COMBINED EFFECTS OF TEMPERATURE, ACIDIFICATION, AND DIAFILTRATION ON COMPOSITION OF SKIM MILK RETENTATE AND PERMEATE

Commercial pasteurized skim milk was concentrated five times by UF with a Romicon membrane of .56 m2 having a molecular weight cutoff of 50,000 Da. Ultrafiltration was conducted under various pH and temperature conditions and could be followed by a diafiltration step. Skim milk ultrafiltered at 50-degrees-C was used for control. For each UF treatment, the composition and buffer capacity of the five times retentate and the permeation flux during concentration were determined. The final composition of the retentate was different for each UF treatment. Retentate obtained after diafiltration at 4-degrees-C and pH 5.3 had the highest protein content and the lowest ash. Under these conditions, 78.3% of the Ca, 93.9% of the P, 75% of the Mg, and 95.3% of the K were removed from skim milk compared with 17.5% of the Ca, 45.4% of die P, 60.4% of the Mg, and 79.1% of the K for the control retentate. The removal of these minerals lowered the buffer index from .151 for the control retentate to .063 for the retentate after treatment. Permeation flux was affected by temperature and acidification. These retentates with different buffering capacities could be dried and used to standardize or to enrich milk used for cheese making.