Asi-Caseins

South Uist in the Outer Hebrides. A number of positive results were obtained and these compared well with the analysis of the fatty acids. However, some samples gave a negative result for ocsl-casein but a positive one for milk fat based on the A13C value. The possibility exists that some of the residues represent sheep milk (which would give a negative result for bovine asi-casein), or that in some cases the casein molecule is degraded with consequent depletion of the immunological response. 

Semenova, M.G., Antipova, A.S., Belyakova, L.E., Dickinson, E., Brown, R., Pelan, E., Norton, I. (1999). Effect of pectinate on properties of oil-in-water emulsions stabilized by asi-casein and P-casein. In Dickinson, E., Rodriguez Patino, J.M. (Eds). FoodEmul-sions and Foams Interfaces, Interactions and Stability, Cambridge, UK Royal Society of Chemistry, pp. 163-175. 

Figure 3.4 Effect of polysaccharide on protein-stabilized emulsions. The diameter ratio, j43nuxtlire / J43protem is plotted against the molar ratio R (moles polysaccharide / moles protein). Here J43nuxtlire is average droplet diameter in fresh emulsion prepared with protein + polysaccharide, and d43pTOtQm is average diameter in emulsion stabilized by protein alone. Key , , legumin + dextmn (48 kDa) or legumin + dextran (500 kDa), respectively (0.5 w/v % protein, 10 vol% oil, pH = 8.0, /= 0.1 M) (Dickinson and Semenova, 1992) O, , asi-casein + pectinate and p-casein + pectinate at pH = 7.0, / = 0.01 M (2.0 w/v % protein, 40 vol% oil), respectively , p-casein + pectinate at pH = 5.5, / = 0.01 M (2.0 w/v % protein, 40 vol% oil) (Semenova et al, 1999). Reproduced from Semenova (2007) with permission.

Figure 6.3 Representation of casein self-association structures according to die simple copolymer model (a) p-casein, (b) asi-casein. Reproduced from Home (1998) with permission.

Another way to interpret the above observations would be in terms of the general principle that effective steric stabilization of polymer-coated droplets requires that the continuous phase be a good quality solvent for the polymeric stabilizer. Under poor quality solvent conditions (asi-casein at high ionic strength), the required entropic stabilizing repulsion of the adsorbed protein layer is converted into a destabilizing polymer-mediated attraction (Dickinson and Stainsby, 1982 Dickinson, 2006). 

Figure 7.9 Effect of pectin (DE = 76%) on (a) creaming of protein-stabilized emulsions (11 vol% oil, 0.6 wt% protein, 0.28 wt% pectin, I = 0.01 M) containing (A) asi-casein (pH = 7), (A) p-casein (pH = 7), and ( ) o i-casein (pH = 5.5) and (b) steady-state shear viscometry of casein-stabilized emulsions (40 vol% oil, 2 vt% protein). Apparent shear viscosity at 22 °C is plotted against stress pH = 7.0, / = 0.01 M, (A) -casein, (A) p-casein, ( ) ocsi -casein + 0.5 wt% pectin, ( ) p-casein + 0.5 wt% pectin, ( ) p-casein + 1.0 wt% pectin, (O) as[-casein + 1.0 wt% pectin pH = 5.5,1 = 0.01 M, (x) ocsi -casein, (O) as[-casein + 0.5 wt% pectin, ( ) oc -casein + 1.0 wt% pectin. Reproduced from Semenova (2007) with permission.

Figure 7.10 Effect of the thermodynamic incompatibility of otsi/p-casein + high-methoxy pectin (pH = 7.0, / = 0.01 M) on phase diagram of the mixed solutions and elastic modulus of corresponding casein-stabilized emulsions (40 vol% oil, 2 wt% protein), (a) (O) Binodal line for p-casein + pectin solution with critical point ( ) ( ) binodal line for asi-casein + pectin solution with critical point ( ). (b) Complex shear modulus G (1 Hz) is plotted against the pectin concentration (O) p-casein ( ) o i -casein. Dotted lines indicate the range of pectin concentration for phase separation in the mixed solutions. The pectin was added to the protein solution before emulsion preparation. Data are taken front Semenova et al. (1999a).

Surface shear rheology at the oil-water interface is a sensitive probe of protein-polysaccharide interactions. In particular, there is considerable experimental evidence for a general increase in surface shear viscosity of protein adsorbed layers as a result of interfacial complexation with polysaccharides (Dickinson et al., 1998 Dickinson and Euston, 1991 Dickinson and Galazka, 1992 Semenova et al., 1999a Jourdain et al., 2009). One such example is the case of asi-casein + pectin at pH = 5.5 and ionic strength = 0.01 M (Ay = - 334 x 10 cm /mol) the interfacial viscosity after 24 hours was found to be five times larger in the presence of pectin (i.e., values of 820 80 and 160 20 mN m 1 with and without pectin, respectively) (Semenova et al., 1999a). 

The other major casein monomer in bovine milk is asi-casein. The SCF theory suggests that a loop-like protein conformation is favoured for adsorbed asi-casein (see Figure 8.1a) (Dickinson et al., 1997 Home, 1998). This implies a reduced hydrodynamic thickness of the adsorbed layer for as]-casein as compared with p-casein. 

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-... 

Anand, K., Damodaran, S. (1996). Dynamics of exchange between asi-casein and p-casein during adsorption at air-water interface. Journal of Agricultural and Food Chemistry, 44, 1022-1028. 

Dickinson, E., Horne, D.S., Pinfield, V.J., Leermakers, F.A.M. (1997). Self-consistent-field modelling of casein adsorption. Comparison of results for asi-casein and p-casein. Journal of the Chemical Society, Faraday Transactions, 93, 425 132. 

Figure 3.1. Primary Structure of Bos asi-casein B-8P. (From Mercier et al. 1971 Grosclaude et al. 1973 Eigel et al. 1984. Reprinted with permission of the American Dairy Science Association.)...

The association of asi-casein B in the neutral range, pH 6.6, appears to occur in a series of association steps at ionic strengths greater than 0.01 (Figure 3.11) (Schmidt 1970A, 1982). As the ionic strength in-... 

Figure 3.11. The association of asi-casein B at different values of pH and ionic strength. Molecular weights determined at 20°C using the light-scattering technique. (From Schmidt 1982. Reprinted with permission of Elsevier Applied Science Publishers, Ltd.)...

Electrophoretic Methods. Several electrophoretic procedures have been developed to fractionate or purify the various caseins (McKenzie 1971C Thompson 1971 Whitney 1977). Wake and Baldwin (1961) fractionated whole casein by zone electrophoresis on cellulose powder in 7 M urea and 0.02 ionic strength sodium phosphate buffer at pH 7 and 5°C. Payens and co-workers employed several somewhat different electrophoretic conditions for the fractionation and purification of the caseins on cellulose columns (Payens 1961 Schmidt and Payens 1963 Schmidt 1967). Three fractions, as-, k-, and /3-caseins, were separated at pH 7.5 and 30°C with 4.6 M urea-barbiturate buffer. The purification of asi-casein and the separation of the genetic variants of K-casein were accomplished by altering the electrophoretic conditions. Manson (1965) fractionated acid casein on a starch gel column stabilized by a density gradient at 25 °C. 

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