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.
Optimization of process parameters for vacuum drying ofCTC tea
Vacuum drying process could be used for drying of black CTC or orthodox tea for better quality. The level of vacuum was maintained at 28.5 inch ofHg inside the vacuum chamber. The drying parameters, such as, plate temperature during drying, the loading rate of dry solid on the plate and duration of drying were varied on the basis of Rotatable Central Composite Design (RCCD). From preliminary experiments, the ranges for drying temperature, loading rate and duration were fixed as 60 to tOO'C, 0.2 to 0.35 kg dry solid/m2 and 20 to 45 min; respectively. Moisture content ofthe product and liquor colour values (brightness, redness and yellowness) was measured. The independent parameters were optimized on the basis ofthe desired values ofthe quality parameters of product, using Design Expert Software. The optimized values ofthe drying parameters were obtained to get moisture content of product within 3 to 4% (wet basis), maximum redness and yellowness value and the brightness value closest to the ECP dried tea. The optimized alues ofthe independent drying parameters, viz. plate temperature, loading rate and duration were 88.9'(" 0.212 kg dry solid/m2 and 42 min respectively. INTRODUCfION The processing steps in manufacturing of black tea are plucking, withering, maceration, fermentation and drying. The qual ity of final tea is very much dependent on these processing steps. Chemical changes in the leaves starts at the moment they are plucked. Leaves are kept for withering in wired troughs with wired mesh with or without air flow for 12-16 h. During withering the leaves become flaccid and loose moisture and precursors of different aroma compounds develop. Withered leaves are macerated either by crush-tear­ curl (CTC) method or Olthodox process. In CTC maceration, leaves are smashed and granulated using CTC rollers; whereas, in orthodox processing, leaves are twisted and rolled on rolling table by application of pressure. Oxidation or fermentation of the leaves starts immt'diately after maceration. Characteristic colour and flavour compounds are produced by different chemical reactions in the presence of oxidative enzymes. Oxidized leaves are dried to stop the oxidation process by deactivating the enzymes and to reduce the moisture content to a lower value for increasing the storability of the material. In the tea industry, mainly two types of dryers are used. These are conventional Endless
The chemical basis of quality in tea. III.—correlations of analytical results with tea tasters' reports and valuations
Theaflavin and thearubigin contents of manufactured tea depend not only on the flavanol content, protein content, and oxidase activity of the green leaf, but also on the resistance of the leaf to mechanical damage. This resistance has less influence in C.T.C. manufacture than in conventional manufacture. Tea tasters' marks for colour and strength are related to theaflavin and thearubigin contents. Marks for briskness and quality depend to some extent on the theaflavin content, with caffeine contributing to briskness, and unidentified substances, including volatile aroma constituents, influencing quality.