The effect of membrane processes on the structure and renneting behavior of casein micelles.

摘要

This research is an investigation of the effects of membrane processing (ultrafiltration and diafiltration) on the structure and renneting behavior of casein micelles. During membrane processing, the casein particles are concentrated and the ionic equilibrium of calcium in the colloidal and serum phases is affected. The work carried out in this thesis carefully monitored the ionic equilibrium and tested the renneting properties of the casein micelles after reconstituting these particles to the original volume fraction and serum environment. By having comparable conditions, it was therefore possible to determine if processing history had an impact on the functionality of the casein micelles. The relevance of this topic to the dairy industry is shown in the study of the renneting properties of reconstituted commercial milk protein concentrates. Depending on the extent of filtration (i.e. higher concentration of proteins reflect higher extent of processing) casein micelles showed changes in dispersability and renneting properties. Most of the work in this thesis was carried out with raw milk and very controlled processes, in order to be able to better identify key factors involved in the processing changes. Small angle neutron scattering experiments were performed on ultrafiltered as well as diafiltered samples, and the results clearly demonstrated that not only diafiltration, but also extensive concentration by ultrafiltration affected the native structure of the casein micelles. The main effect identified was the change in the distribution of the colloidal calcium nanoclusters inside the casein micelle. This research clearly demonstrated that ultrafiltration and diafiltration induced changes in the light scattering and renneting properties of casein micelles, and these changes are mostly attributed to the changes in the levels of colloidal calcium phosphate.