αs-Casein-PE6400 mixtures: surface properties, miscibility and self-assembly.

Surface properties, miscibility and self-assembly of mixtures of a food-grade αs-casein and the triblock copolymer PE6400 (PEO13-PPO30-PEO13) were examined. The properties at the surface were determined by surface pressure measurements for a 1:1 molar mixture. Comparison of the measured with the calculated isotherms show attractive interactions at surface pressures above 9mN/m. The miscibility gaps of solutions containing 0.004-0.2mmol/l αs-casein and 0.02-0.1mol/l polymer were examined. It was found that a one-phase region exists at distinct mixing ratios and temperatures. Comparison of the cloud points of mixtures of αs-casein and PE6400 with pure αs-casein showed that the presence of the triblock copolymer enhanced the solubility of the protein. The ζ-potential of the αs-casein solution decreased by addition of PE6400 to zero. Our results thus suggest that αs-casein and PE6400 are miscible. The results of the cloud point and ζ-potential measurements were explained by formation of a mixed aggregate where the PPO chains are anchored inside the hydrophobic part of the αs-casein while the PEO chains cover the charged hydrophilic part of the αs-casein thereby leading to an increase of the cloud point and a decrease in ζ-potential. This is in agreement with the attractive interactions between αs-casein and PE6400 as observed via surface pressure measurements at the surface.

Increased loading of vitamin D2 in reassembled casein micelles with temperature-modulated high pressure treatment.

Native casein micelles were treated (incubated) with vitamin D2 at different hydrostatical pressures (0.1, 200, 400, and 600MPa) and temperatures (10-50°C) in order to load the vitamin. Pressure induced the release of αs-, ß- and κ-casein and calcium from the micelle into the soluble phase. During the pressure stable reassembled micelles were created due to a structural rearrangement of hydrophobic interactions between the hydrophobic domains of the caseins and vitamin D2. Treatment at 600MPa and 50°C increased the loading of vitamin D2 per casein from 2.2±0.2µg/mg (native sample) to 10.4±0.2µg/mg. The reassembled micelles presented an average hydrodynamic diameter of 272±10nm and contained 8.1, 10.3 and 0.8mg of α-, ß- and κ-casein, respectively, per 100mg casein.

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

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 % ß-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.

Alphas-casein-PE6400 mixtures: a fluorescence study.

Micelles are a potential encapsulation system for bioactive compounds. In previous studies we were able to show that the triblock copolymer PEO13-PPO30-PE013 and alphas-casein are miscible within distinct temperature and concentration ranges. In this study, we wanted to test our hypothesis that mixed micelles are formed which, in turn, are able to solubilize hydrophobic compounds. Additionally we want to gain insight into the specific arrangement of individual molecules in these micelles. For that purpose, mixtures containing increasing mole fractions of casein (alphacas = 4.3 x 10(-4) to alphacas = 7.2 x 10(-3)) were examined at 30 degreeC. The hydrophobic fluorescence probe pyrene was used as a model solute. Emission spectra were recorded and I1/I3 and lex/I1 ratios were evaluated. Furthermore, the emission spectra of tryptophan were recorded and the maximum emission wavelength was evaluated. The determined parameters showed that micelles are formed in all solutions and that the solubilization of pyrene occurred. The calculated interaction parameter betaindicated that the mixing was antagonistic, possibly due to steric hindrances between the casein and the polymer. It seems that the mixed micelles are formed in such a way that the hydrophobic part of the polymer is attached to the hydrophobic parts of the caseins (a necklace and bead model) since the tryptophan residues are located in a hydrophobic environment above the CMC. We suggest a first structural arrangement model, recognizing that further studies will be required to prove and refine it.

Isothermal titration calorimetry as a tool to determine the thermodynamics of demicellization processes.

Demicellization of a 90 mM sodium dodecyl sulfate (SDS) solution in water at 10, 22, and 30 °C was studied by isothermal titration calorimetry (ITC). ΔH of the demicellization process was strongly temperature dependent, having an exothermic progression (-20.4 ± 0.9 kJ∕mol, max) at 10 °C and an endothermic one (3.7 ± 1.2 kJ∕mol, max) at 30 °C. ΔH for micelle dilution followed a slightly endothermic progression (0.9 ± 0.5 kJ∕mol at 30 °C, 0.7 ± 1.3 kJ∕mol at 22 °C, and 0.0 ± 0.5 kJ∕mol at 10 °C) at all studied temperatures. No differences in ΔH for micelle dilution and demicellization was observed at 22 °C. The temperature dependence of ΔH measured by ITC could be related to hydrophobic interactions. Therefore, ITC was shown to be a useful tool to describe the thermodynamics of demicellization processes and in addition to determine alterations in ΔH caused by changes in hydrophobic and steric∕electrostatic interactions.

Cambios en las actividades de alfa-amilasa: pectinmetilesterasa y poligalacturonasa durante la maduración del maracuyá amarillo passiflora edulis var. flavicarpa degener) / Changes in enzymatic activity of alfa-amylase pectinmethylesterase and polygalacturonase during ripening in yellow passion fruit (passiflora edulis var. flavicarpa degener)

El maracuyá amarillo (Passiflora edulis var. Flavicarpa Degener) es una fruta tropical muy apreciada por su sabor. Los componentes responsables del sabor se desarrollan durante la maduración, como resultado del incremento en la actividad metabólica. En el presente trabajo se evaluó la producción de CO2 y el desarrollo del color como indicadores de la maduración, así como la actividad de la a-amilasa, poligalacturonasa (PG) y pectinmetilesterasa (PME), con relación a la presencia de diversos componentes responsables del sabor agridulce (sólidos solubles, azúcares, ácidos orgánicos y pH) característico del maracuyá. Los resultados mostraron que la maduración del fruto continuó después de su separación de la planta en la semana 8 después de la antesis (DA). La actividad de a-amilasa y PG se elevó en la semana 9 DA y la de PG se incrementó nuevamente al final del desarrollo del fruto. La actividad de PME también presentó dos valores máximos, en las semanas 8 y 11 DA. El incremento conjunto en la actividad de las enzimas asociado a la maduración de la fruta, indicada por el cambio de color y la elevación en la producción de CO2, generan un incremento en los azúcares y ácidos orgánicos, algunos de los cuales pueden ser responsables del sabor característico del maracuyá