Mechanistic Insights Into Fouling Behavior During Ultrafiltration of Watermelon Rind Juice for L‐Citrulline Recovery

Watermelon rind, an underutilized agro‐waste rich in L‐citrulline, represents a sustainable source of functional ingredients. This study applied an ultrafiltration polysulfone membrane with 50 kDa molecular weight cut‐off (MWCO) to recover L‐citrulline from watermelon rind juice, and investigated fouling behavior under different operating pressures (2–8 bar). Permeate flux exhibited the typical three‐stage decline: a rapid initial drop, a transitional phase, and a final stabilization. Among the investigated pressures, operation at 6 bar was associated with the highest volume concentration factor (VCF = 1.69), indicating improved concentration efficiency under moderate‐pressure conditions. Hermia model analysis demonstrated that cake layer formation predominated at 2–6 bar, whereas model fitting at 8 bar showed greater agreement with intermediate blocking characteristics ( R 2 > 0.92). Fouling resistance accounted for over 80% of the total hydraulic resistance, suggesting a pressure‐dependent transition in dominant fouling behavior. L‐citrulline rejection remained low (< 11%) across all conditions, confirming its predominant recovery in the permeate stream. The appropriate pressure selection (4–6 bar) enables effective macromolecular clarification while preserving low–molecular‐weight bioactive compounds such as L‐citrulline, small amino acids, and soluble phenolics, providing practical guidance for membrane‐assisted valorization of watermelon rind in functional food processing applications.