Impact of membrane channel diameter on limiting flux and serum protein removal during milk protein concentrate microfiltration

Abstract Text: The design of a ceramic microfiltration membrane will impact its ability to remove serum proteins (SP) from milk. Our objectives were to determine the limiting fluxes and SP removal as a percentage of theoretical (SPR) of 0.1 µm ceramic graded permeability membranes with either 3mm or 4mm diameter flow channels. A microfiltration process was fed with an 85% milk protein concentrate that had been standardized to 5.62±0.06% protein with reverse osmosis water. Retentate and permeate were continuously recycled to the feed tank. Limiting fluxes were determined by incrementally increasing flux once per h from 55 kg/m² per h until flux became independent of transmembrane pressure. Experiments were replicated 3 times and the Proc GLM procedure of SAS was used for statistical analysis. Temperature, longitudinal pressure drop (ΔP), and protein concentration in the retentate recirculation loop were maintained at 50°C, 220 kPa, and 9.16±0.08%, respectively. Because the graded permeability membranes are designed to operate at ΔP between 200 and 220 kPa, ΔP was controlled. Consequently, 3mm cross-flow velocity was lower (P < 0.001) than 4mm cross-flow velocity (5.48 vs 7.00 m/s). In both membranes, cross-flow velocity decreased (P < 0.05) between the initial and limiting fluxes. The 3mm membrane limiting flux was lower (P < 0.001) than the 4mm membrane limiting flux (105 vs 133 kg/m² per h). SPR was calculated by dividing true protein in the permeate by SP in the permeate portion of the feed to describe the ease of SP passage through the membrane. In the 4mm membrane, SPR decreased (P = 0.03) between the initial and limiting fluxes due to fouling. No decrease in SPR was detected (P > 0.10) in the 3mm membrane between the initial and limiting fluxes. Experimental variation and the fact that 3mm SPR was lower (P = 0.07) than 4mm SPR at the initial flux contributed to this finding. Despite a lower limiting flux and a higher rejection of SP, the modular SP removal rate (kg SP removed/module per h) of 3mm membranes would be higher than that of 4mm membranes because 46% more membrane surface area can be housed in a 3mm membrane module. This relationship could change if the retentate protein concentration were different. A processor should consider both the increased performance of the 4mm membrane and the reduced cost per module of the 3mm membrane before proceeding with a purchasing decision.

Keywords:

microfiltration, serum protein removal, limiting flux