Julien Andrieu

Characterization of ice cream structure by direct optical microscopy. Influence of freezing parameters

Abstract The main objective of this study was to develop and to set up a new optical direct microscopy method, based on the reflected light flux differences, with episcopic axial lighting to characterize the different phases structure of commercial overrun ice creams. Firstly, the results obtained have been validated by two others methods, a destructive method by dispersion and observation by light microscopy and, an indirect method, by scanning electron microscopy after freeze-drying sample. It was observed that the three methods were in agreement and led to the same conclusions concerning the main freezing parameters influence. So, this technique has been principally used to investigate the effects of the freezing conditions on the ice crystal structure. One of the most important parameters is the freezing rate that governs not only the size, but also the ice crystals texture of the ice cream. The air introduction (overrun) tends to limit the ice crystal growth. Besides, by this method, it was also possible to observe and to quantify the air bubbles size distribution. This direct microscopy method could be extended to other frozen food structure analysis.

Experimental study and modelling of the ice crystal morphology of model standard ice cream. Part I: Direct characterization method and experimental data

Abstract A standard commercial dairy ice cream was used to study the ice crystal texture by a direct method namely by optical microscopy with episcopic coaxial lighting and with image analysis software (IPS). The exactness, precision and reproducibility of the method was checked. A mean ice crystal size at different locations inside the food samples was determined for different freezing conditions (initial temperature, freezing rate) and different compositions with and without stabilizers. This relatively new method for foodstuffs was compared with the freeze-drying method (indirect method) previously used in our laboratory and some discrepancies were observed. Because it preserved at best the frozen original texture, we think that this direct optical microscopic method is more reliable and more precise than the other methods previously used. We observed that the mean crystal size increased with the distance to the cooling plate and, on the other hand, decreased with the cooling rate or the cooling temperature. Due to its numerous advantages – i.e., low cost, rapidity, reliability and precision – we forecast, that the use of this direct method, quite unknown to the frozen food technologist, will increase in the near future.

Kinetics and slurry-type reactor modelling during catalytic hydrogenation of o-cresol on Ni/SiO2

Abstract The effect of kinetic parameters (reactant concentrations, temperature) was investigated on the initial reaction rate for the catalytic hydrogenation of o -cresol on Ni/SiO 2 , carried out in a batch or semi-batch agitated slurry-type reactor. The data were interpreted by a kinetic model based on the Langmuir-Hinshelwood mechanism with non-dissociative and non-competitive adsorption of o -cresol and hydrogen on different sites, where the limiting step is due to the reaction of adsorbed reactants. The activation energy ( E a = 82 kJ/mol) is in good agreement with previous literature values reported for the catalytic hydrogenation of phenol. Taking into account thermodynamic (solubilities) and mass transfer kinetics ( k L a ) data measured in situ , the integral reactor conversion rate of this three-phase catalytic reaction was simulated accurately in the physical regime by taking into account external and internal mass transfer resistances.

High-intensity infrared drying study: Part II. Case of thin coated films

Abstract This study describes the combined drying (convective and infrared) of aqueous coated films under high infrared densities (i.e. 5–18 kW m −2 ). This shrinking, highly hygroscopic and transparent material exhibited specific characteristics which are very different to those observed in a pure convective drying process. Thus, the strong influence of the radiative properties of the support (absorptivity, reflectivity) on the drying kinetics and on the film temperatures of the material was observed. Modelling of the drying process, based on a diffusional model written using a mobile frame of coordinates (Lagrangian coordinates) and associated with an overall heat balance in the support + coated film system, has proved capable of expressing the drying curves and temperature evolution very precisely. Before simulation, two main coating properties were deduced from drying experiments: (i) the mean absorptivity as a function of the mean moisture content, and (ii) the apparent diffusivity as a function of the temperature and of the local moisture content. So, this simulation lets us approach quantitatively the process control and the risk of bubbling (or swelling) during the final drying period.

High-intensity infrared drying study: Part I. case of capillary-porous material

Abstract Experiment involving the drying of a thin horizontal unconsolidated wet sand layer under intensive infrared radiation (IR) have been performed in a laboratory-scale dryer. The drying rates and solid temperature data were much higher than in conventional convective processes. Coupled heat and mass-transfer phenomena have also been analyzed. It was observed that medium-infrared radiation (medium-IR) was more efficient than near-infrared radiation (near-IR) and that the analogy between heat and mass transfer is still valid in the case of high mass fluxes. A simple receding model based on the assumption that IR radiation is absorbed at the surface of the product could predict the drying rates and the temperature profiles up to the zero moisture content. The experimentally determined parameters of the main model were the mass transfer and infrared absorption coefficients.

A CONTRIBUTION TO THE CHARACTERISTIC DRYING CURVE CONCEPT: APPLICATION TO THE DRYING OF PLASTER

ABSTRACT Implicit equations relating the dimensionless drying rate to the reduced moisture content for porous non-hygroscopic slabs have been determined. This study is based on the receding evaporative front model and on the assumption of a parabolic moisture content profile in the diffusional zone of the wet region. The characteristic functions derived depend on the relative position of the receding evaporative front. Two cases were considered : the drying of thick products and the drying of thin products. It has been observed that the relationships corresponding to the C.D.C obtained for thick products depend on the initial. On the other hand, it was observed that experimental CDC. of plaster slabs determined for many thicknesses and various plaster textures depend strongly on these two parameters. Furthermore, the model presented gives good results only for thin products.

Experimental study and modelling of the ice crystal morphology of model standard ice cream. Part II : Heat transfer data and texture modelling

Quantitative analysis of the mean size of ice crystals is mainly based on heat transfer modelling during the freezing process. So, during this work a standard dairy ice cream was frozen in a freezing device that ensured a uni-directional heat transfer. The temperature profiles inside the frozen ice cream were recorded; these data obtained for different cooling temperatures and for different initial temperatures led to the freezing front rate and the freezing front temperature. Then, these results were interpreted using the Neumann model that allowed to calculate the thermal gradients near the interface in the frozen or in the unfrozen zone. Finally, these results were used to fit empirically the ice crystal mean size as a function of the freezing front rate and of the thermal gradient in the frozen zone. Another type of empirical correlation including the distance to the cooling plate and the Neumann parameter δ was proposed.

Heat and mass transfer relationship in combined infrared and convective drying

ABSTRACT Results from three experimental studies are reviewed in order to examine the relationship between the heat and mass transfer coefficients in the combined infrared and convective drying process. These results demonstrared that the heat

EXPERIMENTAL INFRARED DRYING STUDY OF A MODEL WATER-BASED EPOXY-AMINE PAINTING COATED ON IRON SUPPORT

ABSTRACT The aim of this work is to study the infrared drying and curing kinetics of a model water-based epoxy-amine painting coaled on iron support. After a presentation of the model painting elaborated in our laboratory, experimental and theoretical data of the curing kinetics are presented. Then, we described the special laboratory-scale infrared dryer set up in our laboratory. This dryer was able to carry on drying experiments with uniform infrared flux densities between 0 to 25 kW,m-2 The study of the painting drying as a function of the infrared flux density and of the film thickness shows the influence of these two main physical parameters.

Drying-Induced Cracking of Abrasive Rings:Risk Prediction and Process Optimisation by Numerical Simulation

ABSTRACT The dependence of the mechanical stresses distribution on the water content and temperature profiles has been numerically investigated in a porous unsaturated hygroscopic abrasive agglomerate of annular shape. The thermophysical, kinetic and mechanical properties of the abrasive agglomerate were determined experimentally. The simulations have been applied to unfired abrasive rings convective drying optimisation by fitting operating conditions in order to avoid cracks formation.