Mass transfer and polyphenols retention in the clarification of black tea with ceramic membranes

Abstract A mass transfer analysis of ultrafiltration of black tea was carried out using inorganic membranes (NMWCO=40 kDa) at different operating conditions ( T =50 °C, TMP=70÷170 kPa, u =0.49÷3.2 m/s). Based on the rejection data and on the film theory, a relationship between the mass transport coefficient and cross-flow velocity was identified. A decreasing flux with increasing pressure was observed. Accordingly a modified in-series resistance model was provided by introducing a concentration polarisation resistance given in terms of two indexes both depending on the feed velocity. The apparent rejection coefficient increases, linearly, with increasing transmembrane pressure up to 170 kPa, but at the highest flow condition ( Re =33 000) it was independent of pressure. The effect of ultrafiltration on the stability of clarified tea was evaluated. The polyphenols concentration and the colour parameters (CIE L , a , b) remained stable with time and no visible haze was observed at least for 2 month.