Thermal stability of microfiltered and ultrafiltered retentates

Abstract Text: Membrane filtration technologies are widespread unit operations in the dairy industry, and often employed to obtain ingredients of tailored processing functionalities.  The objective of this work was to better understand the effect of partial removal of whey proteins by microfiltration (MF) on the heat stability of the retentates. Control retentates were obtained using ultrafiltration (UF). Pasteurized milk was microfiltered (80 kDA polysulfone membrane) or ultrafiltered (30 kDa cellulose membrane) in a plate and frame membrane system, to reach two and four times concentration (based on volume reduction).  Concentrates showed no differences in pH, casein micelle size, or minerals in the serum phase, before heating; as diafiltration was not employed in this study. The reduced amount of whey protein in the MF retentates caused a significant increase in the heat stability of the retentates, compared to UF retentates. This difference was not due to ionic composition differences, nor pH. Heat coagulation time decreased with protein concentration, but significantly increased in MF retentates, containing less whey proteins than the corresponding UF controls. In 2x concentrates, retentates prepared with MF, containing 20% less whey proteins than UF control, showed an increase in the heat coagulation time of about 11 min . 4xMF retentates contained 17 ± 3 mg/ml of whey proteins, about 40% less whey proteins than the 4xUF control retentates. A 4x concentrate prepared by MF showed statistically similar heat stability to that of a 2xUF concentrates, with a heat coagulation time of about  38 min. The turbidity parameter 1/l*, measured by diffusing wave spectroscopy, increased after heating, with the UF retentates showing a higher value than the MF retentates, at the same protein concentration. In addition, 4x MF concentrates showed a 1/l* value comparable to that of 2xUF concentrates. In conclusion, this work demonstrated that partial removal of whey proteins by MF could be employed as a means to increase heat stability of milk concentrates.

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Keywords: heat coagulation time, milk concentrate, whey protein