Congélation de produits alimentaires : amélioration de la cristallisation par applications de micro-ondes

National audienceLa qualité d'un produit congelé est liée à la cristallisation ; en règle générale, plus la taille des cristaux de glace est petite, meilleure sera la qualité du produit. Un procédé innovant a été évalué dans le cadre du projet européen (FREEZEWAVE). Une assistance micro-ondes (2450 MHz) a été utilisée pour perturber le changement d'état de l'eau. Sur un prototype de laboratoire, les expériences ont confirmé que, quels que soient le mode d'application des micro-ondes et les paramètres du procédé, on observe une réduction de la taille des cristaux. Cet impact positif a été obtenu sur diverses matrices alimentaires, dont des pommes de terre. Un modèle de prédiction a par ailleurs permis de qualifier ces cinétiques. Deux concepts sont avancés pour expliquer ces effets bénéfiques. Abstract FREEZEWAVE project (SUSFOOD ERA-net project 2015-2018

Numerical and experimental study of a microwave assisted freezing process

Ce travail s’inscrit dans le cadre duprojet européen FREEZEWAVE (ERA-NetSUSFOOD) et porte sur l’étude et lamodélisation d’un procédé de congélationassistée par micro-ondes. Il a pour objectif unemeilleure compréhension des phénomènesphysiques associés à ce procédé. Un modèlecouplant thermique, changement de phase etélectromagnétisme a été développé. Une étudenumérique préliminaire a permis d’appréhenderles interactions complexes entre micro-ondes etmatière durant le changement de phase,notamment l’importance de la diminution despropriétés diélectriques sur la modification desphénomènes de résonance entrainant ledéplacement des « points chauds » et sur laquantité de chaleur générée.Un gel modèle a été caractérisé puis utilisé pourvalider le modèle sur un pilote développé pourl’étude. La micro-tomographie à rayon X apermis la mesure de la taille des cristaux et amis en évidence, lors d’une congélationconventionnelle, l’impact du tempscaractéristique de congélation et du gradient detempérature sur la taille des cristaux de glace.Cette étude a permis de constater une nettediminution de la taille des cristaux de glace lorsd’une congélation assistée par micro-ondes parrapport à une congélation conventionnelle. EnfinIl a été montré que la réduction de taille descristaux n’était pas due aux oscillations detempérature provoquées par des créneaux depuissance micro-ondes mais à la quantitéd’énergie apportéeThis work was performed in thecontext of the European project FREEZEWAVE(ERA-net SUSFOOD) and focuses on studyingand modelling the microwave assisted freezingprocess. The main objective was to acquire abetter understanding of the correspondingphysical phenomena. A model coupling thermal,phase change and electromagnetism wasdeveloped. A preliminary numerical studypermitted to grasp the complex interactionsbetween microwaves and matter, especially theimportance of the dielectric properties reductionon resonance phenomena, leading to the movingof “hot spots”, and on the generated heat.A methylcellulose gel was characterised andused for the model validation on a prototype pilotspecially designed for this study. A protocolbased on X ray tomography was developed todetermine the ice crystal size. It highlighted, fora conventional freezing process, that thethermal gradient and characteristic freezing timeimpacts the ice crystals sizes.In the case of microwave assisted freezing, thestudy permitted to show a clear decrease in icecrystal size compared to conventional freezing.Finally, it has been shown that the ice crystalsize reduction was not a consequence oftemperature oscillations due to microwavespulses but the consequence of the energyquantity brought to the system by microwaves

Impact on dough aeration of pressure change during mixing

International audienceThis study aimed to provide a better understanding of the impact on dough aeration of pressure change during dough mixing with a spiral dough mixer with the possibility of controlling the temperature and the overhead pressure from À960 mbar to þ500 mbar during mixing. The objectives were to understand the effect of pressure on dough during kneading in order to optimize dough kneading conditions. The well-known experimental strategy was to knead with overpressure to incorporate gas into the dough and maximize dough aeration then to subdivide the gas bubbles introduced in the previous step by applying a vacuum in the mixer's overhead. The results showed that dough aeration was proportional to the number of rotations of the spiral. The time to reach equilibrium was longer for a larger pressure drop. The kinetics of disentrainment were slower with the highest pressure drop

Numerical study of an innovative microwave-assisted freezing process

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Micro-CT imaging and analysis for evaluating the quality of frozen food

International audienc

Evaluation d'un procédé de congélation assistée par micro-ondes

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Modélisation d’un procédé de congélation assistée par micro-ondes

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Microwave assisted freezing part 1: Experimental investigation and numerical modeling

International audienceThe aim of this study was to develop an innovative process dedicated to enhancing the quality of frozen food using microwaves. A microwave assisted freezing device was designed at the laboratory scale to perform experiments in controlled conditions. Small samples of methylcellulose gels were frozen using nitrogen gas in a TE 10 waveguide, where microwaves at 2.45 GHz were emitted intermittently or continuously. A numerical model was also developed to obtain results difficult to measure such as the local electric field and the corresponding energy density. The phase change part of the model was based on an enthalpy formulation and on the growth of spherical ice crystals. An original "hybrid 2D-3D" solving methodology was used to reduce the duration of the simulations. Favourable comparisons between the predicted temperatures and the experimental data highlighted the relevance of the models used for the thermophysical and dielectric properties. The analysis of the interactions between microwaves and matter, performed with numerical simulations, revealed the role of the freezing front as a boundary. The strong influence of sample size and of dielectric properties on the power density distribution were also illustrated when comparing our results with those published previously. The scientific knowledge obtained through this study and the original structure of the numerical model will be used to optimize microwave assisted freezing and link the process parameters to the reduction of ice crystal size highlighted in the companion paper. Industrial relevance: Microwave assistance during freezing can improve frozen product quality by reducing ice crystal size. This innovative and promising process has not yet been given much attention. Developing an accurate model which describes microwave-matter interactions during phase change permits numerical simulations that can facilitate the design of industrial equipment, and determine optimal product dimensions

Microwave Assisted Freezing: Experiments and Modelling

International audienceMicrowaves assisted freezing. Experiments and modelling. Freezing is widely used in the food industry to preserve product quality thanks to the low storage temperature. It is well known that a faster freezing enhances frozen food quality while forming smaller ice crystals. Indeed large ice crystals damage cells membranes. However fast freezing processes are energy demanding. It is therefore a challenge to reduce process cost without increasing ice crystals size or to reduce ice crystal size without a dramatically increase of cost. Some experimental studies have shown that it is possible to obtain smaller ice crystals by using electromagnetic waves assisted freezing [1–3]. The freezing process could thus be carried out with less intensive conditions. Nevertheless the optimisation of this new process requires that all involved phenomena responsible of improving ice nucleation are well understood. Two hypothesis are proposed to explain the lower crystals size: i) Water molecules rotations due to microwaves interfere with H-bond network, which could be a crystalline structure precursor [1]; ii) Microwaves heating induces a partial melting of ice crystals. The small temperature rise induced by microwaves pulses is followed by a rapid temperature decrease. The partial melting due to these temperature oscillations could induce a secondary nucleation [3]. This study aimed to model an innovative process of pulsed microwaves assisted freezing of a model gel (methylcellulose). The phase change model is based on spherical ice crystal growth and an original enthalpy formulation. The objective is to get a better understanding of thermal interactions between microwaves and product during freezing. The experimental device is characterized by a TE10 waveguide at 2.45 GHz (43 x 86 mm) in which gas nitrogen at-40°C is introduced. The sample of 9 g is placed at the centre of the waveguide crosssection where the electric field is the highest. Both nitrogen and microwaves reach the sample on the same surface. Other surfaces are insulated. Temperature is measured by optical fibres at several depths. Both microwave incident and reflected powers are measured. The 2D model was validated for both microwave heating and phase change independently first, then with the entire process. It allowed to observe spatial and temporal evolutions of the thermal and electromagnetic variables (temperature, electric field strength) and their effects, like resonance or development of hot spots

Modélisation d’un procédé de congélation assistée par micro-ondes

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