Nanofiltration for Separation of L-Citrulline From Watermelon Rind Juice

Nanofiltration is a green purification method for juice valorization; however, its efficiency is often limited by membrane fouling. Ultrafiltration is commonly applied as pretreatment to mitigate fouling during juice nanofiltration. This study evaluated the separation of L-citrulline from watermelon rind juice using nanofiltration with and without a prior ultrafiltration step. The juice was pre-clarified using a 50 kDa ultrafiltration membrane and subsequently filtered through a polyamide nanofiltration membrane at pressures of 10-40 bar. Membrane performance and fouling behavior were analyzed in terms of permeate flux, solute rejection of L-citrulline and D-glucose, total soluble solids (TSS), total dissolved solids (TDS), salinity, electrical conductivity (EC), and Hermia's fouling models. During nanofiltration without pretreatment, rapid cake layer formation limited the permeate flux to below 15.0 L·m^{- 2}·h^{- 1} and the volume concentration factor (VCF) to approximately 7. In contrast, the use of ultrafiltration prior to nanofiltration significantly improved process performance, increasing the flux to 84.7 L·m^{- 2}·h^{- 1} at 35 bar and enabling concentration up to VCF 11.56. L-citrulline exhibited negative rejection (R_i < 0), resulting in higher permeate concentrations after the combined process (1,773 mg·L^{- 1}), while D-glucose remained strongly rejected (R_i > 74%). The permeate obtained from the combined process also showed reduced levels of TSS, TDS, salinity, and EC (2.0 °Brix, 2,203.0 ppm, 0.22%, and 4,370.0 µS·cm^{- 1}, respectively). Fouling analysis revealed a transition in the dominant mechanism from cake filtration to complete blocking when ultrafiltration was applied prior to nanofiltration. These findings highlight the effectiveness of integrating ultrafiltration as a pretreatment step to enhance nanofiltration performance for selective L-citrulline recovery from watermelon rind. PRACTICAL APPLICATIONS: This study demonstrated that integrating ultrafiltration with nanofiltration effectively reduced sugars while concentrating L-citrulline in watermelon rind juice, making the resulting permeate suitable for functional beverage and nutraceutical formulations. The proposed membrane-based process offers a scalable, clean-label strategy for valorizing fruit-processing by-products, thereby supporting sustainable manufacturing and the development of health-oriented food products.