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Food Chemistry
Food Chemistry 87 (2004) 103–110 www.elsevier.com/locate/foodchem

Properties of casein micelles in high pressure-treated bovine milk
Thom Huppertz, Patrick F. Fox, Alan L. Kelly
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Department of Food and Nutritional Sciences, University College Cork, Cork, Ireland Received 18 July 2003; received in revised form 28 October 2003; accepted 28 October 2003

Abstract Highpressure (HP) treatment of milk alters many of its constituents and properties; in this study, HP-induced changes in casein micelles were examined. HP treatment of milk at 250 MPa increased casein micelle size by $30%, whereas treatment at 400 or 600 MPa reduced it by $50%. Pre-denaturing whey proteins in milk by heat treatment or preventing interactions between b-lactoglobulin and j-casein, by additionof sulphydryl-blocking or reducing agents to milk prior to HP treatment, had little influence on HP-induced changes in casein micelle size. Addition of ethanol to milk, prior to HP treatment, resulted in the formation of large casein aggregates. On treatment at 250 MPa, larger increases in casein micelle size were observed in milk reconstituted from larger micelles, suggesting that increases inmicelle size at this pressure are probably due to the formation of casein aggregates. HP treatment reduced the LÃ -value of milk; these changes were rapidly reversed on storage of milk at 37 °C, but maintained on storage at 5 °C. Similar to untreated milk, micelles in HP-treated milk were susceptible to dissociation by urea and tri-sodium citrate. Altered properties of casein micelles in HP-treatedmilk may influence the properties of products made from such milk. Ó 2003 Elsevier Ltd. All rights reserved.
Keywords: High pressure; Milk; Casein micelles; Aggregation

1. Introduction Under high pressure (HP), considerable changes occur to a wide variety of properties and constituents of milk (for review, see Huppertz, Kelly, & Fox, 2002). Casein micelles are influenced considerably by HPtreatment. Under HP, the light transmittance of milk increases (Kromkamp, Moreira, Langeveld, & Van Mil, 1996), suggesting disruption of the casein micelles, probably as a result of solubilisation of colloidal calcium phosphate (CPP) and disruption of hydrophobic and electrostatic interactions (Needs, Capellas, Bland, Manoj, MacDougal & Paul, 2000a; Schrader & Buchheim, 1998). Hydrogen bonds, anotherimportant structural feature in casein micelles, are thought not to be disrupted by HP treatment (Hendrickx, Ludikhuyze, Van den Broek, & Weemaes, 1998). On release of pressure, the increase in light transmittance of milk is at
Corresponding author. Tel.: +353-21-4903405; fax: +353-214270213. E-mail address: a.kelly@ucc.ie (A.L. Kelly). 0308-8146/$ - see front matter Ó 2003 Elsevier Ltd. Allrights reserved. doi:10.1016/j.foodchem.2003.10.025
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least partially reversible (Kromkamp et al., 1996), suggesting reassociation of casein particles, probably due to the reformation of hydrophobic bonds (Needs et al., 2000a) and possibly also electrostatic interactions. At 20 °C, treatment at a pressure up to 200 MPa had little effect on casein micelle size (Desobry-Banon, Richard, & Hardy, 1994;Huppertz, Fox, & Kelly, 2004a; Needs, Stenning, Gill, Ferragut, & Rich, 2000b), treatment at 250 MPa increased average casein micelle size (Huppertz et al., 2004a; Huppertz, Fox, & Kelly, 2004b) and treatment at >300 MPa reduced micelle size by $50% (Desobry-Banon et al., 1994; Gaucheron, Famelart, Raulot, Mariette & Le Graet, 1997; Huppertz et al., 2004a, 2004b; Needs et al., 2000b). Large caseinaggregates were observed in milk treated at 200–400 MPa at 40–45 °C (Garcia-Risco, Olano, Ramos, & Lopez-Fandino, 2000; Gaucheron et al., 1997; Law et al., 1998). HP-induced increases in micelle size may be due to interactions between casein micelles and denatured whey proteins (Huppertz et al., 2004a; Schrader & Buchheim, 1998) or to the formation of casein aggregates (Huppertz

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