Ultrafiltration of Low-Heat and UHT Skim Milks with a Shear-Enhanced Vibrating Filtration System

Abstract

A vibratory shear-enhanced filtration system (VSEP) was used for ultrafiltration of UHT skim milk and a skim milk reconstituted from low-heat powder, which has a similar protein content as fresh milk. Two polyethersulfone membranes of 10 and 50 kDa cutoff were used, respectively, for total protein concentration and for α-lactalbumin (α-LA) separation from β-lactoglobulin (β-LG). With the 50 kDa membrane, casein micelles were completely rejected after 40 min of filtration while α-LA transmission rate (ratio of permeate to retentate concentrations) remained between 22% and 28%. The β-LG transmission rate was around 1% and the stabilized permeate flux at initial concentration (Volume Reduction Ratio, VRR=1) and a transmembrane pressure of 250 kPa was between 60 and 70 L h⁻¹ m⁻². Permeate flux data on this membrane were found to be very close to those obtained under the same conditions with UHT skim milk until a VRR of 3. At higher concentrations, the reconstituted milk yielded higher permeate fluxes, 29 L h⁻¹ m⁻² at a VRR of 6 vs. 15 L h⁻¹ m⁻² for UHT milk. The 10 kDa membrane retained whey proteins completely and yielded higher fluxes—up to 82 L h⁻¹ m⁻² at initial concentration and a transmembrane pressure (TMP) of 1500 kPa—than data reported for conventional ultrafiltration. The maximum VRR reached in concentration tests was 13 with a final flux of 8 L h⁻¹ m⁻², corresponding to a maximum theoretical VRR by extrapolation to zero flux of 17. The critical shear stress at which a steady TMP could be maintained for a constant flux of 30 L h⁻¹ m⁻² was found to be 13.7 Pa.

Keywords:

• Skim milk
• Ultrafiltration
• Shear-enhanced filtration
• Filtration stability
• Vibrating membrane

Acknowledgments

This research has been supported in part by the CNRS-INRA Program Prosetia and by the Regional Council of Picardy (scholarship of O. Akoum). The authors thank Dr G. Gésan-Guiziou from LRTL of INRA Rennes for her scientific assistance and acknowledge the loan of the VSEP pilot from Vivendi Waters Co., and Alting Co. (Strasbourg) for supplying the membranes.