teas, there existed a relationship between the theaflavin levels and quality as measured by sensory evaluation and/or prices. Several studies were therefore conducted to evaluate the possibility of relating Kenyan tea quality to theaflavin content[xiii]. Thesestudies revealed that although the linear relationship between theaflavin levels and sensory evaluations and/or price of tea were positive, the regression coefficient was not significant (234889). Theaflavin content was therefore concluded to be important[xiv] for Kenyan black tea quality, but it was not the sole objective quality indicator[xv]. It was recognised that there were problems with thetheaflavin analysis as the results were not inter-laboratory reproducible. The methods of theaflavin analysis were therefore examined[xvi]. Factors causing the non-reproducibility were investigated and were found to be related to infusion temperatures, altitude at which infusion was done, size and shape of the infusion vessel, and pH of the infusing water[xvii]. The methods of assaying...
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