Multi-criteria evaluation of graphene oxide nanofiltration membranes for decaffeination

Nanofiltration (NF) offers a promising low-energy approach for producing decaffeinated instant coffee, while requires membranes that balance caffeine removal, water permeance, and retention of key coffee matrix compounds. Here, graphene oxide membranes (GOMs) were modified by sodium alginate (SA) intercalation, Ca2+ crosslinking, and their combined strategy to tune nanochannel structure and separation performance. The results show that membrane modification does not follow a simple monotonic trend; instead, small structural and chemical changes generate competing effects on permeance and selectivity through coupled transport, adsorption, and confinement mechanisms. To identify the most suitable membrane, a multi-criteria evaluation framework (MCEF) combining entropy weighting and TOPSIS was established using five key performance indicators: caffeine removal, flavour-related marker retention index, choline retention, water permeance, and concentration increase. This framework identified CGSM-1 as the best-performing flat membrane, which also showed promising performance in hollow-fibre form. In coffee filtration tests, CGSM-1 reduced caffeine concentration from 24.64 to 12.95 μg/mg, corresponding to a 47.4% reduction, while retaining measurable levels of HMF, trigonelline, NMP, and choline. These findings demonstrate the potential of engineered GOMs for selective caffeine reduction in coffee liquor and provide a practical evaluation strategy for membrane-based decaffeination.