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The formation of whey protein complexes during heating enhances the acid gelation of milk; hydrophobic interactions may play an important role in this acid gelation. To investigate this, the surface hydrophobicity of model heat-induced whey protein complexes was modified using acylation with various carbon chain lengths. It was determined that the size and thiol/disulfide distribution of the complexes were unaffected, and that the change in their apparent isoelectric point could be restricted within 0.5 pH unit. These complexes were added to whey protein-free skim milk systems and the resulting acid-gelation behaviour of the milk samples was measured. Increasing the hydrophobicity of the heat-induced whey protein complexes significantly increased the pH of gelation of the milk samples and strongly affected the final properties of the acid gels, demonstrating both the importance of the heat-induced complexes and the relevance of hydrophobic interactions in the acid-induced gelation of preheated milk.

During the manufacture of dairy products, heat treatments of milk result in the conversion of whey proteins into protein aggregates. These heat-induced aggregates provide new functional properties to dairy products. A protein tailoring approach is developed at UMR-STLO to produce heat-induced aggregates with targeted physicochemical and structural properties, and then to introduce them in a recombined milk to test their functional properties for the formation of dairy acid gels, used as models of yoghurts. It is also well known that denaturation and heat treatments improve interfacial and foaming properties of proteins. The BIA research unit works on understanding the role of heat-induced aggregates in the formation and stability of foams. While non-aggregated proteins adsorb rapidly at the interface, the heat-induced aggregates present in the continuous phase build a protein network that stabilizes the foam. The objective of theses studies is to determine the properties of aggregates that promote the building of new interactions in products that will result in the improvement of custom properties of dairy products and to produce new functional ingredients.; formation de nouvelles entités protéiques ou agrégats thermo-induits. Ces agrégats confèrent des propriétés fonctionnelles aux produits laitiers. Une approche d’ingénierie des agrégats thermo-induits a été développée à l’UMR-STLO qui vise à produire par des moyens variés des agrégats thermo-induits de propriétés physico-chimiques et structurales ciblées, puis à les introduire dans un lait recombiné afin d’évaluer leur propriétés fonctionnelles pour la formation des gels acides, pris comme un modèle du yaourt. Il est connu que la dénaturation et les traitements thermiques améliorent également les propriétés interfaciales et moussantes des protéines. Le laboratoire BIA mène des recherches pour comprendre le rôle des agrégats thermo-induits dans la formation et la stabilisation des mousses. Alors que les protéines non agrégées s’adsorbent rapidement à l’interface, la présence d’agrégats thermoinduits stabilise la mousse par formation d’un réseau de type gel dans la phase continue. L’objectif de ces travaux est de déterminer les propriétés des agrégats qui sont favorables à l’établissement de nouvelles interactions dans les produits, ces dernières étant à l’origine de l’amélioration de leurs propriétés d’usage et de créer de nouveaux ingrédients techno-fonctionnels.

The heat treatment of milk greatly improves the acid gelation of milk and is therefore largely applied in yoghurt manufacture. During the heat treatment, soluble and micelle-bound whey protein/κ-casein complexes are produced in milk. The complexes and their physico-chemical properties have been held responsible for the early gelation point, the increased final firmness and for the serum retention capacity of the acid gels made of heated milk. They are suspected to bring new functionalities to the casein micelles and to help the formation of interactions when building the gel network. In order to investigate the type of interactions that the complexes can affect throughout the acid gelation of milk, an original strategy would be to control the physico-chemical properties of the whey protein/κ-casein soluble complexes and to use them as vectors to modify the possible interactions in the milk. In that perspective, the different physico-chemical properties of the whey protein/κ-casein soluble complexes that are thought to significantly affect the acid gelation behaviour of the casein micelles are listed. Then, the physical, chemical and biological means that could possibly be applied to the formation of complexes in order to modulate each of the targeted property are reviewed and evaluated. In order to open a large choice for future investigation, these methods were found in a larger literature resource than milk, including other protein systems like model whey protein solutions or non-dairy globular protein systems. The food-compatible character of some of these means is indicated, for their potential technological interest.

Heat treatment of milk at 85–95 °C has long been reported to increase the pH of gelation and firmness of acid milk gels; hence its wide application in yoghurt manufacture. These changes have been attributed to the formation of heat-induced whey protein/κ-casein complexes in the milk, to which heat-denatured whey protein ingredients may be substituted. However, variations in resulting gels show that a possible role of κ-casein in determining the functional acid-gelation property of the complexes needs investigating. Model heat-induced whey protein/κ-casein complexes were produced of κ-casein content from 0 to 40% (w/w), but of similar size, secondary structure, surface hydrophobicity and thiol/disulphide distribution. These complexes were added to whey protein-free skim milk systems and the resulting acid-gelation behaviour of the milks was evaluated. The results showed a modification of the pH of gelation that was explained more by variation of the pI of complexes than by the κ-casein content.