Estimation of tea quality by infusion colour difference analysis
The colour differences in terms of deltaL, deltaa, deltab and deltaE between distilled water and infusions of 29 green teas, 16 black teas and 13 oolong teas and their correlation with sensory quality attributes of tea were investigated. The results showed that the deltaL value decreased but the deltaa, deltab and deltaE increased with the degree of fermentation of the three kinds of tea. The deltaL was positively correlated to quality attributes of green tea and oolong tea but negatively correlated to those of black tea. The deltaa of tea infusions is a reliable descriptor to differentiate among green and black teas. The regression of total quality score of green tea (TQSg) upon the deltaa was TQSg = -2.26deltaa + 77.2 (R2 = 0.22, p < 0.01), that of total quality score of black tea (TQSb) upon deltab was TQSb = 0.70deltab + 44.6 (R2 = 0.31, p < 0.05) and that of total quality score of oolong tea (TQSo) upon deltaL was TQSo = 1.09deltaL + 94.5 (R2 = 0.75, p < 0.01). Because the infusion colour difference indicators of deltaa, deltab and deltaL can be done rapidly, it could be very useful for on-site or on-line quality monitoring during tea manufacture and storage.
The effect of fermentation temperature on the quality parameters and price evaluation of Central African black teas
Tea was fermented at 15, 25 and 35°C and otherwise manufactured according to commercial practice on several occasions in early May and mid-November 1977. Fermentation at 15°C resulted in teas containing greater amounts of the desirable theaflavin class of pigments. Teas produced at the higher temperatures contained more of the thearubigin pigments, including the polymeric fraction known as the non-dialysable material. Colour development was also more rapid at 25 and 35°C. Teas produced at the low temperature were accorded higher evaluation and a linear relationship between evaluation and theaflavin content was established which explained > 70% of the variation in value data. It is suggested that, since the price premium for quality is high and the market stable, the installation of temperature control equipment to regulate the production of oxidation products would improve the quality of the product and benefit Central African tea industry.
Studies on the Mechanism of the Oxidation of Tea Leaf Catechins:Part II Formation of Diethyl 4, 4', 5, 5', 6, 6'-Hexahydroxydiphenate by the Enzymic Oxidation of Ethyl Gallate
In order to understand the mechanism of the enzymic oxidation of epigallocatechin, ethyl gallate was taken up as a model substrate on account of its close structural similarity to the w-trihydroxyphenyl group, which indeed is supposed to be attacked by oxidase, when epigallocatechin is oxidized enzymically. By the action of tea or apple oxidase at pH 5.5, it gave a characteristic polyphenolic substance, which was obtained in prisms, C18H18O10·2H2O, and proved to be diethyl 4,4′,5,5′,6,6′-hexahydroxydiphenate, as the product was converted into ellagic acid by hydrolysis of ester linkage and then lactone formation.