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Acid milk gels with fat (3.5 or 10%) or without fat containing 0.2–1% of whey proteins (WP) or 0.2–1.5% of protein aggregates (fractal aggregates, microgels and mixed casein/WP aggregates with an 80/20 ratio) were investigated. The fat-containing systems were homogenized and the systems were preheated (90 °C, 11 min) and acidified to a pH of 4.6 by adding glucono-delta-lactone. The protein composition of the fat droplet interface was characterized by SDS-PAGE and the textural and rheological properties, microstructure, and whey separation of acid networks were determined. WP and fractal aggregates showed the best ability to improve the textural properties and microstructure of all acid milk gels and reduce whey separation. Increasing the concentrations in microgels and mixed aggregates did not lead to an increase in gel firmness or generate a strong impact on the protein network, but a high concentration in mixed aggregates could reduce whey separation. The fat droplet interface was made almost exclusively of caseins, even if the fractal aggregates could also be adsorbed when the interfacial surface was increased either through fat content or through an increase in homogenization pressure. Adding proteins changes the textural properties of the acid gels in all the systems, mainly due to their role in the continuous phase.

Combined effects of trisodium citrate (TSC), transglutaminase (Tgase) and heating were studied with relation to the mechanical properties of acid milk gels. Acidification of skimmed microfiltered milk samples was performed at 30 °C by addition of glucono – δ lactone to achieve final pH of 4.6 in 4 h. Increasing levels of TSC (0, 10 and 20 mM) were added to the samples, followed by addition of a constant level of Tgase (3 U/g of protein), and subsequent heating to 85 °C for 30 min. Prior to acidification, suspensions were analyzed in terms of amount of sedimentable material, hydration, hydrodynamic diameter (Dh) and ζ-potential (ζ) of the particles. Acid-induced gelation was studied using small deformation rheology followed by large deformation test. The water holding capacity of the gels (WHC) was performed at pH 4.6. Gels microstructure were investigated by confocal laser scanning microscopy at pH 4.6 and determination of mean pore size was performed by 2D-correlation analysis. Results showed that combined Tgase and TSC treatments had significant effect on rheological properties, WHC and gel microstructure. Higher stiffness of the gels was observed with Tgase and TSC at 10 mM, whereas a homogenous network distribution with smaller pore size was observed in the presence of Tgase and TSC at 20 mM.

International audience; Water-in-water (W/W) or aqueous two-phase system (ATPS) emulsions are finding increasing applications in diverse fields of technology, for instance as protocells [1], and reactors for the synthesis of hydrogel particles [2], and enzyme-laden microgels [3]. Successful ATPS emulsification depends on thermodynamic incompatibility between two (bio)polymers, causing segregative phase separation [4]. Herein, we demonstrate that hydrophobization of whey proteins, by grafting acetyl moieties and heat denaturation, makes the proteins immiscible with a co-charged polysaccharide solution (alginate). Addition of erythritol, which is a low-calorie and zero-glycemic sugar alcohol, to the hydrophobized protein solution, enhanced emulsification and increased the stability of the resulting emulsion. Subsequently, the acid-induced gel properties of the emulsion was studied by dynamic rheometry and confocal microscopy. Erythritol addition reduced the surface tension (at the air-water interface) of the hydrophobized protein solution, enhancing the incompatibility between protein and alginate. It also postponed the gelation time of the hydrophobized protein solution and resulted in formation of a softer gel. Confocal imaging of the emulsion gel confirmed micro-phase separation of alginate and the droplets aggregation in the protein-rich matrix.

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

When skim milk (SM) and whey-protein-enriched skim milk (WPE-SM) were heated (80 C, 30 min)with N-ethylmaleimide (NEM, 0e0.8 mM), the levels of residual native whey proteins increased to w70%,whereas the levels of disulphide-bonded whey proteins decreased to

In acidified dairy products, the size of the whey protein particles could play a key role in the final structure of the gel. In the present study, small (SM; 2.5 ± 1.2 μm), medium (MM; 4.2 ± 2.2 μm), and large (LM; 18.4 ± 7.2 μm) whey protein microbeads were produced by mixing a 150 g.kg−1 whey protein isolate (WPI) solution and n-dodecane in the presence of polyglycerolpolyricinoleate (PGPR) surfactant at different shear rates and were then stabilized through heat gelation. The microbeads were then washed by centrifugation, dispersed at 70 or 90 g.kg−1 in milk ultrafiltrate, and acidification was performed at 35 °C by adding glucono-δ-lactone to achieve the final pH of ~4.5 in 6 h. Acid gelation was monitored using small deformation rheology, while the gel microstructure was investigated microscopically. The results showed that smaller size of microbeads promoted gels with a higher stiffness and a smaller pore size distribution. The effects were particularly significant at SM microbeads as the number of particles in this system was higher than in LM or MM, hence more connectivity between particles.

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.

Milk contains a colloidal particle, called the casein micelle (CAS) and globular proteins, called whey proteins. If a whey protein solution is heated, heat-induced whey proteins denature and form colloidal aggregates, called whey protein aggregates (WPA). These compounds are present in milk when it is heated, as for the manufacture of yoghurts. Indeed, CAS and WPA form particular gels by acidification, either in mixture or even separatly. The aim of this aim was to understand how the gel formed from these 2 colloids. The CAS and WPA were dispersed in a milk permeate at different protein concentrations ranging from ~15 to ~105 g kg-1 separately and in a mixture at 80/20 on a weight ratio, respectively and as in milk. Acidification was performed at 35 °C by addition of glucono- -lactone to achieve a final pH at ~ 4.5 in 6 h. Acid gelation was studied by rheology, namely by small amplitude oscillation, followed by large deformation measurement at pH 4.5. Viscoelastic moduli versus time or pH were compared for the different systems at different concentrations, while the microstructure of the gels at their final pH value was studied by confocal scanning laser microscopy. Results show that increasing the protein concentration for the 3 systems promoted a faster gelation, led to higher moduli and to gels with smaller pore sizes, which is due to the increase in the number of particles and consequently probably in the number of potential bonds between them and to the reduction in the distances between colloids. Replacing 20 % of CAS by WPA increased the moduli of the acid gels as long as the protein concentrations was < 90 g kg-1 and increased their pH of gelation as compared to pure CAS gels. We fitted the model of Shih et al. (1990) on the elastic modulus and the yield strain to determine the fractal dimension (Df) of the gel structure and compared the mixture gel to the 2 other ones. From the strain yield behavior, we concluded that the mixture gel and the CAS gel were of the weak-link regime, i.e. that bonds inside the protein clusters making the gel are stronger than bonds between clusters, with Df > 2.6, and probably built under a reaction-limited mechanism. The WPA gel was of the strong-link regime, with lower Df value around 1.7 and produced by a diffusion-limited mechanism.We suspect that these differences were possibly due to the different surface interactive properties of these 2 colloids, together with their different internal structure that lead to a different mechanism of acid gelation.

Thermo-induced aggregates from whey proteins and casein micelles assembled to build the acid gel of milk, commonly known as the milk yoghurt. This study aims understanding the respective role of these two kinds of particles in the building of yoghurt gels by changing their ratio in model systems and observing how they interact, depending on their respective numbers. Thermo-induced aggregates were produced by heating a whey protein solution. Mixtures of the two particles were prepared in a permeate solution of milk at different weight ratios and at a constant total protein content ( 45 g/kg). Acidification to a final pH value of 4.5 was obtained by the addition of glucono-delta-lactone at 35°C for 6 h. Gelation was followed by dynamic rheology and a large deformation test at pH 4.5. Microstructure and whey retention of the gels at pH 4.5 were also studied. Results confirm that particles behaved differently during the acidification, depending on their respective number. Increasing the proportion of thermo-induced aggregates in model systems leads to a large increase of their pH of gelation, and of the gel visco-elastic modulus, “solid-like” properties and whey retention and to a reduction in the gel pore size. This effect is dramatic for a 10% increase in the proportion of aggregates as compared to a pure casein micelle gel. The model system containing 10% of aggregates could consist in an equimolar mixture of the 2 particles and coverage of the casein micelle surface of about 8%, assuming that aggregates interacted with the surface of the casein micelle. These results are discussed through physical interactions built between the 2 kinds of particles having different physicochemical properties. A preferential interaction between casein micelles and whey protein aggregates could explain these results and bring new structural and mechanical properties to acid milk gels.; Les agrégats thermo-induits de protéines sériques et les micelles de caséine s’assemblent pour former le gel acide de lait ou yaourt. Cette étude vise à évaluer le rôle respectif des 2 types de particules en changeant leur ratio dans des systèmes modèles et à observer comment ces 2 types de particules colloïdales interagissent selon leur nombre respectif. Les agrégats thermo-induits de protéines sériques sont produits par chauffage d’un isolat de protéines sériques. Les mélanges des 2 particules sont préparés dans un perméat de lait à différents ratios d’agrégats et à concentration constante en protéines ( 45 g/kg). L’acidification est obtenue par ajout de glucono-delta-lactone à 35°C afin d’atteindre un pH final de 4.5 en 6 h. La gélification est suivie par rhéologie dynamique, suivie d’un test en forte déformation à pH 4.5. La microstructure des gels à pH 4.5 est étudiée, ainsi que leur aptitude à la rétention de sérum. Les résultats confirment que les particules se comportent différemment pendant le processus de déstabilisation, selon leur nombre respectif. L’augmentation de la proportion en agrégats entraine une forte augmentation du pH de gélification, des modules élastiques finaux et du caractère « solid-like » des gels, une réduction de la taille des pores du gel et une meilleure rétention du sérum. Cet effet est particulièrement remarquable pour une proportion d’agrégats de 10%, correspondant à un mélange équimolaire des 2 types de particules et à un taux de couverture de la micelle par les agrégats estimé à 8%. Ces résultats sont discutés vis-à-vis des interactions physiques qui se bâtissent entre les 2 populations de particules de propriétés physicochimiques différentes. Nous confirmons qu’une interaction préférentielle existe entre les micelles de caséine et les agrégats de protéines sériques et qu’elle permet de conférer des propriétés structurelles et mécaniques spécifiques aux gels acides de lait.

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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

Ingénierie des nanoparticules protéiques et propriétés rhéologiques des gels acides laitiers. 5. Rencontres de Biologie-Physique du Grand Ouest

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

Ingénierie des nanostructures protéiques et propriétés rhéologiques des gels acides de lait. 45 iéme colloque du GFR Rhéologie des matériaux nanostructurés

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

Stirred yoghurt is obtained by pumping and cooling of set yoghurt at pH 4.6 followed by filling. During storage, the texture of the stirred yoghurt improved, with a large increase in viscosity or viscoelastic properties, which is known as rebodying. We studied the nature of bonds established between protein particles after stirring that are responsible for this rebodying.

Comparison of set or stirred acid gels fermented at 42°C and 30°C. IUFoST 13th World Congress of Food Science and Technology

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"