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International audience; During filtration of skimmed milk, the casein micelles accumulate at the membrane surface and their presence has a crucial impact on the performances of the operation and on the products properties. Information is still scarce, especially at ‘low’ temperature (≈8-12°C), about how the casein micelles behave at the membrane surface and interact in such concentrated regimes. The aim of this work was to understand the structural organization and behavior of concentrated casein micelles accumulated at the membrane during crossflow filtration (concentration, sol-gel transition, cohesiveness and reversibility) with a focus on impact of temperature. The strategy consisted in combining in-situ measurements by Small-Angle X-Ray Scattering coupled to crossflow filtration, with ex-situ characterization of concentrated dispersions of casein micelles using osmotic stress technique and rheology. Swelling-re-dispersion experiments were carried out to characterize the cohesiveness of micelles concentrates. During in-situ filtration performed at constant pressure, increasing temperature (12, 25, 42°C) increased permeate flux (which was mainly attributed to the decrease of viscosity of permeate) and led to higher accumulation of casein micelles and a thicker deposit. The relaxation of pressure showed that casein micelles deposit was more cohesive at low temperature. With osmotic stress and rheology experiments, it was observed that compressibility (concentration reached at given osmotic pressure) and sol-gel transition concentration of casein micelles increased with the temperature increasing from 7 to 20°C. Moreover, swelling of casein micelles gels at 7°C resulted in lower redispersion rate as compared to the swelling at 20°C regardless of the gel formation temperature. It was also observed that casein micelles gels obtained at 7°C are more cohesive than those prepared at 20°C. This work shows that carrying out filtration at low temperature led to a lower accumulation of casein micelle (lower compressibility of casein micelles), but to a higher cohesiveness of the accumulated micelles (lower sol/gel transition and swelling properties). These results were attributed to physico-chemical modifications of casein micelles (voluminosity, hydration) under temperature changes.

Les caséines sont présentes dans le lait sous forme de micelles qui s'apparentent à des agrégats colloïdaux poreux, déformables et compressibles [1,2]. La compréhension des propriétés de ces objets et des interactions qu’ils bâtissent (i.e. compressibilité, transition sol-gel, cohésion des gels) en fonction de leur concentration est nécessaire à la compréhension des phénomènes de colmatage qui gouvernent les performances des opérations de filtration du lait largement utilisées industriellement. En effet, lors de la mise en œuvre du procédé de séparation membranaire destiné à séparer les micelles de caséines des protéines sériques et d'autres espèces moléculaires, les micelles se concentrent au voisinage de la membrane. Cette accumulation de matière entraîne une diminution du flux de perméation et de la transmission des protéines sériques. Les propriétés de compressibilité des micelles et de viscoélasticité de leur dispersion concentrée en relation avec les mécanismes de structuration induits lors de l'augmentation de leur concentration, ont été étudiées par compression osmotique et par rhéométrie à 20°C [3-5]. Les dispersions de micelles de caséines présentent une transition sol-gel à une concentration de gélification (Cgel) de 170-180 g/L à 20°C [3-5]. Déterminer cette Cgel permet de définir des conditions de filtration limitant le colmatage des membranes [6]. Actuellement, l’utilisation de membranes organiques spiralées à basse température (≈8-12°C) pour la filtration de lait rencontre un succès grandissant. Il est connu que la dynamique, la structure et la composition chimique des micelles de caséines évoluent avec la température [2], mais peu d’études de son comportement à basse température en milieu concentré ont été réalisées [7]. Il est donc nécessaire d’élucider l’influence des basses températures sur la compression, la structure et la transition sol-gel des dispersions de micelles de caséines concentrées afin d'améliorer notre connaissance des conditions optimales de filtration du lait à basse température. Le but de ce travail est d’analyser l’influence de la température sur les interactions et le comportement rhéologique (i.e. compressibilité, comportement d’écoulement, transition sol-gel, cohésion des gels) des dispersions de micelles de caséines concentrées. Les expériences ont été menées à 7°C et 20°C. Des dispersions de micelles de caséines à différentes concentrations (de 25 à 700 g/L) ont été préparées, soit par dispersion de poudre d’isolat de micelles de caséines, soit par compression osmotique. La compressibilité des micelles de caséines a été caractérisée par des expériences de compression osmotique (de 0.01 à 13 bar). La transition sol-gel a été caractérisée par rhéométrie via des mesures d’écoulement et de cisaillement harmonique. Les deux techniques utilisées apportent également des informations sur les interactions micelle-micelle. Une équation d’état de la pression osmotique en fonction de la concentration en caséine a été déterminée à 7°C : il a été mis en évidence que la compressibilité, c’est-à-dire la concentration atteinte à une pression osmotique donnée des micelles de caséines augmente quand la température augmente de 7°C à 20°C. Par exemple pour atteindre une concentration de 450 g/L, il faut appliquer une pression osmotique de 2.6 bar à 7°C contre 1 bar à 20°C. De plus, la transition sol-gel à 7°C est observée à 150 g/L (correspondant à une compression de 0.18 bar) contre 170 g/L (correspondant à une compression de 0.073 bar) à 20°C. Les micelles de caséines sont plus hydratées et ont donc une plus forte voluminosité [7] à basse température, elles s’opposent davantage à la compression, mais elles interagissent et gélifient à plus faible concentration qu’à 20°C. Ces résultats mettent en avant la nécessité d'une modification de la conduite des protocoles de filtration appliqués lors du fonctionnement de ce procédé à basse température, afin de maintenir des performances équivalentes à celles actuellement obtenues à température élevées.

National audience; Ultra- and microfiltration of skim milk are widely used in the dairy industry for concentration and the fractionation of milk proteins. However their performances are limited by formation of a concentrated fouling layer of casein micelles at the membrane surface. Some precedent results on the possibly effect of operating conditions have been highlighted by Qu et al. (2012) and Jin et al. (2014). Nevertheless, there is a lack of knowledge on the behaviour of concentrated layer (sol-gel transition and reversibility) at different operating conditions. Consequently, the aim of this work is to elucidate the local structure and time dependant organization of casein micelles fouling layer under various operating conditions thanks to in-situ small-angle X-ray scattering SAXS during cross-flow ultrafiltration. This allowed to (i) distinguish the thickness and local solid concentration profiles in flowing (sol) and stagnant (gel) parts of the fouling layer; (ii) discriminate the effect of time, transmembrane pressure TMP, shear and temperature on the sol-gel transition; (iii) study the reversibility of the sol-gel transition under the moderate laminar cross flow. Material and Methods Dispersion with initial casein micelle concentration of 54.7 g.L-1 (2.1 C0 with C0 = 26 g.L-1) was prepared from casein micelle powder (Promilk 802 B, Ingredia, Arras, France) dispersed in preliminary obtained milk ultrafiltrate (Qu et al., 2012). The dispersions were prepared by thoroughly mixing the casein powder in a UF permeate for 15h at 35°C. SAXS experiments were performed at ID02 High Brilliance beamline of ESRF (Grenoble, France) with the help of specially developed SAXScross-flow ultrafiltration cell (Jin et al., 2014). Polyethersulfone ultrafiltration membrane with nominal MWCO of 100 kDa (Pleiade, Orelis Environnement, France) was used for filtration. Those experiments allowed to access to the structural organization at nanometer length scales and concentration profiles at the vicinity of the membrane surface. The profiles of casein micelles concentration in the fouling layer (local casein concentration versus distance to membrane) were obtained with spatial resolution of 20 μm. The impact of transmembrane pressure (10 and 110 kPa), cross-flow velocity (0.03, 0.05 and 0.15m.s-1) and, notably, temperature (12, 25 and 44°C) on the local structure of fouling layer was studied during different stages of filtration (layer formation → relaxation → erosion). Results and Discussion Filtration run in figure 1 was carried out at v = 0.03m.s-1 and TMP = 110 kPa. A second step of relaxation was performed by reducing the TMP at 10 kPa. Finally, erosion step at 10 kPa and 0.15m.s-1 was applied to try to eliminate the fouling layer. Such conditions allow to build a thick casein micelle layer of around 125 μm (Fig. 1) with a maximal concentration of 7.5 C0 (around 195 g.L-1) close to the membrane (z = 30 μm) above the sol- gel transition (180g/L), according to Bouchoux et al. (2009) and Jin et al. (2014). The SAXS analysis revealed that this highly concentrated layer is mostly reversible since it lasts a 50-70 μm of casein micelles layer reaching 3.5 C0 (around 91 g.L-1) close to the membrane. The effect of temperature on the concentration profile and its reversibility has been studied for three independent filtrations (Fig. 2). Concentrated layer thickness increased with temperature: from ∼100 μm at 12°C to 150 μm at 25°C and 200 μm at 44°C (after 183 ± 9min of filtration) and decreased with shear. However, the concentration distribution profile did not evolve with layer thickness. A maximal concentration is observed for filtration performed at 25°C. Irreversibility is similar for all the temperatures since a 3.5 C0 (around 91 g.L-1) fouling layer close to the membrane is lasting after pressure relaxation. Conclusion In-situ SAXS measurements during cross-flow ultrafiltration of casein micelles allowed to follow the formation and reversibility of concentration polarization layers as a function of time, TMP, cross-flow and temperature conditions. It leads to identify sol-gel transition of casein micelles near the membrane surface and to distinguish which part was reversible and eliminated by TMP relaxation. Those results are important for understanding the mechanisms of concentration during cross-flow filtration and they could be used to adjust the filtration parameters

The aim of this work is to understand the influence of operating conditions (time, transmembrane pressure(TMP), cross-flow velocity (v)) on the casein micelles fouling layer properties (build-up, sol-gel transition andreversibility behaviour during TMP relaxation) during cross-flow filtration. Small-Angle X-Ray Scattering(SAXS) has been coupled to cross-flow ultrafiltration to determine the behaviour of casein micellesconcentrated layers at the membrane surface. Concentration profiles have been deduced from the scatteringintensities registered in-situ in the course of the cross-flow ultrafiltration of casein micelles dispersions. SAXSand filtration results for a reference experiment (TMP = 110 kPa, v = 0.03 m∙s−1, 2h30 of filtration, 25°C)show the formation of a thick fouling layer of 275 μm and reaching 307 g∙L−1 at the membrane surface, whichis above the sol-gel transition of casein micelles (≈180 g∙L−1). Furthermore, results show some reversibility ofthe outer fraction of fouling layer and irreversibility of its fraction close to the membrane surface. Thoseresults are important for understanding the concentrated layer build-up of casein micelles during cross-flowultrafiltration but more originally the TMP relaxation behavior of casein micelles fouling layer linked todifferent operating conditions

Ultra- and microfiltration of skim milk are widely used in the dairy industry for concentration and the fractionation of milk proteins. However their performances are limited by formation of a concentrated fouling layer of casein micelles at the membrane surface. Some precedent results on the possibly effect of operatingconditions have been highlighted by Qu et al. (2012) and Jin et al. (2014). Nevertheless, there is a lack of knowledge on the behaviour of concentrated layer (sol-gel transition and reversibility) at different operating conditions. Consequently, the aim of this work is to elucidate the local structure and time dependant organization of casein micelles fouling layer under various operating conditions thanks to in-situsmall-angle X-ray scattering SAXS during cross-flow ultrafiltration. This allowed to (i) distinguish the thickness and local solid concentration profiles in flowing (sol) and stagnant (gel) parts of the fouling layer; (ii) discriminate the effect of time, transmembrane pressure TMP, shear and temperature on the sol-gel transition; (iii) study the reversibility of the sol-gel transition under the moderate laminar cross flow.Material and MethodsDispersion with initial casein micelle concentration of 54.7 g.L-1 (2.1 C0 with C0 = 26 g.L-1) was prepared from casein micelle powder (Promilk 802 B, Ingredia, Arras, France) dispersed in preliminary obtained milk ultrafiltrate (Qu et al., 2012). The dispersions were prepared by thoroughly mixing the casein powder in a UF permeate for 15h at 35°C. SAXS experiments were performed at ID02 High Brilliance beamline of ESRF (Grenoble, France) with the help of specially developed SAXScross-flow ultrafiltration cell (Jin et al., 2014). Polyethersulfone ultrafiltration membrane with nominal MWCO of 100 kDa (Pleiade, Orelis Environnement, France) was used for filtration. Those experiments allowed to access to the structural organization at nanometer length scales and concentration profiles at the vicinity of the membrane surface. The profiles of casein micelles concentration in the fouling layer (local casein concentration versus distance to membrane) were obtained with spatial resolution of 20 μm. The impact of transmembrane pressure (10 and 110 kPa),cross-flow velocity (0.03, 0.05 and 0.15m.s-1) and, notably, temperature (12, 25 and 44°C) on the local structure of fouling layer was studied during different stages of filtration (layer formation → relaxation → erosion). Results and DiscussionFiltration run in figure 1 was carried out at v = 0.03m.s-1 and TMP = 110 kPa. A second step of relaxation wasperformed by reducing the TMP at 10 kPa. Finally, erosion step at 10 kPa and 0.15m.s-1 was applied to try toeliminate the fouling layer. Such conditions allow to build a thick casein micelle layer of around 125 μm (Fig. 1) with a maximal concentration of 7.5 C0 (around 195 g.L-1) close to the membrane (z = 30 μm) above the sol- gel transition (180g/L), according to Bouchoux et al. (2009) and Jin et al. (2014). The SAXS analysis revealed that this highly concentrated layer is mostly reversible since it lasts a 50-70 μm of casein micelles layer reaching 3.5 C0 (around 91 g.L-1) close to the membrane. The effect of temperature on the concentration profile and its reversibility has been studied for three independent filtrations (Fig. 2). Concentrated layer thickness increased with temperature: from ∼100 μm at 12°C to 150 μm at 25°C and 200 μm at 44°C (after 183 ± 9min of filtration) and decreased with shear. However, the concentration distribution profile did not evolve with layer thickness. A maximal concentration is observed for filtration performed at 25°C. Irreversibility is similar for all the temperatures since a 3.5 C0 (around 91 g.L-1) fouling layer close to the membrane is lasting after pressure relaxation.ConclusionIn-situ SAXS measurements during cross-flow ultrafiltration of casein micelles allowed to follow theformation and reversibility of concentration polarization layers as a function of time, TMP, cross-flow andtemperature conditions. It leads to identify sol-gel transition of casein micelles near the membrane surface and to distinguish which part was reversible and eliminated by TMP relaxation. Those results are important for understanding the mechanisms of concentration during cross-flow filtration and they could be used to adjust the filtration parameters

International audience; The bottleneck of skim milk ultrafiltration (UF) is the fouling phenomenon that affects the process performance thus limits the productivity. Several strategies have been chosen to control fouling. Some proposed to increase the shear rate close to the membrane [3], others suggested different procedures such as backpulsing[4], pulsating or reversed feed flows[5]. The ultrasonic-assisted filtration processes have been also proposed to enhance the cleaning process or the permeate flux. Several successful examples can already be found in literature [6-7]. Our recent work has shown a great interest to apply in-situ ultrasound (US) in colloidal suspensions filtration: with a designed ‘Cross-Flow US-coupled Filtration Cell’ at low US power (2 W.cm-2), a significant increase of permeate flux was observed without damaging the membrane structure [8]. Using the designed ‘SAXS Cross-Flow US-coupled Filtration Cell’, this study is devoted to enhance skim milk ultrafiltration by applying in-situ ultrasonication and to characterize effects of ultrasound at multi-scales: the macroscopic results, presented by the permeate flux, have been combined with simultaneous detections at nanometer length scale, revealed by in-situ SAXS measurements of synchrotron radiation [9]. Skim milk suspensions were prepared from ‘low heat’ Bovine Skim Milk Powder at C0=27 g.L-1 in casein micelle content. Figure.1a displays the evolution of permeate flux Jv, inlet/outlet pressures of filtration cell and cross-flow rate Qv over time. The filtration run is divided into 4 steps according to different operating conditions

National audience; Dans ce travail, il est proposé de combiner in-situ des ondes ultrasonores basse fréquence aux champs de pression et aux effets hydrodynamiques lors de l’ultrafiltration tangentielle du lait. Une nouvelle cellule de filtration tangentielle a été développé afin d’une part, d’appliquer des ultrasons au voisinage de toute la surface membranaire au moyen d’une lame vibrante à 20 kHz, et d’autre part d’observer in-situ l’organisation des colloïdes dans les couches de polarisation par diffusion de rayons X aux petits angles (SAXS) [1–4]. L’observation in-situ des profils de concentration permet d’apporter des informations précieuses sur les modifications des couches concentrées sous ultrasons aux échelles nanométriques. L’objectif est de corréler les performances du procédé de filtration à l’échelle macroscopique (flux de perméat, taux de rétention, caractéristiques rhéologiques...) avec les résultats de ces observations aux échelles des particules (profils de concentration et de structure au voisinage de la membrane).

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The main obstacle limiting the performance of ultra and microfiltration processes is the accumulation of matter at the membrane surface (concentration polarization, gelation and/or deposit build-up). Improvement of these processes and possible prediction of the performance depends then on the understanding of mechanisms involved in the formation of deposits in the vicinity of the membranes. In the past, experiments, modeling and numerical simulations have been attempted to accurately explain and quantify the decrease of flux due to the development of concentrated layers at the membrane surface. Some of them give satisfactory results concerning the macroscopic behavior but have limited validity at the microscopic scale. Moreover the lack of pertinent experimental data prevents direct validation of these models.The focus of this paper is to characterize, in-situ, at nanometer length scale, the structures and concentration profiles of colloids accumulated in the vicinity of the membrane. Casein micelles, which represent 80% of the proteins in cow milk, are a good candidate for this study since ultra- and microfiltration are widely used for the concentration of milk proteins. Small-angle X-ray scattering (SAXS) has been applied on filtration cells to scan the interior of the concentrated layers during its growth and relaxation. Experiments were performed using a frontal filtration cell, equipped with a polyethersulfone membrane (100 kDa) mounted on a motorized sample stage that enabled to translate the cell across the incident beam vertically and horizontally. The spatial and temporal resolution of the measurements enabled access of structure and colloid concentration profiles between 100 μm and 2 mm from the membrane surface. The results obtained with SAXS have been compared to characterization of casein micelles done in dense regime [1].This study shows that in dead-end filtration mode, the high reduction of permeation flux in the early stage of filtration was associated with an exponential increase of the concentration at the membrane surface. The analysis of concentration profile highlights three main regimes:In the first concentration regime (C115g/L), the micelles strongly interact and progressively get into close contact. At a concentration > 150 g/L the micelles are in close packing and then tend to deform and deswell as concentration increases. In this last regime, casin micelles form an gel, that does not disappear after pressure relaxation and rinse of the filtration cell.

International audience; Understanding the mechanisms that control the filtration of complex colloidal suspensions is a major challenge in the development of membrane-based processes in industry. One of the main limiting factors for the development of these processes lies in the formation of a polarization layer and/or deposit near the separating membrane. Elucidating the structure of the first layer of deposited colloids during the first steps of filtration, constitutes a considerable technological jump in the understanding of the physical mechanisms implied in the formation of these deposits. It is also important to identify the effects of the physicochemical properties, the rheological behavior and the hydrodynamic fields on the organisation of the colloids during filtration. The focus of this work is the in-situ characterization of the induced structures and flow properties of the polarization layers of colloidal suspensions when simultaneously subjected to a transmembrane pressure and tangential flow over the membrane. To fulfill this challenge a new tangential ultrafiltration SAXS cell has been developed at the "Laboratoire de Rhéologie" Grenoble, France. This cell allowed combining small angle x-ray scattering (SAXS) at the European Synchrotron Radiation Facility (ID02 beamline) with membrane separation processes. Systems studied are anisotropic colloidal aqueous dispersions composed of sepiolite fibers 1 micrometer long and 0.01 micrometers in diameter, a natural non-swelling fibrous clay. The initial concentration of the filtered suspensions was in the semi-dilute domain for which the suspensions exhibit a shear thinning rheological behavior. During filtration, a transmembrane pressure was applied and successive tangential flows were imposed. The permeation flux was continuously measured and simultaneously the x-ray beam (40 x 250 micrometer) crossed the lateral dimension of the cell to probe the structure of the deposit with time at different heights above the membrane. As already described in previous studies concerning frontal filtration mode [1-2], a calibration curve relating the absolute scattering intensity to the particles concentration has allowed to deduce the concentration profiles in the deposits at distances to the membrane above 300 micrometers. During these experiments the high level of concentration reached in the deposit up to 50 times the initial concentration, and the highly anisotropic structure formed has been identified as one of the main mechanisms controlling the filtration flux decrease. The results prove the possibility to obtain pertinent structural information in the vicinity of membrane surfaces during ultrafiltration. It offers essential experimental data necessary for improvement in theoretical and numerical modeling of the filtration process. [1] F. Pignon, A. Alemdar, A. Magnin, and T. Narayanan, Langmuir, 19, 8638 (2003). [2] F. Pignon, G. Belina, T. Narayanan, X. Paubel, A. Magnin and G. Gésan-Guiziou, The Journal of Chemical Physics, 121(16), 8138 (2004).

International audience

Concentrated suspensions of anisotropic or deformable particles show partial orientation when submitted to an external field, like shear, as for instance, transition from isotrope to nematic phase. Apart from these two extreme phases, partial orientation can occur. Direct information of this intermediate regime is difficult for micrometric suspensions, since the medium is generally turbid. Indeed, microscopy provides a partial information at interface, that is not representative of the average volumic orientation at the sample scale. A good and pertinent tool is 2D Small Angle X Scattering (SAXS), leading to the determination of the average orientation of single particles. The aim of this presentation is to show that anisotropic (suggested: scattering) of incoherent light can also give an average information on the anisotropy of the medium. The experimental device is composed by a focused laser (100µm) at the sample surface and incoherent light transport is then measured by a camera in the backscattering geometry (1,2). The acquired image size is 1cm, very large compared to the impinging laser. For spherical particles, or randomly oriented anisotropic particles, the acquired image possesses an angular symmetry. When the scatterers are globally oriented, this incoherent image is deformed. We have performed experiment on a sepiolite suspension (0.5%), known as developing a fiber like system under shear (3). We show here that the measurement of its anisotropy through incoherent light transport is very similar to the order parameter obtained by 2D SAXS measurements performed in shear flow geometry at the European Synchrotron Radiation Facility, on beamline ID02 (fig.1). This shows that the average time and volumic information obtained by both techniques are identical. This very simple technique is then used in another system composed by non Brownian calibrated glass fibers (6µm diameter, 12µm length). Average orientation of particles is followed versus shear at various concentrations.