Analyse du rôle de la transition vitreuse dans les procédés de conservation agro-alimentaires

The glass transition of foodstuffs is of particular importance because it is closely linked to the quality of products. For food industries, particularly those concerned with dehydration processes (drying, spray-drying, freeze-drying, …) or low temperature processes (freezing, quick-freezing, …), this parameter is of prime interest. This paper presents an easy to understand approach to the glass transition phenomenon, explains its influence according to different food processes, and reviews several methods to determine it experimentally or to estimate its value.     

Modèlisation de la consommation de puissance dans des echangeurs à surface raclée traitant des fluides Newtoniens et non-Newtoniens

The power consumption of a scraped-surface heat exchanger operating on Newtonian and non-Newtonian liquids in isothermal laminar flow has been studied in relation to the rheological behaviour of the liquids, the annular gap width and the number, position and speed of the blades.The part of the power input which was dissipated in the bearings of the rotating shaft was determined experimentally. Excluding those bearing losses, the power consumption consists of two parts: the power required to maintain the rotational flow in the annulus and the power required to rotate the blades. For a Newtonian liquid, each part of the power consumption was calculated theoretically and the results were found to be in agreement with experiment.With non-Newtonian liquids, the power required to maintain the rotational flow in the annulus is close to the theoretical value. The power required to rotate the blades was found to be very small: the friction conditions between the edge of the blades and the wall are different. Evidental dry contacts can occur.     

Etude de la perte de charge dans un echangeur de chaleur à surface raclée traitant des produits newtoniens et non-newtoniens

The pressure drop in a scraped-surface heat exchanger operating on Newtonian and non-Newtonian fluids has been studied in relation to the flow rate, the rheological behaviour of the fluids, the rotational speed and the number of blades. For Newtonian fluids, the pressure drop is independent of the rotational speed and can be predicted by the theoretical equation for flow in an annular space modified by a geometrical correction factor taking into account the effect of the blades. For shear-thinning fluids, an increase in rotational speed causes a decrease in pressure drop due to a decrease in the apparent viscosity of the fluid. The authors adapt the method of Metzner and Otto for predicting the power consumption in agitated vessels in order to calculate an apparent shear rate in their exchanger. This apparent shear rate is a linear function of the rotational speed. The parameters of this function are correlated to the operating conditions.     

Evolution de la chaleur specifique apparente des fromages fondus entre 40 et 100°C. Influence de leur composition

The technique of differential scanning calorimetry was used to determine the apparent specific heat of mixtures for processed cheese manufacture.In the ranges of: dry matter content, 31·6–57·5% (kg kg^?1); fat content, 13·5–40·5% (kg kg^?1) and temperature, 40–100°C; the following relationship was obtained:

$\text{c}{}_{\mathrm{p}}\text{= 4,101+0,0012θ ? (1,673+0,00027θ)x}{}_{\mathrm{f}}\text{?(2,716?0,0011θ)x}{}_{\mathrm{s}}$

where c_p = apparent specific heat (kJ kg^?1 °C^?1), θ = temperature (°C), x_f = mass fraction of fat (kg kg^?1) and x_s = mass fraction of non-fat solids (kg kg^?1).After comparison of these experimental results and extrapolated values with those from the literature, it is concluded that this relationship is appropriate to describe the specific heat of processed cheeses.