Microfiltration Of Skim Milk To Separate Micellar Casein From Serum Protein: Theoretical Influence Of Five Factors And Performance Of A Microfiltration Unit

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The production of serum protein (SP) and micellar casein (CN) from skim milk can be accomplished using microfiltration (MF). There are potential commercial applications for both the SP and micellar CN. Our 1st objective was to demonstrate the impact of: skim milk composition, heat treatment of skim milk, concentration factor (CF) and diafiltration factor (DF), control of CF and DF, and SP rejection of membrane on the performance of a MF system designed to process skim milk and separate CN from SP. A mathematical model of a skim milk MF process was developed and used to predict the effect of the 5 factors on retentate and permeate composition, SP removal, and micellar CN concentrate (MCC) and milk SP isolate (MSPI) yield for a 3 stage process. When skim milk TP increased from 3.2 to 3.8%, the yield of MCC and MSPI increased by 19% and 18%, respectively. Increased heat treatment (72.9 to 85.2oC) of skim milk caused CN as a percentage of TP in skim milk as measured by Kjeldahl analysis to increase from 81.97 to 85.94% and the yield of MSPI to decrease 22%, while the 3rd stage cumulative SP removal decreased from 96.96 to 70.08%. A CF and DF of 2X gave a 3rd stage retentate TP concentration of 5.38% compared to 13.13% for a CF and DF of 5X. Variation in control of the balance between CF and DF (unequal CF and DF) caused either an increase or decrease in TP concentration in the retentate across stages depending if CF was greater than DF (increasing TP in retentate) or CF was less than DF (decreasing TP in retentate). An increased rejection of SP by the membrane from a SP removal factor of 1 to 0.6 caused a reduction in MSPI yield by 17%, 3rd stage cumulative SP removal decreased from 96.96 to 79.74%. Within the ranges of the 5 factors studied, the TP content of the 3rd stage retentate was strongly impacted by the target CF and DF and variation in skim milk composition. Cumulative SP removal was strongly impacted by the heat treatment of skim milk, SP removal factor, and target CF and DF. The MCC and MSPI yield was most strongly impacted by initial skim milk composition. MSPI yield was also impacted by the heat treatment of milk and SP removal factor. Our 2nd research objective was to determine the efficiency of SP removal for a 3X continuous feed and bleed uniform transmembrane pressure (UTP) system with 0.1 µm ceramic membranes, when processing pasteurized skim milk at 50oC with two stages of water diafiltration (for a total of 3 stages). For each of 4 replicates about 1100kg of skim milk was pasteurized (72oC, 16s) and processed at 3X through the UTP MF system. Retentate from stage 1 was diluted with reverse osmosis water back to a 1X concentration and again processed through the MF system (stage 2) to a 3X concentration. The retentate from stage 2 was diluted with reverse osmosis water back to a 1X concentration, before running through the MF system at 3X for a total of 3 stages. Theoretically, from the 1st part of our research a 3-stage 3X MF process could remove 97% of the SP from skim milk. The total SP removal in this experiment was 98.27 +/- 2.25%, when SP removal was calculated using the mass of SP removed in the permeate of each stage.
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