Martin Obanda

Changes in the chemical and sensory quality parameters of black tea due to variations of fermentation time and temperature

Abstract The changes in theaflavins and residual catechin compositions, thearubigins TRSI and TRSII, sensory characteristics of total colour, brightness and briskness of black tea were investigated. It was demonstrated that the degradation of individual theaflavins varied during fermentation. Decline in the levels of individual theaflavins was influenced by both duration and temperature and coincided with decline in brightness and briskness. ECG and EGCG were the main residual catechins in black tea. The formation of thearubigins TRSI and TRSII, differed in their response to fermentation duration and temperature. The effects of processing parameters, fermentation temperature and duration, on chemical quality and tea liquor sensory characteristics are discussed.

Changes in thearubigin fractions and theaflavin levels due to variations in processing conditions and their influence on black tea liquor brightness and total colour

Increase in coarser plucking standards significantly (P<0.05) depressed levels of theaflavin (TF; μmol/g), total colour and brightness (spectrophotometer) of black tea. But total thearubigins (TR) TRSI, TRSII and brightness (taster) levels did not significantly (P<0.05) change with plucking standards. Increasing fermentation duration led to rise in TF (μmol/g), total TR, total colour, TRSI and TRSII, liquor brightness (taster) but not liquor brightness determined by spectrophotometer method (P<0.05). Correlation analysis showed that total TR and TRSII had negative correlations (r=−0.66 and −0.77, P<0.01), respectively, while total TF levels had positive correlation with taster brightness (r=0.57, P<0.01). Total TF level had highest correlation with spectrophotometer liquor brightness (r=0.87, P<0.01) for a single substance. TRSII and total TR gave an r of 0.86 indicating that the two groups of substances were strongly correlated to each other. Regression analysis showed that the direct linear model gave the best fit for the sample data studied. The coefficient of multiple determination (R2) was 0.606 in linear model I for the tasters’ liquor brightness. Thus, the independent variables TF and total TR explained 60.6% of the total variation in liquor brightness scores observed. When TRSI and TRSII were included in the linear model II instead of total TR, but with TF maintained, the coefficient of multiple determination improved to 78.9%. This confirmed that the brightness attribute of black tea, assessed by the taster, could best be explained by the combination of TF and TRSII and that TRSI had a lesser role in the tasters’ evaluation of liquor brightness. Indeed, the test statistic in linear model I showed that the coefficient of TF positively and significantly influenced liquor brightness at P<0.01 whereas the coefficient of total TR negatively and significantly influenced liquor brightness (P<0.0001). However, in linear model II the effect of total TR on the taster brightness was clearly unmasked and the influence of each individual component well elucidated. The coefficient of TRSII was negative and significantly explained liquor brightness at 0.01% level (P<0.0001). The coefficient of TRSI was negative but insignificant. Hence it has no discernible influence on the taster liquor brightness. The coefficient of TF was positive and significantly explained taster liquor brightness (P<0.01). Thus, TF and TRSII explain taster liquor brightness and the lower the level of TRSII the higher the score for brightness. The situation for spectrophotometer brightness was somewhat different. The coefficient of multiple determination was 0.896 in linear model I. Thus, TF and total TR explained 89.6% of the variation in spectrophotometer brightness. The coefficient of TF was positive whereas that of total TR was negative. Both coefficients were significant (P<0.0001). When TRSI and TRSII were included in the linear model II instead of total TR, R2 improved to 91%. Unlike for the tasters’ brightness, the coefficient of TRSI was now significant (P<0.05). The coefficient of TRSII was negative and significant (P<0.01) whereas the coefficient of TF was positive and significant (P<0.0001). The differences in the contributions of TRSI and TRSII to black tea liquor brightness and the observed discrepancy between the test methods, due to variations in plucking standards and fermentation duration, are discussed.

Clonal variation in the individual theaflavin levels and their impact on astringency and sensory evaluations

Abstract HPLC analysis of Kenya clonal black tea liquors revealed the presence of four major theaflavins in the following order: theaflavin > theaflavin-3-gallate > theaflavin-3,3′-digallate > theaflavin-3′gallate. The total amounts and the ratios of the individual theaflavins varied with clones resulting in variations in the astringency of the teas as measured by theaflavin digallate equivalent. Theaflavin digallate and theaflavin digallate equivalent showed a better relationship with sensory evaluation than did total (Flavognost) theaflavins.

COMPARATIVE RESPONSES IN PLAIN BLACK TEA QUALITY PARAMETERS OF DIFFERENT TEA CLONES TO FERMENTATION TEMPERATURE AND DURATION

Abstract The amounts of black tea theaflavins, brightness, and sensory evaluations varied with clones in the order clone 6/8>SC12/28>S15/10, while thearubigins and total colour changed in the reverse order. The rates of change of these plain tea quality parameters varied in all clones causing significant (P⩽ 0.05) fermentation duration and clone interactions. Thus, the total amount and rate of development of each quality parameter is unique to a clone and a change in fermentation duration for optimal quality parameter achievement in one clone cannot be extrapolated to another clone. Although processing of black tea at low fermentation temperatures improved black tea quality, there was no significant difference between fermenting at 15 and 20°C. Long fermentation duration and high temperature favoured production of more intense coloured black teas with high thearubigin levels, which were less bright and had lower theaflavin levels. There were significant (P⩽ 0.05) interactions between fermentation duration and temperature in all the plain black tea parameters development indicating that their rates of formation and amounts formed varied with time at different temperatures. Clones 6/8 and SC12/28 plain tea quality parameters were more sensitive to temperature and duration changes than clone S15/10. Thus there are clones, which can withstand high temperature and long fermentation duration without drastic impairment of their plain black tea quality parameters.

The Impact of Withering Temperature on Black Tea Quality

High leaf temperatures during the withering process of black tea manufacture decrease the theaflavins, brightness, flavour index and sensory evaluation scores of black tea. Black teas manufactured with withering temperature above 30°C have high thearubigins and total colour levels but lack briskness. Results suggest a need to control temperatures below 30°C during withering.

Clonal variations in the response of black tea quality due to plucking standards

Abstract Black tea quality, as measured by chemical quality parameters and sensory evaluations, declines with coarse plucking standards. The extent of the decline varies with clone. Clone 6 8 , with a high polyphenol content in the green leaf had a higher decline than clone S 15 10 with low polyphenol content results suggest a possible variation of recommended plucking standard for different clones.

Quality response of clonal black tea to nitrogen fertiliser, plucking interval and plucking standard

Variations in the black tea quality of high-yielding clone S15/10 in response to rates of NPKS 25:5:5:5 fertiliser of 200 and 400 kg Nha ˇ1 year ˇ1 , plucking intervals of 7, 14 and 21 days and a selective plucking standard of up to two leaves and a bud or an unselective plucking standard were studied. Generally, quality declined with longer plucking intervals and unselective plucking. Although there was a general decline in quality with increasing nitrogen rate, only the black tea total colour declined significantly on increasing the nitrogen rate from 200 to 400 kg Nha ˇ1 year ˇ1 . For each nitrogen rate and each plucking interval, unselective plucking reduced the black tea quality. No significant interactions between any two of the three (nitrogen rate, plucking interval and plucking standard) or all three factors were noted, indicating that the patterns of response were similar. The results demonstrate that black tea quality changes due to the factors studied occur in the same pattern with variations in treatments. Poor black tea quality due to any of the factors studied cannot therefore be corrected by varying the other factors. # 2000 Society of Chemical Industry

The effects of blending clonal leaf on black tea quality

Abstract Although most clones are selected for quality and yields, there are clones in production with superior yields but producing inferior black tea quality and vice versa . Some of these clones with superior yield give very poor black tea quality characteristics. Such clones are normally not progressed in selection programmes. Blending the clonal leaf prior to maceration results in the production of black teas with average quality relative to the amount of the clonal leaf in the mixture. There is no evidence of clonal incompatibility. Slow fermenting clones with superior yields may therefore be selected and planted, provided they are not planted in single stands. ©