Fouling of polyethersulphone ultrafiltration membranes during the decaffeination of ground coffee brews

Abstract

The production of reduced caffeine coffee beverages (so called ‘half-decaf’) has recently gained increased commercial attention. Ultrafiltration (UF) can offer a viable alternative to the use of solvents in the decaffeination process, also operating at ambient temperatures, and a low operational cost. This paper reports the development and evaluation of the fouling occurring during the selective reduction of caffeine from coffee brews using a commercially available synthetic tight ultrafiltration (TUF) polyethersulphone (PES) membrane (GP95PP – Alfa Laval) with a 2 kDa molecular weight cut off (MWCO). Performance is reported in terms of fluxes, hydraulic resistances, and component rejection ratios. A cross-flow rig was operated at transmembrane pressures of 2–9 bar and cross-flow velocities of 0.04–0.1 m/s at temperatures in the range 25–50 oC. The process is viable: a retentate is produced with a much reduced caffeine concentration whilst still rich in higher molecular weight bioactives. Commercial PES membranes (GP95PP – Alfa Laval) had a permeate flux of 6.5 L m-2 h-1 and a fouling index (FI) of ca 60%. Rejection ratios were ca 30% for caffeine, ≥ 90% for both polyphenols & proteins, & ≥ 80% for melanoidins, over a 29 hr filtration period. Membrane surface modification due to fouling takes place, altering the hydrophobicity. An effective cleaning protocol is reported comprised of 0.5% w/v NaOH at 500 C. The flux decline and recovery are reported for multiple fouling and cleaning cycles.