Micellar casein concentrate production with a 3X, 3-stage, uniform transmembrane pressure ceramic membrane process at 50°C

The production of serum protein (SP) and micellar casein from skim milk can be accomplished using microfiltration (MF). Potential commercial applications exist for both SP and micellar casein. Our research objective was to determine the total SP removal and SP removal for each stage, and the composition of retentates and permeates, for a 3x, continuous bleed-and-feed, 3-stage, uniform transmembrane pressure (UTP) system with 0.1-mu m ceramic membranes, when processing pasteurized skim milk at 50 degrees C with 2 stages of water diafiltration. For each of 4 replicates, about 1,100 kg of skim milk was pasteurized (72 degrees C, 16 s) and processed at 3x through the UTP MF system. Retentate from stage 1 was cooled to <4 degrees C and stored until the next processing day, when it 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 stored at <4 degrees C, and, on the next processing day, 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. The retentate and permeate from each stage were analyzed for total nitrogen, noncasein nitrogen, and nonprotein nitrogen using Kjeldahl methods; sodium dodecyl sulfate-PAGE analysis was also performed on the retentates from each stage. Theoretically, a 3-stage, 3x MF process could remove 97% of the SP from skim milk, with a cumulative SP removal of 68 and 90% after the first and second stages, respectively. The cumulative SP removal using a 3-stage, 3x MF process with a UTP system with 0.01-mu m ceramic membranes in this experiment was 64.8 +/- 0.8, 87.8 +/- 1.6, and 98.3 +/- 2.3% for the first, second, and third stages, respectively, when calculated using the mass of SP removed in the permeate of each stage. Various methods of calculation of SP removal were evaluated. Given the analytical limitations in the various methods for measuring SP removal, calculation of SP removal based on the mass of SP in the skim milk (determined by Kjeldahl) and the mass SP present in all of the permeate produced by the process (determined by Kjeldahl) provided the best estimate of SP removal for an MF process.