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How protein aggregates can modulate the texture of emulsified and acidified dairy model systems?. 3. Food structure and functionality forum symposium

The effect of heat treatment of milk on the formation of acid gels was investigated using model protein systems. Protein systems contained micellar casein alone (35 g kg −1 ) (systR) or in the combination with either β -lactoglobulin ( β LG) (4 g kg −1 ; systB) or egg ovalbumin (4 g kg −1 ; systO). Proteins in a milk ultrafiltrate (UF) were heat-treated at 90°C for 24 min. The same heat-treated systems without casein were also prepared: the UF alone (systUFR), with β LG (4 g kg −1 ; systUFB) and with ovalbumin (4 g kg −1 ; systUFO). Proteins in pellet and supernatant fractions were analysed using reverse-phase high-pressure liquid chromatography and sodium dodecyl sulphate polyacrylamide gel electrophoresis. The solubility of globular proteins in heated systUFB and heated systUFO was determined at pH 1.6–6.5 by absorbance measurements. Rheological changes during gelation with a starter at 42°C were determined and microstructures of gels by scanning electron microscopy. High quantities of κ -casein and β LG in the form of complexes (disrupted by 2-mercaptoethanol) were found in supernatant of heated systB and not in that of systO. The gelation pH was related to the solubility of the globular proteins, and the gelation pattern for systB and systO resembled that of heated milk and the systR pattern resembled that of unheated milk. The gels of systR were coarse and consisted of large particles. Particles in gels from systB and systO were smaller and less clustered. Although the two globular proteins behave differently during heating, they both confer to the casein micelles their solubility properties as a function of pH. A model is presented where both soluble and colloidal complexes including the globular protein interact at pH 5.5 and initiate the acid gelation.

Low levels of β-mercaptoethanol (β-ME) or N-ethylmaleimide (NEM) were added to preheated skim milk (SM) and preheated whey protein-enriched skim milk (WPE-SM). Addition of NEM did not affect the heat-induced interactions between the proteins in heated SM and WPE-SM; addition of β-ME reduced most disulphide bonds of κ-casein. Neither NEM nor β-ME affected the distribution of proteins between the colloidal and serum phases. The heated then treated SM and WPE-SM were acidified to form acid gels. Acid gels containing β-ME had higher yield stress and G′ values than those made from control heated milk. In contrast, adding NEM to heated SM and WPE-SM lowered the yield stress values of the acid gels, but affected the final G′ values only slightly. The rheological results suggested that thiol-disulphide exchange reactions occurred during acid gelation. However, the newly formed disulphide bonds influenced only the yield properties, not the G′ values of the gels.

Cream cheese is a good model for studying the effect of process and formulation changes on the modification of product texture. In this study, the intensity of heat treatment (72 degrees C/20 s or 94 degrees C/40 s), the final pH of acidification (pH 5.2 or 4.9) and the homogenization pressure (0, 5, 20, or 60 MPa) were studied. Special attention was paid to the whey protein denaturation, casein micelle dissociation, fat globule size and their relations with structural, rheological, and sensory characteristics of the cream cheese model. Rheological properties of final cream cheese mainly depended on homogenization pressure. Increasing the homogenization pressure led to a decrease in fat globule size and consequently in an increase in the cream cheese firmness. This result was modulated by heat treatment temperature and the subsequent whey protein denaturation. Cream cheese final pH between 5.2 and 4.9 had a low impact on rheological properties but was the most discriminating factor for sensory perception by strongly affecting product appearance especially its brightness and its shade.

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The heat-treatment of skim milk at 90◦C/10 min has long been reported to improve its acid gelation; hence its wide application in yoghurt manufacture. These changes have been attributed to the formation of heat-induced whey protein/kappa-casein complexes that interact with the casein micelles on heating, or shortly on acidification. Through this interaction, the complexes are believed to functionalize the casein micelles as to provoke their early destabilisation on acidification, to increase their bridging connectivity and to enhance their ability to engage stronger interactions on formation of the network. However, the exact nature of the colloidal attractive/repulsive interaction forces that drive gelation and are affected by the complexes is not known. In the seek for a comprehensive view of the acid gelation process in heated milk, a range of artificial protein complexes were produced, having one candidate property largely modulated. The success of the modification was assessed through the complete characterisation of the artificial complexes. The complexes were then introduced into a model milk system, and the acid gelation of these systems was studied. Examples are shown on repulsive electrostatic and attractive hydrophobic interactions

Heating milk at temperatures above 70 °C leads to the exposure of the free thiol group of β-lactoglobulin (β-lg). Free thiol group can then initiate thioldisulphide exchange reactions with other whey proteins or caseins (especially κ-casein, κ-cn) that contain disulphide bonds. The intermolecular aggregation reactions can play a significant role in determining the functional properties of milk. In this study, skim milk was treated with low levels of disulphide reducing agent (β-mercaptoethanol, β-ME) prior to heating. Then the treated milk was made to acid gels. • The functionality of the acid gels were examined using rheology. • The proteins interactions and distributions between the serum and colloidal phase were investigated using electrophoresis.

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.

Diplôme : Dr. Ing.; Le traitement thermique du lait à 85-95°C pendant quelques minutes améliore ses aptitudes à la gélifi cation acide. Le lait chauffé gélifi e à un pH plus élevé et le gel acide est plus homogène, plus élastique et présente moins d’expulsion de sérum qu’un lait cru. Ces nouvelles propriétés sont attribuées à la formation de complexes protéiques thermo-induits. Selon une hypothèse récente, ces complexes interagissent avec la micelle de caséines au cours du chauffage puis de l’acidifi cation. Ils fonctionnaliseraient la micelle en modifi ant la nature ou l’intensité des interactions colloidales qu’elle peut établir, et donc sa stabilité. Pour autant, la nature de ces interactions protéiques ne sont encore pas expérimentalement identifiées. L’objectif de cette étude est d’établir le lien entre les propriétés physico-chimiques des complexes et la nature des interactions établies au cours de l’acidifi cation. Le rôle de la caséine-κ, impliquée dans ces complexes, ainsi que les rôles des interactions hydrophobes et électrostatiques ont été testés. Selon une approche originale, chacune de ses propriétés a été largement modulée sur des complexes modèles, sans que les autres propriétés ne soient modifi ées. Les conséquences sur la formation du gel acide ont été mesurées par rhéologie et microscopie confocale. Il a été montré que les complexes thermo-induits imposent leurs propriétés fonctionnelles de gélifi cation acide à la micelle de caséines. Au cours de l’acidifi cation, la réduction des répulsions électrostatiques entre les complexes permet le rapprochement des particules et l’établissement d’interactions attractives, à l’origine de la déstabilisation du lait et de la construction des gels acides. Parmi les interactions attractives, les interactions hydrophobes jouent un rôle majeur. Ces résultats participent à élucider les mécanismes de formation du gel acide de lait.

Designing the heat-induced whey protein complexes: a key to engineer the interactions in acid milk gels. IDF World Dairy Summit

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.

Effect of heat treatment on the formation of acid milk gel: a kinetic study using rheology and confocal microscopy. 4. International Dairy Federation (IDF) Dairy Science and Technology Week

Gelation of heat-treated milk protein solutions by acid fermentation was performed using a model system of micellar casein alone (systR), micellar casein and β -lactoglobulin (systB) or micellar casein and egg ovalbumin (systO) dissolved in a milk ultrafiltrate and heat-treated at 90°C for 24 min. Solubility of globular proteins at given pH values and their interactions with casein were determined. Particle size and ζ -potential of the protein systems were measured before and after heat treatment. Acid gelation of the heated systems was monitored using dynamic low-amplitude strain oscillation. Ovalbumin alone or with casein produced larger particles than casein alone or with β -lactoglobulin upon heat treatment. The systems gelled at different pH values, i.e. 4.88, 5.47 and 5.88 in systR, systB and systO, respectively. The latter two pH values were clearly related to the pH of the loss of solubility of the heated globular proteins. While the gelation pattern for systR resembled unheated milk, the pattern for systB and systO showed the usual maximum for tan δ of heated milk.

The structure of casein in heated suspensions affects the acid gelification of milk. FIL-IDF "Fermented milk"