-We studied three complex food emulsions which differed only by the weight ratio of casein-to-whey proteins (80:20, 60:40, 0:100). We considered the stability of the fat droplets against aggregation/coalescence, the amount of adsorbed proteins, and the protein composition of the interface. Our results indicated that coalescence was inhibited whatever the protein composition. However, aggregation between fat droplets was observed in the casein-free emulsion (0:100), where the amount of adsorbed proteins at the fat globule surface was the lowest. Quantitative SDS-PAGE experiments showed that preferential adsorption of caseins over globular proteins occurs only in the emulsion based on the intermediate (60:40) casein-to-whey protein ratio. Furthermore, the proportion of adsorbed proteins which were found in polymeric forms seemed to increase with decreasing casein-to-whey protein ratio. The fat globule stability, total fat protein load and proportion of adsorbed protein in the polymeric forms were discussed in terms of adsorption properties of milk proteins at the oil-water interface, in relation to the casein-to-whey protein ratio.Milk protein / emulsion / stability / protein adsorption / disulfide bond Résumé -Composition de la couche protéique adsorbée à la surface des globules gras dans des émulsions alimentaires complexes contenant différents rapports caséine/protéines sériques. Nous avons étudié trois émulsions alimentaires complexes qui ne diffèrent entre elles que par le rapport massique caséine/protéines sériques (80:20 ; 60:40 et 0:100). Cette étude a été réalisée en considérant la stabilité des gouttelettes lipidiques vis-à-vis de l'agrégation/coalescence, la quantité de protéines adsorbées par unité de surface de globule gras et la composition de l'interface. Nos résultats ont montré l'absence de coalescence dans les trois émulsions, et la présence d'agrégats n'a été observée que dans l'émulsion (0:100) ne contenant pas de caséine. Dans ce dernier cas, la quantité de protéines adsorbées à la surface des globules gras est la plus faible. Une étude quantitative par électro-phorèse sur gel de polyacrylamide a indiqué une adsorption préférentielle des caséines par rapport aux protéines sériques dans le cas de l'émulsion préparée à base du ratio intermédiaire (60:40) de caséine/protéines sériques. De plus, la proportion de polymères adsorbés à la surface de globules gras semble augmenter lorsque le rapport massique caséine/protéiques sériques diminue. La stabilité des émulsions, la charge protéique des globules gras et la proportion de polymères adsorbés sont discutées en fonction du rapport caséine/protéines sériques.Protéine laitière / émulsion / stabilité / adsorption des protéines / pont disulfure

The thermal behavior of hydrogenated palm kernel oil-in-water emulsions, which differed in their milk-protein composition, was studied in parallel with other characteristic parameters such as the aggregation/coalescence of fat droplets, and the proportion of adsorbed proteins at the oil/water interface. DSC was applied to monitor the crystallization and melting behavior of nonemulsified and emulsified fat samples. Comparison between nonemulsified and emulsified fat samples showed that in emulsified samples the initial temperature of fat crystallization and the temperature of the completion of melting were invariably lower and slightly higher, respectively. Furthermore, in complex food emulsions the supercooling temperature needed to initiate fat crystallization and the variation in its growth rate in the cooling experiment were dependent on the amount and nature of the adsorbed proteins. Our results indicate that the total replacement of milk proteins by whey proteins affected the fat crystallization behavior of emulsified fat droplets, in parallel with changes in their protein surface coverage and in their physical stability against fat droplet agglomeration.Paper no. J10374 in JAOCS 80, 741-746 (August 2003). KEY WORDS:Adsorbed proteins, DSC, emulsions, fat crystallization, milk proteins.In emulsions, the resistance to physical changes, such as flocculation/coalescence of fat globules, is related to properties of the interfacial layer around the fat globules, different physicochemical interactions or chemical bonds, and the interdroplet medium (1-3). In addition to these properties, fat crystallization has been demonstrated to play a role in emulsion stability (4,5). Numerous techniques such as dilatometry (6,7), ultrasonic velocity measurements (8,9), X-ray diffraction, and DSC (10-15) have been used to study physical state transitions in emulsions. Studies performed on hydrocarbon and TG oil-in-water emulsions, on emulsified milk, or on natural fat have focused on the crystallization mechanism in dispersed fats. They showed that the adsorbed emulsifiers might act as catalytic impurities, leading to a distinction between bulk heterogeneous and surface heterogeneous nucleation, and to a mechanism of secondary nucleation (16). More recently, Hindle et al. (9) showed that the kinetics of the secondary nucleation process in supercooled n-hexadecane and cocoa butter oil-in-water emulsions could be mediated by droplet-droplet collisions. In most of the previous studies, the droplet size, the structure of the hydrophobic chains of the emulsifier, and the nature of the fat were observed to have effects on the degree of supercooling and on the crystallization rate of finely dispersed fat, but without considering the possible effect of the composition of the fat protein layer on the crystallization behavior. Milk proteins are largely used to improve the kinetic stability of food emulsions (3). Caseins (the major protein component of milk) have a micellar structure, and among their molecule components (α s1 -, α s2 -, β-...

No abstract