The effect of commercial sterilization regimens on micellar casein concentrates.

This work focused on evaluating the effects of UHT sterilization and in-container retorting on the stability and physical properties of micellar casein concentrates (MCC). The study was performed on MCC obtained by membrane separation, with casein concentrations between 5 and 10%. The UHT and retorting regimens were designed to achieve the same microbial inactivation effect. Ultra-high temperature treatment was performed in a pilot-scale MicroThermics heating system (MicroThermics Inc., Raleigh, NC), and retorting in an FMC multipurpose laboratory retort (Steritort; FMC Corp., San Jose, CA). The heat-treated and the non-heat-treated MCC controls were evaluated for pH, mineral profile, ζ-potential, particle size, and rheological properties for up to 24h after heat treatment. The treatments were performed in triplicate, and differences among samples were evaluated using statistical analyses. Retorting resulted in slight aggregation in the MCC, whereas UHT caused the formation of visible aggregates and coagulation. The UHT-treated MCC had higher viscosity than retorted MCC, and displayed predominantly solid-like rheological behavior, indicative of structure formation. These effects were, at least in part, attributed to a change in mineral equilibrium, which affected the stability of the casein micelles, but additional mechanisms such as κ-casein dissociation may also play a significant role in these heat-induced changes. Drying of MCC accentuated the observed instabilities, as dried and reconstituted micellar casein concentrates (R-MCC) were more unstable to UHT sterilization than the MCC that had not undergone drying. The results of this study provide valuable information about the sterilization behavior and physical properties of MCC, which can be useful to processors in the development and manufacture of shelf-stable casein-based products and beverages.

Effect of solvent and temperature on the size distribution of casein micelles measured by dynamic light scattering.

The objectives of this study were to investigate the effect of the solvent on the accuracy of casein micelle particle size determination by dynamic light scattering (DLS) at different temperatures and to establish a clear protocol for these measurements. Dynamic light scattering analyses were performed at 6, 20, and 50 degrees C using a 90Plus Nanoparticle Size Analyzer (Brookhaven Instruments, Holtsville, NY). Raw and pasteurized skim milk were used as sources of casein micelles. Simulated milk ultrafiltrate, ultrafiltered water, and permeate obtained by ultrafiltration of skim milk using a 10-kDa cutoff membrane were used as solvents. The pH, ionic concentration, refractive index, and viscosity of all solvents were determined. The solvents were evaluated by DLS to ensure that they did not have a significant influence on the results of the particle size measurements. Experimental protocols were developed for accurate measurement of particle sizes in all solvents and experimental conditions. All measurements had good reproducibility, with coefficients of variation below 5%. Both the solvent and the temperature had a significant effect on the measured effective diameter of the casein micelles. When ultrafiltered permeate was used as a solvent, the particle size and polydispersity of casein micelles decreased as temperature increased. The effective diameter of casein micelles from raw skim milk diluted with ultrafiltered permeate was 176.4 +/- 5.3 nm at 6 degrees C, 177.4 +/- 1.9 nm at 20 degrees C, and 137.3 +/- 2.7 nm at 50 degrees C. This trend was justified by the increased strength of hydrophobic bonds with increasing temperature. Overall, the results of this study suggest that the most suitable solvent for the DLS analyses of casein micelles was casein-depleted ultrafiltered permeate. Dilution with water led to micelle dissociation, which significantly affected the DLS measurements, especially at 6 and 20 degrees C. Simulated milk ultrafiltrate seemed to give accurate results only at 20 degrees C. Results obtained in simulated milk ultrafiltrate at 6 degrees C could not be explained based on the known effects of temperature on the casein micelle, whereas at 50 degrees C, precipitation of amorphous calcium phosphate affected the DLS measurement.