The Study of Commercial versus Lab Developed Polymeric Membrane Filters for Coffee Brew Decaffeination

: (Alternative Format Thesis)

Abstract

Membrane filtration could offer a viable alternative to the use of solvents for the decaffeination of coffee brews. This Ph.D. project aimed on the development and evaluation of a filtration process for the selective reduction of caffeine from coffee brews, targeting on the production of reduced caffeine coffee beverages, so called half-decaf. Α commercially available TUF PES membrane (GR95PP) and a LNF PES lab developed MMM (PSCD) were utilized for this study. A development protocol was reported utilizing the effect of drying as a post-fabrication treatment step, resulting to a LNF MMM comprising a conjugated microporous polymer as a dispersed phase. A cross-flow filtration rig with a total filtration area of 0.00146 m2 was operated in recycle mode. Optimal TMP and temperature settings versus permeate flux along with long-term filtration viability were investigated during coffee brew filtration. Optimal operational TMP was found at 9 bar, where the filtration temperature exhibited negligible effect in permeate flux in the range of 25-50 °C, thus 25 °C was used. The filtration performance was evaluated for multiple fouling and cleaning filtration cycles, in terms of flux, rejection and resistances. The PSCD membrane exhibited superior flux compared to GR95PP during coffee brew filtration. Both membranes exhibited similar flux profiles throughout the filtration cycles, exhibiting increased PWF after cleaning only after 1st filtration cycle. An effective cleaning protocol, flux wise, was reported using 0.5% w/v NaOH. Hermia’s model revealed cake filtration as the dominant fouling mechanism. The selective transmittance of caffeine with a rejection (ca. 30%) while retaining key compounds such as protein (>90%), polyphenols (~90%) and melanoidins (~80%) was achieved stably for both membranes for three consecutive filtration cycles. Membrane surface modification due to fouling and cleaning has been investigated in terms of membrane hydrophobicity, surface roughness, charge and porosity over the filtration cycles. Surface charge was altered after fouling and partially recovered after cleaning. Porosity studies revealed the presence of a porous fouling layer increasing the overall pore size of the membranes due to its macroporosity and blockage of membrane pores in the mesoporous range. Surface roughness exhibited a decrease when membrane were fouled suggesting cavities filling. A decrease in surface hydrophobicity was also observed and associated with phenolic species deposition.

Date of Award	27 Mar 2023
Original language	English
Awarding Institution	University of Bath
Supervisor	Michael Bird (Supervisor) & Davide Mattia (Supervisor)