Optimization of withering time and fermentation conditions during black tea manufacture using response surface methodology

ABSTRACT The central composite design (CCD) technique, a response surface methodology (RSM), was used to study the effect of the withering time and the fermentation conditions of the tea leaves on the black tea quality parameters. A three level, three variable design was adopted. The three independent variables investigated in this experiment, each at three levels, were withering time, the fermentation duration and the fermentation temperature. Responses were represented mathematically by second-order quadratic equation and assessed using polynomial multiple regression model. The highest levels of the theaflavins (TFs) formation (1.25%) was with the withering time at about 19-20 hrs while the fermentation duration was at about 25-30 min (fermentation temperature= 25C). The gradual increase in the thearubigins (TRs) as the function of the withering time and the fermentation temperature was noticeable. The increase in TR content at 11.5% was minor and related to the narrower ranges of the two parameters studied (x1 and x3). The predicted dependence of the TLC evaluation (%) (spectrophotometric measurement) on the fermentation duration (x2) and on the fermentation temperature (x3) was statistically significant. The results of this study suggest that the CCD and RSM can be efficiently used in the fermentation process in black tea manufacture. Keywords: Black tea fermentation, Theaflavins, Thearubins, Total liquor color, Response surface methodology, Experimental design.