Complexes of lutein with bovine and caprine caseins and their impact on lutein chemical stability in emulsion systems: Effect of arabinogalactan.

Lutein is an important xanthophyll carotenoid with many benefits to human health. Factors affecting the application of lutein as a functional ingredient in low-fat dairy-like beverages (pH 6.0-7.0) are not well understood. The interactions of bovine and caprine caseins with hydrophobic lutein were studied using UV/visible spectroscopy as well as fluorescence. Our studies confirmed that the aqueous solubility of lutein is improved after binding with bovine and caprine caseins. The rates of lutein solubilization by the binding to bovine and caprine caseins were as follows: caprine αS1-II-casein 34%, caprine αS1-I-casein 10%, and bovine casein 7% at 100 µM lutein. Fluorescence of the protein was quenched on binding supporting complex formation. The fluorescence experiments showed that the binding involves tryptophan residues and some nonspecific interactions. Scatchard plots of lutein binding to the caseins demonstrated competitive binding between the caseins and their sites of interaction with lutein. Competition experiments suggest that caprine αS1-II casein will bind a larger number of lutein molecules with higher affinity than other caseins. The chemical stability of lutein was largely dependent on casein type and significant increases occurred in the chemical stability of lutein with the following pattern: caprine αS1-II-casein > caprine αS1-I-casein > bovine casein. Addition of arabinogalactan to lutein-enriched emulsions increases the chemical stability of lutein-casein complexes during storage under accelerated photo-oxidation conditions at 25°C. Therefore, caprine αS1-II-casein alone and in combination with arabinogalactan can have important applications in the beverage industry as carrier of this xanthophyll carotenoid (lutein).

Review of the chemistry of alphaS2-casein and the generation of a homologous molecular model to explain its properties.

alpha(S2)-Casein (alpha(S2)-CN) comprises up to 10% of the casein fraction in bovine milk. The role of alpha(S2)-CN in casein micelles has not been studied in detail in part because of a lack of structural information on the molecule. Interest in the utilization of this molecule in dairy products and nutrition has been renewed by work in 3 areas: biological activity via potentially biologically active peptides, functionality in cheeses and products, and nutrition in terms of calcium uptake. To help clarify the behavior of alpha(S2)-CN in its structure-function relationships in milk and its possible applications in dairy products, this paper reviews the chemistry of the protein and presents a working 3-dimensional molecular model for this casein. The model was produced by threading the backbone sequence of the protein onto a homologous protein: chloride intracellular channel protein-4. Overall, the model is in good agreement with experimental data for the protein, although the amount of helix may be over-predicted. The model, however, offers a unique view of the highly positive C-terminal portion of the molecule as a surface-accessible area. This region may be the site for interactions with kappa-carrageenan, phosphate, and other anions. In addition, most of the physiologically active peptides isolated from alpha(S2)-CN occur in this region. This structure should be viewed as a working model that can be changed as more precise experimental data are obtained.

Sugar-casein interaction in deuterated solutions of bovine and caprine casein as determined by oxygen-17 and carbon-13 nuclear magnetic resonance: a case of preferential interactions.

17O NMR spectroscopy and (13)C NMR spectroscopy have been used to study the mechanism of interaction of sugars with bovine and caprine caseins in D(2)O. The (17)O NMR relaxation results showed in all cases an increase in water of hydration, as a result of added sugar; this was predominantly associated with "trapped" water in the caseins. Analysis of the vir al coefficients, obtained from the (17)O relaxation data, suggested that preferential interactions occur in the sugar-protein solutions. This could be the result of either sugar binding or a solute-solute thermodynamic effect, preferential hydration. The addition of sugars to deuterated solutions of bovine casein and caprine casein high in alpha(s1)-casein had little or no effect on either line width or chemical shifts of the (13)C NMR spectra of these milk proteins. (13)C NMR studies of sucrose, at various concentrations (100-300 mM) in the presence of caprine casein high in alpha(s1)-casein, showed no changes in either chemical shifts or T(1) values. This indicates that the sugar molecules tumble isotropically and therefore neither bind to the protein nor affect viscosity in the protein-sugar studies. All of these data suggest that the preferential exclusion of the sugar from the domain of the caseins results in preferential hydration of the caseins.

Quantification of alpha s1-casein in goat milk from French-Alpine and Anglo-Nubian breeds using reversed-phase high performance liquid chromatography.

Samples of isoelectrically precipitated goat casein from the milks of French-Alpine and Anglo-Nubian breeds were separated into four components in a single run by reversed-phase HPLC. The proportion of alpha s1-casein thus resolved was determined quantitatively. The method uses a reversed-phase C-4 column and a linear gradient from 30 to 50% acetonitrile in 30 min with trifluoroacetic acid constant at .1%. Sodium dodecyl sulfate-PAGE was carried out to establish the identity of the isolated components. By a comparison with previously published results for caprine and bovine milk caseins, the four peaks were identified as kappa-, alpha s2-, alpha s1-, and beta-casein. Quantitative variations in the chromatographically resolved alpha s1-casein fraction of goat milk were evident. Some individual goat milks contained high levels of alpha s1-casein (2.70 g/L), but others contained significantly low levels (.12 g/L). There was no statistical difference in the overall means between breeds in alpha s1-casein composition, but cluster analysis statistics showed three distinct categories of alpha s1-producers: high, medium, and low. Interestingly, 6 of 15 French-Alpine goats and only one Anglo-Nubian goat fell into the "low" producer category (.38 +/- .2 g/L). Thus, expression of the alpha s1-component may be genetically regulated but may not be a breed-specific trait.