Sensitivity of emulsions stabilised by bovine β-casein and lactoferrin to heat and CaCl2

Lactoferrin and β-casein represent a large proportion of the proteins in human milk and, as a result, are important ingredients for the manufacture of infant formula. At pH 7, lactoferrin has a net positive charge, compared to the negatively charged β-casein. The effects of CaCl2 and heat treatment on the stability of oil-in-water emulsions stabilised by lactoferrin and/or β-casein were investigated. The ζ-potential values of emulsions stabilised with β-casein or a 1:1 mixture of β-casein/lactoferrin significantly (P < 0.05) decreased on addition of 30 mM CaCl2, while CaCl2 had no significant (P > 0.05) effect on the ζ-potential of lactoferrin-stabilised emulsions. Particle size of β-casein-stabilised emulsions increased significantly (P < 0.05) upon the addition of 30 mM CaCl2, due to flocculation, while emulsions stabilised with lactoferrin and β-casein/lactoferrin remained unaffected by CaCl2 addition. Oscillatory rheology measurements showed that β-casein-stabilised emulsions formed a gel when CaCl2 was added, due to calcium-bridging, compared to lactoferrin-stabilised emulsions, where a weaker gel was formed in the presence of CaCl2. While, 30 mM CaCl2 did not increase the elastic modulus (G′) of β-casein/lactoferrin-stabilised emulsions. Lactoferrin protected β-casein from calcium-induced flocculation by: (1) binding calcium ions via sialic acid groups on lactoferrin molecules and reducing the number of free ions available to form calcium linkages between β-casein molecules; (2) electrostatically interacting with negatively charged phosphate groups on β-casein molecules, blocking calcium–phosphate interactions and (3) providing additional stability through increased steric repulsion between β-casein/lactoferrin conjugates, due to the large molecular size of lactoferrin.