Properties of an αs-casein-rich casein fraction: Influence of dialysis on surface properties, miscibility, and micelle formation

Abstract

In this study, the surface tension, miscibility, and particle size distribution of a solution containing an αs-casein (CN)-rich CN fraction (54 wt % αs-CN, 32 wt % β-CN, and 15 wt % κ-CN) were determined at pH 6.6. The nondialyzed CN fraction was compared with a dialyzed one. In the nondialyzed sample, every charge on the protein was compensated by 0.3 charges coming from counterions, whereas in the dialyzed sample, only 0.2 charges could be assigned to each charge on the protein. This relation was determined by calculating the charges at the proteins, taking the measured mineral content into account. The surface tension was measured as a function of the protein concentration by the du Noüy ring method at room temperature. Results indicated alterations in the surface properties after reduction of counterions. The equilibrium surface tension above the critical micelle concentration increased from 40.1×10(-3) to 45×10(-3) N/m, the critical micelle concentration increased from 0.9×10(-4) to 2×10(-3) mol/L, and the minimal area occupied per molecule at the surface increased from 2.4×10(-18) to 4.6×10(-18) m(2). Cloud points were determined by measuring the absorbance of CN solutions as a function of the temperature. The cloud points were found to be concentration dependent and had a minimum at 0.2 wt % at 34°C for nondialyzed CN and at 0.25 wt % at 28°C for dialyzed CN, again demonstrating the influence of counterion reduction. Below the cloud point, a micellar phase was found to exist. The hydrodynamic diameter of the micelles were characterized by dynamic light scattering in both auto- and cross-correlation mode. However, no influence of reduction in counterions could be observed, possibly due to the fact that dynamic light scattering is not a suitable method for this type of system. The presence of self-assembled structures was verified by freeze-fracture electron microscopy. The observed differences between dialyzed and nondialyzed samples were explained by changes in the counterion cloud surrounding the proteins. Consequently, the electrostatic interactions between as well as within the CN are altered by dialysis, which, in turn, affects the behavior at the surface as well as the properties in the solution.

αs-Casein–PE6400 mixtures: a fluorescence study

“…The a s -casein fraction was extracted from micellar protein 16 and further puried as described previously. 17 The protein content of the a s -casein fraction was 0.689 AE 0.013 g protein per g powder, one gram consists of 0.539 AE 0.013 g a s -casein, 0.318 AE 0.011 g b-casein and 0.15 AE 0.017 g k-casein. 17 A molecular weight of 23 000 g mol À1 is assumed for these casein mixtures.…”

“…17 The protein content of the a s -casein fraction was 0.689 AE 0.013 g protein per g powder, one gram consists of 0.539 AE 0.013 g a s -casein, 0.318 AE 0.011 g b-casein and 0.15 AE 0.017 g k-casein. 17 A molecular weight of 23 000 g mol À1 is assumed for these casein mixtures. This a s -casein rich casein fraction is subsequently referred to a s -casein in the manuscript.…”

“…The value is in line with the results of our previous study, where the CMC of the same a scasein was determined to be 9 Â 10 À5 mol l À1 at 20 C by the du Noüy ring method. 17 The slightly higher value at 20 C may be attributed to the temperature difference since lower temperatures lead to higher CMC values for caseins. 23 At concentrations far above the CMC, the I 1 /I 3 ratio decreases again with increasing casein concentration (region III), meaning that the pyrene environment becomes more hydrophobic.…”

“…For comparison purposes, the solubilization properties of the single surfactants were also examined under the same conditions and it was found that they are miscible as determined previously. 15,17 3.2 I 1 /I 3 and I ex /I 1 ratios at low pyrene concentrations Excitation of pyrene with l ex ¼ 355 nm yielded an emission spectra l em , an example of which is shown in Fig. 2.…”

“…This a s -casein rich casein fraction is subsequently referred to a s -casein in the manuscript. Protein stock solutions were prepared 17 and diluted appropriately. To avoid microbial degradation, 0.03% sodium azide (Carl Roth GmbH, Karlsruhe, Germany) was added to all solutions.…”