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The enzymatic milk gel is a weak physical gel of casein micelles that displays strain-softening and strain-hardening before failure during stress sweep tests.

International audience; Dairy ingredients with highly concentrated protein contents are high added value products with expanding market. The manufacture of such ingredients includes a succession of unit operations of which heat treatment is a key step to guarantee the microbial safety, that induces major changes in protein structures and thus ingredients functionalities. However, due to an incomplete understanding of phenomena taking place at high protein concentrations, shedding light on their mechanisms is a scientific challenge as well as an industrial need. In this study, the influence of heat treatment (74 • C/ 30 s) of highly concentrated milk protein systems (up to 20 % w/ w) on protein denaturation/aggregation and enzymatic coagulation properties was studied using an original semi-industrial approach. 10 % w/w protein solutions constituted with whey protein and casein micelles at milk ratio, standardized in osmosed water or ultrafiltration permeate were used. These protein solutions were processed in different ways prior the manufacture of powders: heat treatment of the 10 % w/w protein solution before vacuum evaporation, heat treatment of the 20 % w/w protein solution after vacuum concentration, two consecutive heat treatments before and after vacuum evaporation. A fourth powder was prepared from unheated 10 % w/w protein solution. An increase in protein concentration led to a higher heat-induced protein denaturation. This phenomenon was reduced when increasing the lactose content. The effect of heat treatment on the extent of protein denaturation was not cumulative. At high protein concentration, interactions between κ-casein and whey protein were modified compared to milk, as mainly micelle-bound aggregates were formed at pH about 6.7. This phenomenon was enhanced at low ionic strength and lactose content. Our study showed that the enzymatic coagulation properties of reconstituted protein powders could be correlated with their physico-chemical compositions. An increase in protein denaturation disrupted the gel reorganization and led to the formation of weaker gels but did not interfere on the micelles aggregation phase and the early gelation. On the contrary, an increase in ionic strength and lactose content led to higher gel time.

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.

International audience; Casein micelles (CMs) are ~100 nm natural colloids found in milk resulting from the complex and unresolved association of casein monomers, phosphorus, and calcium; the latter being either directly bound to the phosphoserine residues of caseins or present as nanoclusters of insoluble micellar calcium phosphate (MCP). Dairy products such as cheese or yogurt are colloidal gels formed by CMs destabilization, for which texture and ability to be processed are essentially determined by rheological properties. Those properties depend on the physicochemical conditions used during gel formation (e.g., pH, temperature, ionic content). However, questions remain about the origin of this dependence at the microscopic scale. In particular, we still do not have a clear picture of the specific contributions of the rigidity of the elementary bricks (CMs) in comparison with the interparticles interactions or their spatial distribution. In this work, Atomic Force Microscopy (AFM) is used to evaluate changes in the nanomechanical properties of single CMs following physico-chemical modifications that are known to affect the MCP content and the rheology of the enzymatic milk gel (i.e. decrease of pH and CaCl 2 addition). AFM is used as a direct indenter that assesses the CMs' elastic deformation and gives an estimate of their Young modulus. We show that for a ±18-24% w/w depletion or enrichment in MCP content, the Young modulus of CMs significantly decreases or increases, respectively. This correlation suggests that variations in the modulus of individual CMs could explain the changes in the macroscopic properties of the enzymatic milk gel upon variations of physico-chemical conditions.

International audience; Background: Since the early 1990s, major health and environmental concerns have developed and driven the emergence of diets involving a lower consumption of animal products. However, the transition towards greener diets is being hampered by the poor acceptance of vegan foodstuffs among western consumers. Mixed animal/ plant alternatives to familiar dairy or egg products offer a new field of innovation. Scope and approach: This review focuses on innovative mixes of egg or milk with plant ingredientsespecially legumesto develop products in which interactions between animal and plant are not usually expected, such as dairy or egg gels, emulsions or foams. The opportunities offered by such products in terms of consumer acceptance, nutrition, digestibility and techno-functional properties are reviewed and discussed with respect to their risk-benefit ratios. Key findings and conclusions: In many cases, animal/plant mixes offer enhanced protein stability and synergistic interfacial or textural properties that make them a flexible tool for food design. Fermentation offers important prospects for the nutritional and sensorial enhancement of animal/plant mixes, through the multi-criteria application of microbial consortia. Animal/plant mixes enable reduction in animal protein consumption while preserving amino acid and micronutrient intakes and sensory properties. However, their acceptability to consumers and society will also depend on controlled safety, especially regarding allergies or contaminants, on affordability, their degree of novelty or (ultra)processing, their actual environmental footprint and whether they meet consumer expectations for innovative foods in the transition towards greener diets.

International audience; A process of dry heating alkaline powder containing whey protein (WP) and lactose is used to produce micro-particles that can trap large amounts of water and thus be used in many food applications as a thickening agent. The objective of this study was to optimise the yield of particles formed and the amount of water they trap as well as to minimise the heating duration. The parameters studied were the contents of WP and lactose, the pH, storage time at that pH, water activity (a w) of the powder, and the temperature and duration of dry heating. The best parameters to reach the objective were WP at 100 g/kg with 0.5 g lactose/10 g protein, adjusted and stored at pH 10 for 3 h, and dry heating at 120 °C for ca. 30 min at a w ¼ 0.33. Approximately 70% of the WP converted into wet particles that contained 96% water. This result can help advance large-scale production of particles in a multi-stage spray-dryer that uses an in-line heating fluidised bed.

International audience; The use of plant proteins to design colloidal food systems is a hot topic in the current context of the protein transition. However, replacing animal-derived proteins (in particular, dairy proteins) that have been traditionally used for this purpose by plant proteins is a challenge from various perspectives, and in particular, because of drastically different solubility and functionality. A possible route to mitigate these issues is to combine plant and dairy proteins, providing that their interactions can be understood from the molecular to the macroscopic scale. This review addresses the major advances that have occurred in the field of such blend-based systems, all the way from their behaviour in aqueous dispersions to their potential applications in gels, foams and emulsions.

3.1 Water activity is related to water vapor pressure in a material, and it is a useful parameter relating to glass transition, water content and water plasticization. The glass transitions of dairy solids, particularly that of lactose, determine the dehydration characteristics and successful spray-drying conditions of dairy liquids. Water and glass transition relationships assist in the selection of spray-drying parameters as well as required storage conditions. A critical water activity and glass transition-related strength of dairy solids are useful in the formulation of dairy food with desired physicochemical stability in processing and storage. 3.2 Caking is a common root of many problems encountered when handling powders and can lead to unwanted shutdowns, client claims and product recycling/rejection due to the clogging of conveying lines, feeding devices and storage silos. Caking can be defined as a formation of material bonds at the points of contact between particles that leads to undesired and uncontrolled agglomeration of powders. It is a very complex process bringing into play a multitude of interactions and mechanisms resulting from variations in environmental conditions, in particular those of temperature and relative humidity. The objective of this chapter is to describe the fundamental mechanisms of powder caking and especially those involved in the caking of dairy powders. 3.3 This chapter presents the structural changes at protein/aggregate scale and the changes of functionalities (water-holding capacity, viscosity, foaming, emulsifying and gelling properties) resulting from the pre-texturizing of whey proteins by heating in dry state. The first part of this chapter describes the main chemical/structural modifications occurring during protein dry-heating (limited protein unfolding, Maillard reaction, dehydration reactions) and the specificities of the aggregation reactions that are controlled by molecular mobility (or a w ) and the presence of heat-sensitive compounds (type and amount of sugars). Then, the second part of the chapter describes the functionalities of the pre-texturized whey protein by dry-heating. The change of functionalities can be huge, as shown for the modification of viscosity of protein solutions after whey protein dry-heating at alkaline pH (protein particles are able to trap up to 30g of water per g of protein). The strength of the whey protein gels is increased two-fold by the dry-heating treatment, favoring the formation of soluble aggregates. The change in the foaming properties after whey protein dry-heating is much more limited. 3.4 The perceived quality of dairy powders obtained by spray-drying depends on their functional properties, i.e., their ability to flow easily and rehydrate quickly without lump formation. These requirements can be achieved provided that the physicochemical properties of the powders are mastered by an adequate choice of formulation and spray-drying conditions. On one hand, this chapter presents the impact of the spray-drying conditions on the physicochemical characteristics of the resulting dairy powders and highlights the tremendous role of the evaporation rate in the particle structure and surface composition. On the other hand, the influence of the main powder physical properties (particle size, shape, microstructure and surface mechanical properties) on flowing properties and rehydration capacity is discussed through examples of dairy powders produced by spray drying. In order to complete this discussion of the link between spray-drying conditions, physicochemical properties and functionalities, the main analytical methods used to assess spray-dried dairy powder characteristics are briefly reviewed throughout the chapter. 3.5 Initial milk contamination may occur due to intramammary infections, microorganisms on the external udder surface, poorly sanitized handler hands and milking equipment. During the refrigeration, storage and transport stages, lapses in hygienic-sanitary conditions can lead to milk contamination from pathogenic and spoilage microorganisms. Once raw milk reaches the processing plant for dehydration, it undergoes processes including cooling, heat treatments, evaporative procedures and membrane filtration in order to inhibit microbial growth and destroy microorganisms. Following thermal treatments and membrane filtration, caution should be taken to ensure the hygienic-sanitary conditions of equipment and utensils to avoid product recontamination. All the stages of dried dairy products (from milking to packaging) should be monitored to guarantee the quality and microbiological safety of these foods.

International audience; In casein micelle (CM), Ca is either precipitated in the colloidal calcium phosphate (CCP) stabilized by clusters of phosphoserine (SEP) residues, or is directly bound to SEP (or glutamic and aspartic acids) of ca-seins without inorganic phosphate. However, it is currently not possible to titrate separately the different micellar Ca forms, making it difficult to assess their respective importance for CM properties and behavior. Both Ca 2+ and Mg 2+ have the same binding constants with SEP. Moreover, MgHPO 4 is more soluble than CaHPO 4 , and its natural concentration in milk is lower. Thus, upon addition of MgCl 2 , Mg is mainly exchanged with CM in the bound form, whereas upon addition of CaCl 2 , Ca is mainly exchanged in the precipitated form. Our objective was to assess the role of the 2 forms of micellar cations (bound and precipitated) during the enzymatic coagulation of cow milk. Magnesium chloride , CaCl 2 , or KCl (10 mM) were added to milk and pH was adjusted to 6.6 after overnight equilibration. The KCl-supplemented milk was a positive control to assess the effect of the increased ionic strength after MgCl 2 and CaCl 2 addition. Mineral partition between soluble and colloidal phases after salt addition was assessed both experimentally and by using computer simulation. Enzymatic coagulation was proceeded at 30°C. Hydrolysis of κ-casein was followed by the quantitative determination of caseinomacropeptide released by RP-HPLC, aggregation of para-κ-casein micelles was measured through the evolution of backscattering properties of milk and the gel time and gel firming kinetics were determined using a Chymograph (Chr. Hansen, Horsholm, Denmark). The KCl addition did not affect mineral partition. As expected, CaCl 2 addition mainly increased the CCP content, whereas the addition of MgCl 2 mainly increased the bound divalent cations content. The kinetics of κ-casein hydrolysis was slowed down after CaCl 2 and MgCl 2 addition, and was negatively correlated with the concentration of diva-lent cations in the soluble phase of milk. Aggregation and gel firming curves plotted versus the progress of κ-casein hydrolysis were similar for both CaCl 2-and MgCl 2-supplemented milk. In view of the dual-binding model for CM assembly, this means that both Ca forms reduce electronegative repulsions between para-micelles by specific charge shielding. Inclusion of 2 Ca forms in structural models for CM allows a more detailed comprehension of how mineral equilibria affect CM properties.

International audience; Background: Today's consumers demand natural and healthy functional food products with clean labels and less added ingredients and exogenous hydrocolloids. The goal is to produce dairy foods only made by using milk ingredients, while maintaining good sensory properties. Thanks to their multifunctional properties, dairy proteins are used to enhanced the texture of dairy products. However, their addition into dairy products can also have a great influence on taste and aroma perception. Scope and approach: This review concentrates on recent information regarding the impact of dairy proteins in dairy products with a focus on yogurts. The aim of the review is to highlight recent findings relating the composition and the physico-chemical properties of yogurts to their sensory properties. Key findings and conclusions: The addition of dairy proteins in yogurts induces changes in their microstructure, which can explain the modifications of their texture properties. Modifying the protein content also tends to decrease the release of most flavor compounds, which can impact the overall aroma balance and change aroma perception. Modifications of texture and aroma perceptions can also be due to cognitive interactions. Dairy protein aggregates are particularly promising as dairy texture enhancers, however almost no sensory studies have been performed to investigate their impact on texture and aroma perception. More studies focusing on the impact of dairy protein aggregates on the organoleptic qualities of yogurts and dairy products are needed to obtain natural and low-fat products while maintaining a desirable texture quality.

This study aimed to perform Temporal Dominance of Sensations by modality (M-TDS) combined with a multi-intake approach to investigate texture and flavor perception in semi-solid products. Trained panelists (n = 15) evaluated fat-free strawberry yogurts enriched with functional proteins involving texture modifications. As yogurt is a semi-solid product, its in-mouth residence time is short. A multi-intake approach was therefore expected to give more reliable information about the sensory properties perceived by panelists. The two modalities of texture and flavor were analyzed separately to characterize the effect of added proteins. Trials were made according to an experimental design with two factors (protein type and concentration) and three levels each. Different statistical treatments, taking or not the temporality of attributes into account, were performed on standardized and non-standardized data. The implementation of M-TDS was essential to highlight differences of flavor perception in addition to the more evident texture modifications. The study of sensory trajectories evidenced that texture modifications, induced by the use of different whey proteins, slightly modified the perception of flavor and sweetness. The global flavor perception of the samples varied with the number of spoons, which particularly impacted the taste attributes. This study highlighted the importance of using M-TDS when studying texture and flavor in semi-solid products, and the relevance of the multi-intake approach to characterize flavor perception. This methodology enabled panelists to evidence both marked texture differences and subtler flavor modifications, and these useful data were emphasized by combining different statistical treatments.

Hydrophobisation of whey proteins, followed by erythritol addition into the hydrophobised protein dispersion, enables formation of water-in-water (W/W) emulsions of the proteins and alginate. In this study, we investigated the interfacial and gel rheology of whey proteins as affected by hydrophobisation, heat denaturation and erythritol addition. Additionally, the gel rheology of the resultant W/W emulsion was assessed. Hydrophobisation shortened the linear viscoelastic region and decreased the surface storage modulus ( G s ' ), the flow stress and the flow strain of protein layer at the air-liquid interface. Erythritol addition into the hydrophobised protein caused a further reduction of G s ' . In accordance with interfacial rheology, protein hydrophobisation and erythritol addition decreased the dynamic moduli of acid-induced protein gels. Frequency sweep tests indicated that the gelled emulsion had higher dynamic moduli than all WPI gels. The higher firmness of the emulsion gel was ascribed, based on microstructural images, to micro-phase separation of alginate droplets.

Whey protein products are of widespread use as food ingredients because of their high nutritional, biological and functional properties. Whey proteins are important structural components in many foods as used in their native form, for example for their heat-induced gelation abilities. Furthermore, they also offer reliable functionalities when modified by heating processes as denatured or aggregated proteins. Heat treatment of whey proteins in a liquid state has received much attention in recent years. While dry heating of whey proteins, say heating whey proteins in the dry state, is frequently cited in the literature as a potential and efficient means to improve the functional properties of proteins, it has received very little attention. We report first on the dry heating of whey products as applied to promote glycation of whey proteins with a low denaturation, and second, to promote their denaturation and aggregation and on their consequences on the functional properties of whey proteins.

Understanding the behavior and structure of food products requires knowledge of the spatial arrangement of the various components and their interactions. Among these components, proteins, beyond their roles in nutrition and health, exhibit specific functional properties inherited from both their molecular flexibility and the plasticity of their diversified assemblies. These structural and functional properties make food proteins versatile and useful ingredients in a wide range of product development. This chapter reviews the role of proteins in the development of food structure with a focus on milk and egg proteins. We first describe the main proteins, their physicochemical characteristics, and the diversity of assembled structures obtained during processing. Through specific examples, including proteins gels, yogurt, and various cheeses, we illustrate how the properties of proteins are explored in the manufacture of food.

The surface of milk fat globules consists of a biological membrane rich in polar lipids and glycoproteins. However, high shear stress applied upon homogenization disrupts the membrane and leads to the adsorption of casein micelles, as the major protein fraction of milk. These changes in the interface properties could affect the interactions between native or homogenized milk fat globules and the surrounding protein matrix, at neutral pH and upon acidification. In this study, macroscale rheometry, microscopic observations, nanoscale AFM-based force spectroscopy and physico-chemical analysis were combined to examine the interfacial composition and structure of milk fat globules and to evaluate their interactions with casein micelles. We showed that the surface properties of milk fat globules (biological membrane vs. caseins) and pH govern their interactions with casein micelles. The adhesion between individual fat globules and casein micelles was higher upon homogenization, especially at acid pH where the work of adhesion increased from 3.3 x 10-18 to 14 x 10-18 J for native and homogenized fat globules, respectively. Consequently, casein-coated homogenized fat globules yield stiffer milk acid gels. These findings cast light on the importance of colloidal particle’s surface properties and pH on their connectivity with the surrounding matrix, which modulates the bulk microstructure and rheological properties with potential functional consequences, such as milk lipid digestion.

The impact of nanoparticulated whey protein aggregates on the texture of fat-free set-type yoghurts was investigated. Monodisperse (MFA) and polydisperse (PFA) fractal aggregates obtained from heated whey protein isolate (WPI) were added to skimmed milk for yoghurt manufacture at four different concentrations (0.2%–1.5%, w/w). The impact of the concentration and the polydispersity of the aggregates on fat-free set-type yoghurts were studied by instrumental measurements (rheology, penetrometry, syneresis and microscopy) and sensory analysis. Yoghurt gel strength and firmness increased with the concentration of WPI, MFA and PFA. However, yoghurts enriched with PFA clearly differed from the yoghurts enriched with WPI. Indeed, yoghurts enriched with PFA were characterised by a weak gel, a low firmness and a low-density of the protein network. Sensory analysis confirmed the results obtained by instrumental measurements. The whey protein aggregates studied are thus promising tools to modulate fat-free yoghurt texture while using milk-derived ingredients.

Dry heating (DH) at 100 °C of a whey protein isolate powder conditioned at pH 9.5 leads to the formation of stable, large and porous whey protein microparticles (WPM), resulting from crosslinking of proteins in the powder. The Maillard reaction (MR) is probably involved in the formation of these WPM. To confirm the role played by the MR and understand the protein changes occurring during particle formation, we analyzed the properties of the powders (such as their water activity (aw) and color) and characterized the WPM formed after 3 and 36 h of DH in powders with increasing amounts of added lactose. Adding lactose and prolonging the time of DH increased the extent of protein denaturation and the rate of formation of WPM and the yield of conversion of powder into WPM, but led to decreases in their water content. Addition of lactose also altered the kinetics of the MR, the increase in aw and the formation of WPM. The MR, as followed by powder browning, was complete within 3 h of DH irrespective of the amount of added lactose. In contrast, the increase in aw, thought to be due to progress of the MR, continued to increase from 3 to 36 h of DH. Formation of WPM went on increasing from 3 to 36 h of DH at low to medium levels of lactose, but was complete by 3 h of DH at high levels of lactose. In conclusion, the formation of WPM was not simply related to the MR and needs further investigation.

Dry heating (DH) at 100 °C for 36 h of a whey protein isolate powder conditioned at pH 9.5 leads to the formation of stable, large and porous whey protein microparticles (PMs), resulting from the crosslinking of proteins inside the powder. These PMs could be used as high-viscosity food ingredients. Casein, present as a contaminant in whey protein powders, has been shown to become incorporated into the PMs. In this study, we investigated the effect of adding increasing amounts of sodium caseinate to whey protein powders on the formation of PMs during DH at 100 °C for 36 h. In addition, we studied PM formation during DH of a micellar casein-enriched milk protein powder (Casmic). The browning index of the dry-heated powders, and the size and water content of the microparticles were also characterized. We confirmed that sodium caseinate was incorporated into the PMs. The highest PM D[4,3] values (270 μm) were observed for powders with around 40% caseinate. Powders without added caseinate displayed D[4,3] values of 150 μm. The yield of conversion of proteins into PMs increased from 0.6 to 0.8 g/g with caseinate addition, whereas the amount of water entrapped in the PMs decreased from around 30 to 20 g/g. PMs were also formed by DH of the Casmic powder, but these particles were smaller, with sizes of around 80 μm. In conclusion, our study shows that the process of DH at pH 9.5 could be applied to all milk proteins to obtain PMs with functional properties that could be used in the food industry.

Modification of whey protein isolate (WPI) powders is used in the food industry to enhance the functional properties of WPI. We investigated the impact of severe dry heating (DH) at 100 °C for up to 36 h on an alkaline-treated (pH 9.5), spray dried (water activity of ~0.24) WPI powder. Dry heated powders and their reconstituted suspensions were analysed. DH for 0–6 h led to 47% loss of native proteins, increases in the levels of soluble aggregates (×2.2) and of advanced glycation end-products of the Maillard reaction (at least ×2.7) and to powder browning (at least ×3) with a 95% decrease in free lactose content. DH for at least 12 h led to a decrease in soluble aggregates with concomitant formation of large, stable and insoluble microparticles. These microparticles had a microsphere structure, contained 98% of water phase and were made of insoluble powder particles resulting from protein cross-links during DH. Microparticle size could be altered by varying the pH of the suspension: at pH 6.5, microsphere size was 3–5 times larger than powder particle size, but decreased as the suspension pH neared the isoelectric point. DH could be a useful method for producing functional protein ingredients as these microparticles had very high water retention properties and high viscosity values.

International audience; We investigated the effect of dry heating (DH) a whey protein powder (water activity 0.24) at pH 3.5, 6.5 and 9.5 for up to 36 hat 100 degrees C on the structural properties of proteins. Powder browning, particle structures in suspensions reconstituted from the dry-heated powders (DHP), residual native protein content and Maillard reaction markers were studied. The browning index of the DHP rapidly increased after 6-10 h of DH and then levelled off at all pH values. beta-Lactoglobulin, alpha-lactalbumin and bovine serum albumin were denatured by DH at all pH values. In contrast to pH 3.5 and 6.5, DH at pH 9.5 led to abundant production of microparticles. The highly heterogeneous size of these particles was related to the initial powder particle size. One g of DHP in suspension led to 20 g of hydrated microparticles made from 0.5 g of dry material. Maillard reactions at pH 9.5 were probably involved in microparticle formation. (C) 2017 Elsevier Ltd. All rights reserved.

Conference Paper in AIP Conference Proceedings · February 2018 DOI: 10.1063/1.5021215Conference Paper in AIP Conference Proceedings · February 2018DOI: 10.1063/1.5021215; International audience; The current paper describes the formation of whey protein microbeads (WPM) having a spherical shapeand a monodispersed size distribution. A microfluidic flow-focusing geometry was used to control the production of whey protein microdroplets in a hydrophobic phase. The microfluidic system consists of two inlet channels where the WPI solution and the lipophilic phase were separately injected towards the flow-focusing (FF) junction where they eventually meet, then co-flow. A whey protein isolate (WPI) solution of 150 g/kg protein and two types of hydrophobic phases, i.e. sunflower oil and n-dodecane, were tested as the continuous phase. The formation of WPM was observed microscopically. The aim of the present study was to describe the production of stable monodisperse WPM in suspension in milk ultrafiltrate using a microfluidic system. Hints to perform the control of the running parameters, i.e. choice of the hydrophobic phase or fluids flowrates, are provided. The results showed that in the sunflower oil, microdroplets had a large polydisperse size distribution, while in n-dodecane, microdroplets with narrow size distribution were obtained. Stabilization of the whey protein microdroplets through heat-gelation at 75 °C for 20 min in n-dodecane produced WPM and no change in shape nor size is observed. Meanwhile replacing the n-dodecane by MUF using centrifugation and washing caused the swelling of the WPM, but dispersity remained low. From this study, microfluidic system seemed to be a suitable method to be used for producing small quantities of monodisperse WPM.

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.

A novel rheological technique is described, for determining the glass–rubber transition temperature ( T gr ) of spray dried dairy powders. The approach involves constant rate heating of powder under compression and measurement of changes in either gap distance (Method 1) or normal force (Method 2). Significant increases in the rate of change of these parameters was shown to correspond with T gr . The techniques were applied to skim milk, micellar casein and whey permeate powders and a range of fat-enriched micellar casein powders. T gr temperatures, so obtained, were compared with glass transition temperatures ( T g ) determined by Differential Scanning Calorimetry (DSC). Methods 1 and 2 gave predictions for non-fat dairy powders of T g endset ( T ge ) with SEP of 8.8 and 4.4 °C, respectively. These novel techniques provide an accurate means of determining glass transitions in dairy powders, including high protein and fat-containing powders, whose relaxation properties can be difficult to measure by DSC.

A model lipoproteic matrix able to mimic hard-type cheese was produced with controlled structural and textural properties. Changes in the microstructural and rheological properties of these model cheeses made from different milk concentrate powder, anhydrous milk fat, salt contents and pH values at renneting were characterised. Rheological properties were measured by texture profile analysis, fat globule and protein aggregate size distributions by laser light scattering. Microstructural properties of the model matrices were studied by confocal laser scanning and scanning electron microscopy. Significant differences between the matrices were found for the structural, physico-chemical and rheological parameters measured. Cheeses with higher dry matter content were significantly harder and contained more insoluble proteins than cheeses with lower dry matter content. The salt concentration and the pH at renneting had significant influence on cheese hardness and adhesiveness of rheological parameters. The model lipoproteic matrix presented air bubbles and powder aggregates which could not be avoided during the manufacture of products. However, compared with classic cheese making with rennet or acid coagulation, the technology used here allows model cheeses to be produced rapidly with a good reproducibility of texture.

International audience; The ripening of industrial soft cheeses manufactured using a liquid pre-cheese produced from the ultrafiltration (UF) of milk was observed to be slow in comparison to that of cheese manufactured by a traditional process. Moreover, in the UF-cheeses investigated in this study, which were produced using Penicillium camemberti as the surface flora, several surface defects were observed: the texture was 'carton' like and the rind frequently detached from the cheese. To gain a fuller understanding of the development of these surface defects in UF-cheeses, the migration of different minerals and ions, and the study of the rind microstructure by scanning electron microscopy and X-ray mapping were performed. The results suggest that the slower diffusion of lactate, possibly due to the mineral layer at the surface of cheeses, acting as a barrier to its diffusion, may have caused an alteration in the metabolism and growth of the surface mould and may explain the surface defects of these UF-cheeses.; Les fromages à pâte molle industriels produits par ultrafiltration (UF) à partir d'un pré-fromage liquide présentent un affinage retardé par rapport à des fromages produits à l'aide d'un procédé traditionnel. De plus, des fromages UF modèles produits avec seulement Penicillium camemberti comme flore de surface montrent une texture de carton et la croûte se détache souvent du fromage. La caractérisation des migrations des minéraux et des ions et l'étude de la microstructure de la croûte de ces fromages modèles par microscopie électronique à balayage et micro-analyse X ont été réalisées afin de comprendre l'origine de ces défauts. La diffusion retardée du lactate, peut-être due à la présence d'une couche minérale à la surface des fromages, pourrait être à l'origine de changements de métabolisme et de la croissance de la moisissure et pourrait expliquer la moindre qualité des fromages.

The objective of this work was to investigate the effect of heat treatment of sweet creams on the physicochemical properties of sweet industrial buttermilks. Creams with three different heat treatments: low, medium and high, were churned and the corresponding buttermilks were characterised. Furthermore, buttermilks were renneted and centrifuged to obtain insoluble pellets and supernatants. The physicochemical properties such as the particle size measured by laser light scattering and composition of creams, buttermilks, pellets and supernatants were determined and compared. Buttermilk had a composition close to that of skim milk but it contained more phospholipids (PL): 958 (± 137) mg·kg−1, compared with 120 mg·kg−1 in milk. The heat treatment induced a significant decrease in soluble protein contents in creams, buttermilks and soluble fractions as well as an increase in the buttermilk PL/fat ratio, and an increase in supernatant protein and total solids. Buttermilk and supernatant particle sizes ranged from 0.03 to 200 μm, with a maximum at 130 nm, which may correspond to small milk fat globules, phospholipid vesicles or protein aggregates. Fat and PL in buttermilks were recovered in supernatants. The use of buttermilk enzymatic coagulation combined with centrifugation may constitute a means to fractionate milk PL.

The use of spray drying for dehydration of acid whey is generally limited by the appearance of uncontrolled thickening and solidifying of the whey mass during the lactose crystallization step. The origin of this physical change is still unknown and probably linked to complex interactions between physical properties and chemical composition of these products. To understand this phenomenon, we simulated the thickening of concentrated acid whey on a laboratory scale by measuring the flow resistance changes as a function of time and whey composition. The thickening process was characterized by an amplitude of torque and a lag time (induction time). Thickening of lactic acid whey concentrate occurred regardless of the presence of whey proteins or lactose crystals. Moreover, this work clearly demonstrated that the thickening process was due to the formation of filamentous structures corresponding to calcium lactate crystals and showed a large dependence on calcium and lactate contents, pH, and phosphate concentration.

It is well known that one key parameter for food digestion is the rate of emptying of the stomach. However, during its transfer through the stomach, due to the physico-chemical conditions of this compartment (pH, content in digestive enzymes, rate and intensity of contraction), the structure of foodstuff and their rheological properties are strongly modified. This affects the rate of transfer of food into the intestine. The gastric step is particularly essential for colloidal emulsions such as dairy liquids that will gel or precipitate during that step, with changes on their residence time in the stomach. So, our aim is to understand how the structure and the rheology of dairy colloidal emulsion evolve during their digestion and impact the hydrolysis rates of proteins and lipids and the bioavailability of these macro-nutriments. Taking into account these structural and rheological changes is particularly crucial in neonatal nutrition to develop optimized infant formulas. We performed in vitro dynamic gastrointestinal digestions of different infant formulas1. At some specific steps during the digestion, we determined the structure and the rheology of the bolus, together with the hydrolysis rates of proteins and lipids. Rheological measurement can be performed on aliquoted samples by shearing at a low rate. But, only limited sampling can be practiced because digestion systems are limited in volume and sampling the bolus for rheological measurements can introduce bias by applying additional shearing to the sample. Thus, we proposed performing continuous rheological measurements in parallel to the dynamic digestion study.Two continuous rheological methods were tested on different infant formulas, by shearing or small amplitude oscillation. The changes evidenced with these two methods and the one observed in the digestion system were compared.In conclusion, these two methods gave relevant results and were shown to be much simpler than the digestion system. They could be applied to predict the rheological behavior of formulas. Moreover, the methods allowed discriminating the infant formulas in relation with the technological process applied for their manufacture.

Dairy propionibacteria (PAB) are used as a ripening starter in combination with Lactic acid bacteria (LAB) for dairy products such as Swiss-type cheese. LAB and PAB have also been studied for their probiotic properties but little is still known about their individual and/or synergistic beneficial effects within dairy matrices. In the context of a rising incidence of Inflammatory Bowel Diseases, it has become crucial to evaluate the immunomodulatory potential of bacteria ingested in large numbers via dairy products. We therefore selected different strains and combinations of technological LAB and PAB. We determined their immunomodulatory potential by IL-10 and IL-12 induction, in human peripheral blood mononuclear cells, on either single or mixed cultures, grown on laboratory medium or directly in milk. Milk was fermented with selected anti-inflammatory strains of LAB or PAB/LAB mixed cultures and the resulting bacterial fractions were also evaluated for these properties, together with starter viability and optimum technological aspects. The most promising fermented milks were evaluated in the context of TNBS- or DSS-induced colitis in mice. The improvement in inflammatory parameters evidenced an alleviation of colitis symptoms as a result of fermented milk consumption. This effect was clearly strain-dependent and modulated by growth within a fermented dairy product. These findings offer new tools and perspectives for the development of immunomodulatory fermented dairy products for targeted populations.

Background: Postprandial lipemia is a risk factor for cardiovascular disease. The potential impacts of the type/nature of dietary protein on postprandial lipemia and associated dysregulations have been insufficiently investigated. Objective: We investigated the postprandial effect of including in a high-fat meal some milk protein fractions that markedly differ in their physicochemical properties and composition [either casein (CAS), whey protein (WHE), or a-lactalbumin-enriched whey protein (LAC)]. Methods: The protein fractions were incorporated as 15% energy in a high-fatmeal in a W3-period, crossover postprandial study of 10 healthy overweight men with an elevated waist circumference (>94 cm). We measured postprandial changes in plasma lipids, amino acids, glucose, and oxidative stress markers, vascular function (using pulse contour analysis), and low-grade inflammation (using plasma markers). We also characterized in vitro the meal structures, including the size of the fat globule, and possible changes during digestion. Results: The type of protein did not affect postprandial plasma glucose, amino acids, insulin, or nonesterified fatty acids, but, compared with WHE and LAC, which did not differ, CAS markedly reduced postprandial triglycerides (TGs), achieving a 22 +/- 10% reduction in the 6-h area under the curve (P < 0.05). Similar trends were shown for plasma chylomicrons [apolipoprotein (apo) B-48; P < 0.05]. However, there were no significant differences between the meals regarding postprandial oxidative stress (plasma hydroperoxides and malondialdehyde), endothelial dysfunction (salbutamol-induced changes in pulse contour analysis), or low-grade inflammation. In vitro studies showed that when the pH of the meal decreased to stomach pH values, the reduction in the solubility of casein resulted in a phase separation between fat and protein, whereas the proteins in the other meals remained suspended with fat globules. Conclusion: In healthy overweight men, casein has specific physical interactions with fat that affect postprandial TGs, leading to the formation of fewer chylomicrons or an increase in chylomicron clearance. This trial was registered at clinicaltrials. gov as NCT00931151.

The native whey proteins have been intensively used in a multitude of food applications due to their high nutritional, biological, and versatile technofunctional properties. The range of applications of whey proteins has further been extended in the last decades by the use of whey protein aggregates offering new technofunctional properties. These properties are directly dependent on the structure of whey protein aggregates, i.e., their size, shape, density, surface properties, and the type of bonds maintaining proteins together in the aggregates. In this review, after a brief description of the major whey proteins, we examine the most important advances reported to date pertaining to the available approaches to modify the structure of whey proteins for specific functionalities. Our laboratory, Science and Technology of Milk and Eggs, has contributed significantly to the advancement of knowledge on the structure-function relationships of whey proteins either in the native or denatured/aggregated forms. Our expertise and research approaches are highlighted throughout some selected results accumulated during the last decade in comparison with results from the literature.

Skim milk used for the yoghurt manufacture contains 2 main colloidal particles, the native micellar casein (NMC) and the heat-induced whey protein aggregates (WPA). The aim of this study was to understand how these 2 colloids organize in space to form binary acid gels. While acid milk gels were considered homogeneous for scales typically above ∼10 μm, shorter length scales were examined to investigate the early stages of particle aggregation. The NMC and WPA were dispersed in milk permeate at different protein concentrations, separately or in an 80/20 w/w mixture (MIX). Acidification was performed at 35 °C with glucono-δ-lactone to achieve a final pH at ∼4.5 in 6 h. Acid gelation was studied by rheology, while the microstructure of the gels at pH 4.5 was studied by confocal scanning laser microscopy and transmission electron microscopy (TEM). Using Shih et al.'s (1990) model on the rheological data, it seemed that aggregation in the NMC and MIX mixtures was driven by the casein micelles and therefore differed from that of the WPA suspension. The different organizations were confirmed using image analysis of confocal or TEM images. The differences in gel formation were discussed in terms of the different interactive properties of the surface of these 2 colloids, together with their different internal structure.

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.

Le comportement du lait ecreme traite thermiquement au cours de l'acidification par des bacteries lactiques est etudie a 30 et 42 °C. Les modifications rheologiques par oscillations dynamiques et le temps de relaxation T2 en RMN du proton sont etudies en fonction du temps et du pH. La fluorescence frontale intrinseque etudiee conjointement avec l'analyse en composantes principales a ete suivie au cours de la fermentation. La gelification du lait apparait a un temps plus court et un pH plus eleve a 42 °C (5,53 ± 0,01) qu'a 30 °C (5,24 ± 0,13). Le pic de maximum de tan δ etait plus marque a 42 qu'a 30 °C, mais le G t a pH 4,6 avait la meme valeur aux deux temperatures. Les changements de T2 au cours de l'acidification etaient peu differents aux deux temperatures etudiees, mais les courbes dT2/dpH montrent quelques differences entre 42 et 30 °C. Les changements de T2 sont dus a des changements de structure de la particule provoques par la fermentation et sont correles a la solubilisation du calcium. La fluorescence frontale detecte un premier decalage des spectres vers des longueurs d'onde plus petites puis un decalage vers des longueurs d'onde plus grandes. Le point associe a l'inversion de l'environnement des tryptophanes des chaines proteiques correspond au point de gel. Ces resultats et les differences avec des gels acides obtenus par l'addition de GDL sont discutes en terme de limitation des transferts d'acides et de protons de la phase aqueuse vers la phase colloidale et de l'heterogeneite du gel qui en decoule.

New, hydrophilic poly(ethyleneglycol)-ionic liquids have beensynthesized and investigated, based on 1,3-disubstituted imidazolium cations and fluorinated anions (BF4 -, PF6 -,(CF3SO2)2N-, or NTf2 -). A series of typical solvent-free reactions have been safely realized using a focused microwave reactor for the preparation of imidazolium chloride precursors in yields ranging from 73 to 94% followed by quantitative anion metathesis exchanges. The poly(ethyleneglycol)-ionic liquid matrices were also characterized by NMR (1H, 13C), mass spectrometry (MS), and their dynamic viscosity was determined at 25 °C. These poly(ethyleneglycol)-ionic liquid phases (PEGILPs) as task-specific ionic liquids are promising tools for synthetic applications in liquid-phase combinatorial chemistry.

Skim milk supplemented with calcium caseinate (Ca-Cas) and whey protein isolate (WPI) at different ratios was preheated at 72e90 C for 7 min and subsequently acidified for the production of set-style and stirred acid gels. Upon heat-treatment of the suspensions, the native whey protein content decreased and particle size increased with increasing temperature and WPI content. Consequently, the pH of gelation, firmness of the set gel, viscosity and particle size of the stirred gel all increased as the content in denatured whey protein increased. However, for preheating temperatures of 80 or 90 C, the viscosity and firmness of the gels reached a plateau, although almost all whey proteins were denatured and largest particles were formed. These results open perspectives for the production of high-protein yoghurts without unusual firmness properties

In acidified milk, heat-induced whey protein aggregates and casein micelle particles assemble to form a soft gel. The present study was set to evaluate the respective roles of whey protein aggregates (WPIA) and native casein micelles (NMC) during acid gelation by means of changing their ratio in model systems. NMC and WPIA were dispersed in milk permeate at different weight ratios ranging from 0% to 100% NMC for a total protein concentration of ∼45 g kg −1 . Acidification was performed at 35 °C by addition of glucono-δ-lactone to achieve the same final pH of 4.5 in 6 h. Acid-induced gelation of these systems was followed using small deformation rheology followed by large deformation test and whey retention measurement at pH 4.5, while their microstructure was investigated microscopically. The results showed that higher content in WPIA promoted faster gelation and led to more elastic gels with smaller pore size and increased whey retention. The effects were particularly dramatic up to ∼10% w/w WPIA, where the aggregates were about equimolar to the casein micelles and covered ∼8% of the micellar surface. The results were discussed in terms of the physical interactions between two populations of colloids of different abilities for acid gelation. It seemed likely that a preferred interaction exists between the casein micelles and the aggregates, and directs the structural and mechanical properties of the acid gel.

International audience; The kinetics of whey drainage of Camembert soft cheese was studied using an instrumented drainage device to record the pH and the weight of the whey with a variation coefficient amounting to 1.8% during the first 100 min and 0.34% at the end of drainage. Drainage data were fit with a two step equation to obtain a descriptive model of the drainage kinetics. The effect of the pH at renneting, the casein concentration and the ionic strength on the drainage kinetics was investigated with a 2 level experimental design. Milk was modified by a combination of ultrafiltration and dilution with a lactose solution. The pH at renneting had a negative effect on the amount of whey expelled in the early drainage (up to 100 min) and a positive one after 400 min. The effect of the casein concentration was negative on the amount of whey expelled and on the rate of drainage. The ionic strength had a slight positive effect on drainage. An equation was obtained allowing the prevision of whey drainage kinetics according to the milk composition. Calculated values fit experimental data with a correlation coefficient of $r = 0.994$.; Caractérisation des cinétiques d'égouttage au cours de la fabrication d'un fromage type pâte molle en relation avec les facteurs physico-chimiques et technologiques : effet du pH d'emprésurage, de la teneur en caséine et de la force ionique des laits. L'égouttage du fromage à pâte molle de type Camembert a été étudié à l'aide d'un prototype d'égouttage instrumenté qui permettait d'enregistrer le poids de sérum et son pH. Le coefficient de variation était de 1,8 % pendant les 100 premières minutes et de 0,34 % à la fin de l'égouttage. Les cinétiques d'égouttage obtenues ont été ajustées à l'aide d'un modèle mathématique descriptif à deux phases. L'effet sur l'égouttage du pH à l'emprésurage et de modifications de la composition du lait, telle la concentration en caséine et la force ionique, ont été testés à l'aide d'un plan d'expérience à deux niveaux. La composition des laits était modifiée en combinant une concentration par ultrafiltration et une dilution par une solution de lactose. L'effet du pH sur le poids de sérum expulsé était négatif au début de l'égouttage puis devenait positif. L'effet de la concentration en caséine était négatif sur la quantité de sérum obtenue et sur la vitesse d'égouttage. La force ionique avait un léger effet positif sur l'égouttage. Une équation a été obtenue permettant de prévoir la cinétique d'égouttage en fonction de la valeur des trois facteurs étudiés. Les valeurs calculées s'ajustaient aux valeurs expérimentales avec un coefficient de corrélation $r=0,994$.

Gels from milk concentrates [milk and caseinate powder, ultrafiltration (UF) and microfiltration (MF)] and whey concentrates (UF) were obtained with high pressure (200 and 400 MPa, 10 and 30 min). The effects of protein concentration (66–114gkg −1 for milk and 97–127g kg −1 for whey), NaCl addition (0–8 g kg −1 ), sodium citrate addition (0–4 g kg −1 ) and pH (5.2–6.6 for milk and 7–9 for whey) were studied with a Box-Behnken design. Milk supplemented with casemate powder did not lead to gel formation by pressure. For UF or MF milk concentrates, a pH decrease towards 5.9 and a protein content increase led to firmer pressure-set gels. Gels of whey concentrates were obtained only at pH 9. A pressure increase from 200 to 400 MPa led to firmer gels, while a protein content increase did not.

This study aimed at determining the kinetics of milk protein digestion and amino acid absorption after ingestion by six multi-canulated mini-pigs of two gelled dairy matrices having the same composition, similar rheological and structural properties, but differing by their mode of coagulation (acidification/renneting). Duodenal, mid-jejunal effluents and plasma samples were collected at different times during 7 h after meal ingestion. Ingestion of the acid gel induced a peak of caseins and β-lactoglobulin in duodenal effluents after 20 min of digestion and a peak of amino acids in the plasma after 60 min. The rennet gel induced lower levels of both proteins in the duodenum (with no defined peak) as well as much lower levels of amino acids in the plasma than the acid gel. Plasma ghrelin concentrations suggested a potentially more satiating effect of the rennet gel compared to the acid gel. This study clearly evidences that the gelation process can significantly impact on the nutritive value of dairy products.

Summary The role of thiol/disulphide exchanges during acid gelation of preheated milk was studied with milk samples with or without N-ethylmaleimide (NEM), a thiol-blocking agent, and acidified to pH 4 by the addition of glucono-delta-lactone at 20 °C. Active or total thiol groups, particle size with light scattering measurements in a dissociating solvent or by SDS-agarose electrophoresis were determined on acidified milk samples. Diffusing wave spectroscopy and rheology in low strain were applied during acidification of sample, while rheology in large strain was applied on final acid gels. The only effect of the presence of NEM was a reduced firmness of acid gels as measured at large strain and a reduced tendency to form large aggregates at pH

This study aimed to determine the kinetics of milk protein digestion and amino acid absorption after ingestion of four dairy matrices by six minipigs: unheated or heated skim milk and corresponding rennet gels. Digestive contents and plasma samples were collected over a 7 h-period after meal ingestion. Gelation of milk slowed down the outflow of the meal from the stomach and the subsequent absorption of amino acids, and decreased their bioavailability in peripheral blood. The gelled rennet matrices also led to low levels of milk proteins at the duodenum. Caseins and b-lactoglobulin, respectively, were sensitive and resistant to hydrolysis in the stomach with the unheated matrices, but showed similar digestion with the heated matrices, with a heat-induced susceptibility to hydrolysis for b-lactoglobulin. These results suggest a significant influence of the meal microstructure (resulting from heat treatment) and macrostructure (resulting from gelation process) on the different steps of milk proteins digestion.

SummaryPreparation, physicochemical characterization and stability of iron-supplemented caseins were studied. When ferrous iron (final concentrations of 0·25, 0·5, 1·0 and 1·5 mM) was added to sodium caseinate (final casein concentration 25 g/l), iron binding to caseins was complete. In parallel, an increase in absorbance at 280 nm, a decrease in fluorescence intensity, modifications of reversed-phase HPLC profile, a decrease in pH and an increase in free sodium concentration were observed. Moreover, no release of iron from iron-casein complexes was found between pH 6·5 and 3·7 or after heat treatments or in the presence of inorganic phosphate. Only EDTA disodium salt and trisodium citrate had significant effects on the release of iron bound to caseins.

L'effet du pH sur les evolutions physicochimiques de suspensions de micelles de caseine, ainsi que sur les temps de coagulation presure, a ete etudie sur une poudre de phosphocaseinate natif reconstituee dans de l'eau, une solution 0,1 mol L -1 en NaCI et un ultrafiltrat de lait. L'augmentation de force ionique entre l'eau et NaCl provoque une augmentation des teneurs en calcium et en phosphore diffusibles, une plus grande voluminosite des micelles, et une augmentation de la solubilisation des caseines et des mineraux intervenant lors de l'acidification. La solubilisation du phosphocaseinate natif dans l'ultrafiltrat conduit a un comportement proche du lait. L'acidification peut etre divisee en cinq phases successives. Les relations entre les evolutions physicochimiques sont discutees, ainsi que la diminution des temps de coagulation presure avec la baisse du pH et de la force ionique.

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.

The diffusion of small solutes in cheese is of key importance for most enzymatic reactions involved in the ripening process. However, only a limited amount of data is available on salt diffusion and practically none on peptide diffusion. Nisin, a bacteriocin peptide, migrated in model cheeses made from ultrafiltered (UF) retentate. A profile concentration device and an enzyme-linked immunosorbent assay (ELISA), specifically developed for nisin quantification in cheese, were used to model the apparent diffusion coefficients for nisin according to Fick's law. This average coefficient was 49.5 μm(2)/s in UF cheese (n = 2). When 10% gelatin was added to the retentate, this value decreased to 34.4 μm(2)/s (n = 2). The two cheeses differed in their macrostructure (rheology) and microstructure (confocal microscopy). This study provides the first apparent diffusion coefficients for a peptide in cheese and supports the hypothesis that composition and structure influence the diffusion of small solutes such as peptides.