Structure function changes of casein micelles were studied as a function of concentration using a non invasive concentration method, osmotic stressing. A combination of serum analysis, light scattering and rheological measurements were used to characterize the physico-chemical properties of casein micelles. In heated and unheated milk, rheological studies indicated that casein micelles behave as hard spheres of similar volume fractions, if the viscosity changes in the serum phase and the particle particle interactions are taken into account. The differences in the distribution of the heat induced complexes between colloidal and soluble phase affected the colloidal properties of casein micelles. Above 70 g L-1 protein, the protein particles were no longer free diffusing. Re-dilution of the suspensions showed no irreversible aggregation. The data suggested that in the range of concentration studied casein micelles behave as hard spheres. Age gelation was also investigated on heated and unheated concentrated milk. In unheated concentrated milk proteolysis played an important role in imparting an increase in viscosity by causing aggregation of the casein micelles. On the other hand, in heated milk, there was a significant effect of the whey protein aggregates, which increased their interaction with the casein micelles over time. This effect, together with proteolysis caused age gelation in heated concentrated milk. The method of concentration used in this research, osmotic stressing, was then compared to ultrafiltration. It was demostrated that these two methods are not equivalent, as shear and mixing during ultrafiltration cause rearrangements to the casein micells. The differences were clearly demonstrated by adding soluble caseins to the milk before or after concentration. This project brings a better understanding on the effects of concentration on the structure-function of casein micelles and the interactions occurring in milk proteins during concentration.

The Western Dairy Center at Utah State University funded this project to investigate cheese research using concentrated milks. Concentrated milk was provided by the South Dakota State University and starter culture for this study was prepared and donated by Vivolac Cultures Corporation, Greenfield, Indiana. The project initiated as a continuation of a previous study on effects of protein concentration, coagulum cut size and set temperature on curd moisture loss kinetics while stirring during cheesemaking. It was aimed at determining the extent to which pH drop prior to draining and final cheese moisture when using microfiltered concentrated milk. We performed twelve cheesemaking trials using recombined milk from micellar casein concentrate, cream and skim milk according to a modified cheddar cheese-make procedure. Four different levels of starter cultures were used to achieve different acidification rates for pH change during cheesemaking. The amount of starter culture added had significant effect on moisture of cheese at whey drainage, moisture and pH of cheese. Thus, it can be said that the pH drop that occurs during the cheesemaking increases rate and extent of whey expulsion.