The influence of hydrophobicity, roughness and charge upon ultrafiltration membranes for black tea liquor clarification

This paper investigates the clarification of black tea using UF membranes. The efficiency of separation and the final product quality were investigated using regenerated cellulose (RC) and fluoropolymer (FP) membranes of different nominal molar mass cut-off values. Changes during filtration in the hydrophobicity, surface charge on the pore walls, surface roughness and chemical properties were investigated. Results indicate that both of the membrane materials studied produced clarified tea liquors with polyphenol transmission rates of ca. 90%. Increased fouling was present on rougher, more hydrophobic FP surfaces. When roughness, charge and hydrophobicity were similar (i.e. for the RC membrane), variation in pore size was not found to affect the filtration properties significantly over the range investigated. Porous structures with isoelectric points at the pH of the liquor being filtered were linked to an increased fouling tendency. All RC and FP100 kg mol(-1) membranes were cleaned relatively easily. FP10 kg mol(-1) and FP30 kg mol(-1) membranes were found to have undergone surface modification, including increased negative charge and changes in hydrophobicity. The dominant fouling mechanism during all ultrafiltration experiments was found to be cake filtration. The cake formed on RC membranes is thought to dominate filtration properties due to similar transmissions detected through varied pore sizes. (C) 2008 Elsevier B.V. All rights reserved.