Casein lactoglobulin, interaction

Heat denaturation of /3-lactoglobulin is accompanied by alterations in the properties of x-casein (Zittle et al. 1962). x-Casein and /3-lactoglobulin interact through disulfide linkages when heated together or when k-... 

When heated in the presence of whey proteins, as in normal milk, K-casein and jS-lactoglobulin interact to form a disulphide-linked complex which modifies many properties of the micelles, including rennet coagulability and heat stability. 

Calculations from SCF theory of the mixed layer structure, and of the interaction potential for a pair of mixed layers as a function of interlayer separation, suggest that the mixed layer has a heterogeneous morphology perpendicular to die interface (Parkinson et al., 2005). This localized segregation arises from the excluded volume interaction between spaced-out casein chains and the dense brush-like layer that was invoked in the simple SCF model to represent the p-lactoglobulin adsorbed monolayer. 

Protein-polysaccharide complexation affects the surface viscoelastic properties of the protein interfacial layer. Surface shear rheology is especially sensitive to the strength of the interfacial protein-polysaccharide interactions. Experimental data on BSA+ dextran sulfate (Dickinson and Galazka, 1992), asi-casein + high-methoxy pectin (Dickinson et al., 1998), p-lactoglobulin + low-methoxy pectin (Ganzevles et al., 2006), and p-lactoglobulin + acacia gum (Schmitt et al., 2005) have all demon-... 

Elfagm, A. A. and Wheelock, J. V. 1978B. Heat interaction between a-lactalbumin, /3-lactoglobulin, and casein in bovine milk. J. Dairy Sci. 61, 159-163. 

Casein micelle proteins are primarily a8i-, as2-, /3-, and -caseins in approximate proportions 3 .8 3 1. asi-Casein has eight or nine phosphate groups, depending on the genetic variant. aS2-Casein is the most hydrophilic of the caseins. It has two disulfide bonds which, by severe heat treatment, can be caused to interact with those of /3-lactoglobulin. It also has 10 to 13 phosphate groups and is very sensitive to the calcium ion concentration (Kinsella 1984 Swaisgood 1982). 

It will be noted that K-casein is the only monomer subunit that contains a disulfide group, and this is undoubtedly responsible for its ability to self aggregate as well as to interact with g-lactoglobulin during heat processing of milk. 

K-casein also contains two Cys residues per monomer subunit and is thus capable of interacting with the whey proteins, e.g., mainly g-lactoglobulin, via the disulfide interchange mechanism at temperatures at or above 65°C. This latter phenomenon is believed to be important in providing colloidal stability to the milk casein micelle system, as well as to the whey proteins, in high temperature processed milk products. It has also been postulated that this latter interaction with g-lactoglobulin may alter the availability of K-casein in the micelle, and thus has a detrimental effect upon the cheese making properties of milk (4). 

Oldfield, D.J., Singh, H., Taylor, M.W., and Pearce, K.N. (2000). Heat-induced interactions of beta-lactoglobulin and alpha-lactalbumin with the casein micelle in pH-adjusted skim milk. Int. Dairy J. 10,509-518. 

Casein is low in sulphur (0.8%) while the whey proteins are relatively rich (1.7%). Differences in sulphur content become more apparent if one considers the levels of individual sulphur-containing amino acids. The sulphur of casein is present mainly in methionine, with low concentrations of cysteine and cystine in fact the principal caseins contain only methionine. The whey proteins contain significant amounts of both cysteine and cystine in addition to methionine and these amino acids are responsible, in part, for many of the changes which occur in milk on heating, e.g. cooked flavour, increased rennet coagulation time (due to interaction between /3-lactoglobulin and (c-casein) and improved heat stability of milk pre-heated prior to sterilization. 

The structure of adsorption layers is of great importance during preparation of food foams and emulsions. These problems have been studied in [144] for protein adsorption at the liquid/gas interfaces and in [145] for liquid/liquid interfaces. Due attention is also paid to the interaction of typical emulsifiers and proteins during preparation of food emulsions [146 - 147]. Addition of an oil-soluble emulsifier to proteins during preparation of w/o emulsions [146] increased the emulsification rate, but at high concentrations decreased it due to the increase in oil viscosity. In this case, the emulsifier displaced (3-casein from the surface easier than P-lactoglobulin. However, there was no complete displacement into the aqueous phase since multiple emulsions were formed, as mentioned above [142 -143]. Hence, the ehoice of the surfactant/protein ratio is important. 

Because protein-ba sed foams depend upon the intrinsic molecular properties (extent and nature of protein-protein interactions) of the protein, foaming properties (formation and stabilization) can vary immensely between different proteins. The intrinsic properties of the protein together with extrinsic factors (temperature, pH, salts, and viscosity of the continuous phase) determine the physical stability of the film. Films with enhanced mechanical strength (greater protein-protein interactions), and better rheological and viscoelastic properties (flexible residual tertiary structure) are more stable (12,15), and this is reflected in more stable foams/emulsions (14,33). Such films have better viscoelastic properties (dilatational modulus) ( ) and can adapt to physical perturbations without rupture. This is illustrated by -lactoglobulin which forms strong viscous films while casein films show limited viscosity due to diminished protein-protein (electrostatic) interactions and lack of bulky structure (steric effects) which apparently improves interactions at the interface (7,13 19). 

Forrest, S. A. Yada, R. Y Rousseau, D., Interactions of vitamin D-3 with bovine beta-lactoglobulin A and beta-casein. Journal of Agricultural and Food Chemistry (2005) 53, 8003-8009. 

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