42 research outputs found
Approche multi-échelles de la congélation des produits alimentaires
Abstract : Freezing is used to increase the shelf life of foods thanks to the lowering of the temperature which reduces the kinetics of the reactions of degradation but also thanks to the solidification of water which reduces its availability for the development of microorganisms. But freezing can also be used to transform and create a desired texture in the case of products that are initially in liquid state and are consumed at frozen state, such as frozen desserts. For these foods, the problem of freezing is different since the crystals are part of the structure of the final product.My research is dedicated to the understanding of the interactions occurring at different scales between strongly coupled phenomena (heat, mass transfer and flow) during the freezing of foods. This work has been carried out since 2002 as a lecturer at AgroParistech within the UMR SayFood and within the FRISE unit. At the beginning, focused on the freezing of semi-liquid foods under shear, my work has gradually expanded to the freezing of non porous and porous solid foods.The developed approach combines laboratory and semi-industrial pilot scale experiments with modelling. The dynamics of food structuring are monitored over time thanks to the development of advanced tools allowing characterization at different scales. The phenomena are studied using a reasoned simplification approach of the studied product and/or of the developed model.The prospects of this research are to make the link between the scales : the one the food subjected to negative cold and that of its microstructure. First by the systematic experimental monitoring of markers at these scales. Then by using modelling tools to move from one scale to another. This approach will be used to deepen the understanding of the crystallization of food containing air and cells (vegetable or animal tissues).It will be applied to other types of crystallization (fats), to the study of the link between freezing and the health risk, and finally to the study of the cold chain of frozen foods.RĂ©sumĂ© : La congĂ©lation est un procĂ©dĂ© qui permet d'augmenter la durĂ©e de conservation des aliments grĂące Ă l'abaissement de la tempĂ©rature, qui rĂ©duit les cinĂ©tiques des rĂ©actions de dĂ©gradation, et grĂące Ă la solidification de l'eau, qui rĂ©duit la disponibilitĂ© de celle-ci pour le dĂ©veloppement des micro-organismes. Mais la congĂ©lation intervient aussi pour transformer et crĂ©er une texture souhaitĂ©e dans le cas de produits initialement Ă lâĂ©tat liquide et destinĂ©s Ă ĂȘtre consommĂ©s Ă lâĂ©tat congelĂ©, tels que les desserts glacĂ©s. Pour ces aliments, la problĂ©matique de la congĂ©lation est diffĂ©rente puisque les cristaux font partie de la structure du produit final.Mes travaux de recherche ont pour objectif de comprendre les interactions se manifestant Ă des Ă©chelles diffĂ©rentes entre des phĂ©nomĂšnes fortement couplĂ©s (transfert de chaleur, de matiĂšre et Ă©coulement) lors de la congĂ©lation des aliments. Ces travaux sont conduits depuis 2002 en tant que maĂźtre de confĂ©rences Ă AgroParistech au sein de lâUMR SayFood et au sein de lâunitĂ© FRISE. Au dĂ©part, centrĂ©s sur la congĂ©lation des aliments semi-liquides sous cisaillement, mes travaux se sont progressivement Ă©largis Ă la congĂ©lation des aliments solides non poreux et poreux. Lâapproche dĂ©veloppĂ©e combine des expĂ©rimentations Ă lâĂ©chelle du laboratoire et du pilote semi-industriel avec une modĂ©lisation. La dynamique de la structuration de lâaliment est suivie au cours du temps grĂące au dĂ©veloppement dâoutils de pointe permettant la caractĂ©risation aux diffĂ©rentes Ă©chelles. Les phĂ©nomĂšnes sont Ă©tudiĂ©s par une approche de simplification raisonnĂ©e du produit Ă©tudiĂ© et/ou du modĂšle dĂ©veloppĂ©. En terme de perspectives, la prioritĂ© est de faire le lien entre les Ă©chelles : celle de lâaliment soumis Ă un froid nĂ©gatif et celle de sa microstructure. Dâabord par le suivi expĂ©rimental systĂ©matique des marqueurs Ă ces Ă©chelles. Puis par le passage dâune Ă©chelle Ă lâautre Ă lâaide de la modĂ©lisation. Cette dĂ©marche sera utilisĂ©e pour approfondir la comprĂ©hension de la cristallisation des aliments en prĂ©sence dâair et de cellules (tissus vĂ©gĂ©taux ou animaux). Lâapproche sera aussi Ă©largie Ă dâautres types de cristallisation (matiĂšres grasses), Ă lâĂ©tude du lien entre la congĂ©lation et le risque sanitaire, et enfin Ă lâĂ©tude de la chaĂźne du froid des aliments surgelĂ©s
ETUDE DU REACTEUR TORIQUE (MODELISATION ET APPLICATION A LA TRANSFORMATION DE PROTEINES VEGETALES)
LE BUT DE CE TRAVAIL EST L'ETUDE DU REACTEUR TORIQUE, LA MODELISATION DE SON COMPORTEMENT HYDRODYNAMIQUE ET SON APPLICATION A DES TRANSFORMATIONS DE PROTEINES VEGETALES. DANS UN PREMIER TEMPS, LES CARACTERISTIQUES HYDRODYNAMIQUES DU REACTEUR SONT DETERMINEES A L'AIDE DE LA DISTRIBUTION DES TEMPS DE SEJOUR (DTS) OBTENUE EXPERIMENTALEMENT DANS LE REACTEUR PAR UNE METHODE DE TRACEUR CONDUCTIMETRIQUE ET PAR LA MODELISATION DE LA DTS A L'AIDE D'UN REACTEUR PISTON AVEC DISPERSION AXIALE ET RECYCLAGE TOTAL OU PARTIEL SUIVANT QUE LE REACTEUR EST FERME OU OUVERT. CETTE DETERMINATION EST EFFECTUEE SUR UNE LARGE GAMME DE NOMBRE DE REYNOLDS ET POUR LES DEUX CONFIGURATIONS DU REACTEUR. IL EST MONTRE QUE LE MODELE REPRESENTE DE FACON SATISFAISANTE L'HYDRODYNAMIQUE DU REACTEUR DANS CES DIFFERENTES CONDITIONS D'UTILISATION. LE REACTEUR TORIQUE SE COMPORTE LORS DE CHAQUE CIRCULATION COMME UN REACTEUR DE TYPE PISTON CARACTERISE PAR UNE FAIBLE DISPERSION AXIALE AUSSI BIEN EN FONCTIONNEMENT FERME QU'OUVERT ET POUR DES REGIMES D'ECOULEMENT DE LAMINAIRE A TURBULENT, CE QUI LE REND INDUSTRIELLEMENT INTERESSANT. LE TRANSFERT DE MATIERE ENTRE SOLIDE ET LIQUIDE DANS LE REACTEUR EST ETUDIE A L'AIDE D'UNE METHODE DE DISSOLUTION ET D'UN MODELE. UNE CORRELATION PERMETTANT DE PREDIRE LES COEFFICIENTS DE TRANSFERTS DE MATIERE ENTRE PARTICULES ET LIQUIDE AU SEIN DU REACTEUR EST ETABLIE. LES PERFORMANCES DU REACTEUR TORIQUE OBTENUES EN TERME DE TRANSFERT DE MATIERE SONT COMPARABLES A CELLES D'UN REACTEUR PARFAITEMENT AGITE CE QUI LAISSE ESPERER DES TAUX DE CONVERSION SATISFAISANTS LORS DE REACTIONS BIPHASIQUES SOLIDE-LIQUIDE. LE REACTEUR EST UTILISE POUR LA TRANSFORMATION DE PROTEINES DE BLE ET DE LAIT A L'AIDE D'ENZYMES IMMOBILISEES SUR DES BILLES. LA FAISABILITE DE CETTE REACTION DANS LE REACTEUR EST DEMONTREE. LES PARAMETRES CINETIQUES DES ENZYMES IMMOBILISEES SONT DETERMINES ET COMPARES A CEUX DE L'ENZYME LIBRE. A L'AIDE DE LA CORRELATION PRECEDEMMENT OBTENUE, ON VERIFIE QUE LE TRANSFERT DE MATIERE N'EST PAS UN FACTEUR LIMITANT POUR LA TRANSFORMATION DES PROTEINES SUR LES BILLES. ENFIN, LA CINETIQUE D'HYDROLYSE DES PROTEINES DANS LE REACTEUR EST MODELISEE A L'AIDE DU MODELE DE REACTEUR UTILISE EN PREMIERE PARTIE ET DES PARAMETRES CINETIQUES DE L'ENZYME IMMOBILISEE.NANTES-Ecole Centrale (441092306) / SudocNANTES-BU Sciences (441092104) / SudocSudocFranceF
Etude de lâĂ©coulement dans un freezer lors de la congĂ©lation de sorbets
La fabrication d'un sorbet implique une Ă©tape dĂ©licate : la congĂ©lation dans un freezer (Ă©changeur de chaleur Ă surface raclĂ©e), pendant laquelle les cristaux ne doivent pas ĂȘtre trop gros. La granulomĂ©trie des cristaux de glace dĂ©pend de l'Ă©coulement du produit dans l'Ă©changeur. Mise en place d'une Ă©tude pour Ă©valuer l'Ă©coulement et dĂ©terminer la DTS (Distribution des temps de sĂ©jour), grĂące Ă une mĂ©thode expĂ©rimentale utilisant des traceurs colorĂ©s
Analyzing the microstructure of a fresh sorbet with X-ray micro-computed tomography: Sampling, acquisition, and image processing
International audienceX-ray micro-computed tomography and image processing techniques were used to analyze fresh frozen sorbets at the outlet of a batch freezer. Sorbets made from water and sucrose were visualized and their microstructure was quantified with a resolution of 9 ÎŒm. Sodium iodide was confirmed to enhance the contrast between the unfrozen water and ice in sorbets. A thermostated box was employed to keep the samples at frozen state and constant temperature (close to â 6 âą C) during imaging. A reproducible quantification of size distributions and volume fractions of ice crystals and air bubbles were obtained. Data concerning ice crystals were in agreement with cryo-SEM imaging. Ice crystals represented approximately 50%wt of the product and their mean size was about 60 ÎŒm whereas air bubbles represented about 6% of the volume. Finally, X-ray microtomography equipped with a thermostated box was found to be a particularly relevant technique for the analysis of the microstructure of frozen desserts
Characterization of the Residence Time Distribution in a Scraped Surface Heat Exchanger during the Freezing of Sorbet
International audienceThe final quality of frozen desserts is highly related to crystal size distribution and apparent viscosity, both of which are determined by the process conditions. During the freezing of sorbet, the increase of the ice volume fraction leads to the increase of the apparent viscosity of the product. This effect modifies the fluid flow behaviour, the residence time distribution (RTD) and the temperature profile inside the equipment. This work aimed at studying the effect of the operating conditions, on the RTD of the product in a SSHE, so as to characterize the product flow behaviour
Rheological properties of stabilizers at low temperatures in concentrated sucrose solutions
International audienceDuring the freezing of sorbets, the concentration of sucrose and stabilizing macromolecules increases in the unfrozen phase. Rheological properties of this unfrozen phase were studied, depending on nature and concentration of hydrocolloids in sucrose solutions at temperatures below zero. Three stabilizers were tested individually in sucrose-rich solutions: locust bean gum (LBG), hydroxypropylmethylcellulose (HPMC) and carboxymethylcellulose (CMC). The parameters of the Ostwald-de Waele model were determined and apparent viscosities of solutions were calculated at shear rates encountered during the process. Intrinsic viscosities and critical concentrations of entanglement were determined to evaluate the expansion of polymers in solutions and the transition of concentration regimes according to process conditions. Viscoelastic properties were also tested to study the possible cryogelation. For HPMC and LBG, the shear thinning behavior increased quite linearly with the concentration in stabilizer whereas CMC was highly shear thinning. Increasing sugar did not reveal large effect on these properties. The increase in apparent viscosity of the unfrozen matrix in process conditions was dependent on stabilizers; solutions containing LBG were less viscous than the others. Intrinsic viscosities revealed that HPMC and LBG became progressively less expanded as the concentration of sucrose increased whereas CMC tended to aggregate. Measurements of viscoelastic properties evidenced a dominant viscous behavior for HPMC and LBG systems whereas CMC systems showed dominant elastic behavior at frequencies higher than 0.1 Hz. The next challenge will be to better understand the potential link between the particular rheological behavior of hydrocolloids in process conditions and their possible influence on crystallization mechanisms
Online Capillary Rheometry of Commercial Sorbet
International audienceSorbet is a complex food colloid consisting of a frozen matrix, containing air bubbles, ice crystals and an unfrozen serum phase. The rheology of sorbet is strongly related to microstructure features, such as air bubbles and ice crystal size distribution. In this work, the rheology of a commercial sorbet has been studied online by means of a pipe rheometer connected at the freezer exit. This work aimed at studying separately the influence of the ice volume fraction (Xv.i.) on the increase in the apparent viscosity (app) of a non aerated frozen sorbet, and the influence of the air volume fraction (Xv.a.) on the increase in app of an aerated frozen sorbet
Modélisation de la nucléation, croissance et attrition des cristaux de glace dans un échangeur de chaleur à surface raclée
International audienceThe objective of this study is to propose a mathematical model of ice crystallisation occurring during the freezing of sucrose solutions in a continuous freezer. The approach developed is a 1-D radial model where ice crystal nucleation, growth and breakage kinetics are described by discrete population balance (PB) equations coupled with computational fluid dynamic (CFD) modeling of the energy conservation equation. In this approach, the problem has three dimensions: one crystal size variable, one position variable, corresponding to the radius between the scraper and the wall and the residence time variable. For the numerical implementation, the population balance equation is discretized by using the class method. Population density is then obtained for each class of crystal size. Ice fraction and sorbet temperature are obtained in function of residence time and exchanger radial coordinate. Assuming spherical particles, the mean crystal diameter is calculated. The results are compared to experimental measurements of temperature and mean crystal size at the freezer outlet for given process conditions: scraper rotational speed, flow rate and wall evaporator temperature. With a first estimated set of model parameters, it is shown that the experimental tendencies are correctly represented by the model. This coupled CFD-PB modelling approach can then be considered as a promising tool for the understanding and the prediction of ice crystallisation for several operating conditions in a continuous freezer
Etude de lâĂ©coulement dans un freezer lors de la congĂ©lation de sorbets
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