Ultrafiltration of black tea using diafiltration to recover valuable components

Abstract

Ultrafiltration of black tea liquor in a constant volume diafiltration regime has been carried out to remove the haze characteristically associated with this product. Commercial polymeric membranes with a range of molecular weight cut-offs (25, 50, and 100 kDa) fabricated from both polysulphone (PS) and fluoropolymer (FP) were used to assess the total solids throughput of the system. A shortfall in the amount of polyphenols transmission over the amount of residual tea solids transmission is the main driver for operating in this regime. The result of this shortfall means membrane permeates are impaired in terms of colour, flavour and health-giving properties. From an initial tea concentration of 1.0 wt.%, diafiltration was carried out, with rejection coefficients for total solids and total solids being assessed continuously. An acceptable (but not ideal) permeate product was the result, which showed excellent clarity (<2.0 nephelometric turbidity units (NTU) in the majority of cases) although the associated transmission in polyphenols meant the tea has lost some of the deep red colour associated with a higher-quality tea infusion. This shortfall in polyphenols transmission gave rise to higher concentrations of polyphenols in the retained side of the process than for original raw tea. In addition, membranes were pre-treated using ethanol, which had a significant effect on both their water permeability and zeta potential. When used in a filtration environment, fluxes increased for all membranes following treatment, with the 100 kDa PS membrane recording a 41% increase in terminal filtration flux. With the exception of the 25 kDa PS membrane, alcohol treatment resulted in all membranes showing increases in the total solids throughput, with the proportion of polyphenols transmitting through the membrane increased to a greater degree over all other components. The disproportionate increase in polyphenols transmission was attributed to reduced charge interactions following ethanol treatment. The results show that as fouling builds up, the separation of polyphenols from all other tea solids increases, meaning that small batches (relative to membrane area) and short-term operation are favoured, in order to maximise polyphenol content in the product stream. In summary, the operation of UF in a diafiltration regime is shown to be an effective industrial processing option for the clarification of black tea liquors.