Alterations to the composition of casein micelles and retentate serum during ultrafiltration of skim milk at 10 and 40 °C

The composition of casein micelles and retentate serum were investigated during four-fold concentration of skim milk by ultrafiltration (UF) at 10 and 40 °C. Analysis of supernatants and pellets obtained by ultracentrifugation showed that while the concentration of soluble casein increased considerably during UF there was a net shift of casein from the serum to micelles at both temperatures. The hydration of casein micelles was higher at 10 °C than at 40 °C and increased slightly during UF. Dynamic light scattering measurements on diluted retentates did not reveal any permanent change in the average micelle diameter. During UF concentration of skim milk the progressive removal of calcium was affected by the partitioning of calcium between the micelles and the serum which was influenced by processing temperature. Performing UF at different temperatures altered the final calcium content of the retentates and composition and hydration of the casein micelles.