High pressure casein micelles

Roach, A. and Harte, F. (2008). Disruption and sedimentation of casein micelles and casein micelle isolates under high-pressure homogenization. Innovative Food Sci. Emerg. Technol. 9,1-8. 

Pressure cycling high hydrostatic pressure (e.g., 500 Mpa) induces disintegration of casein micelles and reassociation on pressure reduction Hydrophobic casein particles, formed under pressure, reassociate into smaller and more irregularly shaped aggregates Dickinson 2006a... 

The self-assembly of caseins may be readily manipulated by processing methods that affect the integrity of native casein micelles and the character of the casein interactions in aqueous media. Examples of such procedures are (Dickinson, 2006) (i) acidification toward the isoelectric point (p/) (pH 4.6-4.8), leading to a neutralization of the net protein charge (ii) enzyme action, as exploited in the production of cheeses and fermented milks (iii) addition of divalent ions, especially, Ca2+ ions (iv) addition of sucrose or ethanol (v) temperature treatment and (vi) high-pressure treatment. 

We have seen earlier in this chapter how the self-assembly of casein systems is sensitively affected by temperature. Another thermodynamic variable that can affect protein-protein interactions in aqueous media is the hydrostatic pressure. Static high-pressure treatment causes the disintegration of casein micelles due to the dismption of internal hydro-phobic interactions and the dissociation of colloidal calcium phosphate. This phenomenon has been used to modify the gelation ability of casein without acidification as a consequence of exposure of hydrophobic parts of the casein molecules into the aqueous medium from the interior of the native casein micelles (Dickinson, 2006). High-pressure treatment leads to a reduction in the casein concentration required for gelation under neutral conditions, especially in the presence of cosolutes such as sucrose (Abbasi and Dickinson, 2001, 2002, 2004 Keenan et al., 2001). 

The casein micelle is an example of a naturally occurring nanoparticle formed when the different types of caseins (asl, 0 2, (5, and k) self-assem-ble around amorphous calcium phosphate. This allows it to be a natural carrier for calcium. The casein micelle also serves as a carrier for hydro-phobic bioactives (Livney and Dalgleish, 2007). Treatments such as ultra-high pressure have been reported to alter the structural characteristics of the casein micelle by partially removing parts of the surface of the casein (Sandra and Dalgleish, 2005). Altering the surface properties of these nanoparticles is expected to alter their functional properties. 

Sandra, S. and Dalgleish, D. G. (2005). Effects of ultra-high-pressure homogenization and heating on structural properties of casein micelles in reconstituted skim milk powder. Int. Dairy ]. 15,1095-1104. 

Needs, E., Stenning, R.A., Gill, A.L., Ferragut, V., and Rich, G.T. High-pressure treatment of milk effects on casein micelle structure and on enzymic coagulation, /. Dairy Res., 67, 31, 2000. 

High pressure induces disruption of casein micelles and denaturation of whey proteins, increases pH of milk, reduces rennet coagulation time, and increases cheese yield, thereby possessing potential application in cheese making (O Reilly et al.,... 

San-Martin et al. (2006) discussed the application of high pressure in cheese making with reference to its effect on milk components such as casein micelles, whey proteins, milk fat globules, as well as its impact on milk color, microbial inactivation, cheese ripening, and brining. 

Huppertz and Smiddy (2008) demonstrated that application of high pressure (250-300 MPa) led to an initial rapid micellar disruption this was followed by a partial reversal of the high-pressure-induced reassociation of micellar fragments. Partial internal cross-linking of casein micelles by transglutaminase prior to pressure treatment considerably slowed down both the disruption and reassociation processes. 

Anema (2008b) showed that application of combined process involving heat and high pressure to skim milk resulted in higher level of whey protein denaturation than that of heat or pressure treatment alone. High-pressure treatment alone decreased the casein micelle size, whereas the change in casein micelle size was not prominent for thermal or combined treatment. 

Anema, S.G. 2008b. Heat and/or high pressure treatment of skim milk Changes to the casein micelle size, whey proteins and the acid gelation properties of the milk. International Journal of Dairy Technology 61 245-252. 

Huppertz, T. and Smiddy, M.A. 2008. Behaviour of partially cross linked casein micelles under high pressure. International Journal of Dairy Technology 61 51-55. 

The ultrafiltered milk (Figure 2.7) shows a concentrated, continuous network, consisting primarily of casein micelles where some individual fat globules (1 to 18 jm in diameter) are included the diameter of the fat globules is greater than that observed in raw milk (1 to 5 jm in diameter) because a mechanical breakdown of the double membrane, followed by coalescence of the original fat droplets, is produced under high ultrafiltration pressures (Hernando et al., 1999). 

Monolayer techniques were used to characterize the interfacial properties of the resultant Fractions. Fraction I contained highly cohesive complexes that did not unfold at the interface and had an average diameter of 9.1 nm. These particles are thought to represent submicelles, previously identified in micelle formation. Fraction II showed interfacial properties that are characteristic of spread casein monomers, and contained mainly a -casein. The results are discussed in relation to casein interactions and micellar formation. Mixed monolayers of sodium caseinate/glyceride monostearate (NaCas/GMS) were also examined at different composition ratios. The results show that for low surface pressures (0-20 mNm ), there is a condensation ascribable to hydrophobic interactions in the mixed film. At high surface pressures, the hydrophobic interaction is modified and the protein is expelled from the monolayer into the subphase. These results are discussed in relation to emulsion stability. 

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