Ultrafiltration (UF) has found wide application in the food industry. In this work, gelatin, as a model food protein, was used to study the influence of pH and ionic strength on fouling during ultrafiltration using 30 kDa polyethersulfone (PES) membrane. The results showed significant water flux reductions after filtration for all feed solution conditions. The maximum fouling rate was obtained near gelatin's IEP, suggesting corresponding electrostatically driven fouling. Addition of salt increased the flux at pH near the gelatin's IEP but it had a negative effect at pH above or below gelatin's IEP. Analysis by the resistance-in-series model suggested that concentration polarization, reversible and irreversible fouling has contributed to the overall fouling. Cake formation was identified as the dominant mechanism for flux decline.