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The use of casein preparations obtained by membrane separation is receiving increasing interest from the dairy and food industry. The objective of this work was to generate information about the steady shear rheological properties of micellar casein concentrates (MCC) and the effect of composition, temperature, and shear rate on these properties. Micellar casein concentrate preparations with 2 levels of serum proteins (SP; 65 and 95% SP reduced, respectively), were obtained from skim milk by microfiltration followed by spray drying. Micellar casein concentrate preparations with casein concentrations ranging from 2.5 to 12.5% were obtained by dispersing the MCC powders in ultrapure water. Steady shear rheological analyses at temperatures ranging from 0 to 80°C were performed using a strain-controlled rheometer. Viscosity versus shear rate curves were used to evaluate the effect of shear on viscosity, and the apparent viscosity at a shear rate of 100 s(-1) was used to make direct comparisons between various concentration and temperature conditions. The 65% SP-reduced MCC had lower viscosity than the 95% SP-reduced MCC at the same casein concentration and temperature. Protein preparations at casein concentrations above 7.5% displayed shear-thinning behavior, which was more pronounced as concentration increased. The viscosity of MCC increased exponentially with casein concentration and decreased with temperature. The dependency of viscosity on temperature followed an Arrhenius relationship. A modified Arrhenius model able to accurately predict rheological properties under desired shear, temperature, and concentration conditions was developed and validated. This study provides critical rheological data necessary for developing practical applications of micellar casein preparations.

The application of high pressure (HP) provides an opportunity for the non-thermal preservation of highquality foods, whereas highly resistant bacterial endospores play an important role. It is known that the germination of spores can be initiated by the application of HP. Moreover, the resistance properties of spores are highly dependent on their physiological states, which are passed through during the germination. To distinguish between different physiological states and to detect the amount of germinated spores after HP treatments, two fluorescence-based methods were applied. A flow cytometric method using a double staining with SYTO 16 as an indicator for germination and propidium iodide as an indicator for membrane damage was used to detect different physiological states of the spores. During the first step of germination, the spore-specific dipicolinic acid (DPA) is released [P. Setlow, Spore germination, Curr. Opin. Microbiol. 6 (2003), pp. 550-556]. DPA reacts with added terbium to form a distinctive fluorescent complex. After measuring the fluorescence intensity at 270 nm excitation wavelength in a fluorescence spectrophotometer, the amount of germinated spores can be determined. Spores of Bacillus subtilis were treated at pressures from 150 to 600 MPa and temperatures from 37 • C to 60 • C in 0.05 MACES buffer solution (pH 7) for dwell times of up to 2 h. During the HP treatments, inactivation up to 2 log 10 cycles and thermal sensitive populations up to 4 log 10 cycles could be detected by plate counts. With an increasing number of thermal sensitive spores, an increased proportion of spores in germinated states was detected by flow cytometry. Also the released amount of DPA increased during the dwell times. Moreover, a clear pressure-temperature-time-dependency was shown by screening different conditions. The fluorescence-based measurement of the released DPA can provide the opportunity of an online monitoring of the germination of spores under HP inside the HP vessel. Implementation can be done using diamond anvil cells, units with inspection glasses or by inserting an optical fiber into the HP vessel. The analytical methods used can help to understand the complex mechanism of germination and inactivation of bacterial spores. Due to its universal, process-independent character, the application of these methods is feasible for established and emerging technologies.

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