Résumé
Few studies are reported in the literature about the kinetics of chemical reactions in liquid food products under high temperature conditions. In most cases, temperatures are above 100°C, temperature gradients are created in the product, transient phase (temperature increase) is quite long compared to the holding phase and cooling phase is not described. Ohmic heating was chosen because it is a direct method allowing a bulk heating of a product. To achieve a homogeneous thermal treatment, three conditions need to be respected: first, a homogeneous electrical field was obtained by using parallel electrodes. Second, a batch configuration prevents thermal gradients due to velocity gradients. Third, the treated products have to be homogeneous in terms of electrical conductivity. Many food solutions like milk, fruit juices etc. can fulfil this constraint.Another challenge was to achieve high temperature levels situated in a sterilisation domain: the reactor was thus put under constant pressure with nitrogen and a sampling system was conceived to be able to carry out kinetic studies at high temperature and under pressure. A large range of temperatures from ambient to 140°C could be reached and hold for given times, covering thermal treatments from pasteurisation to UHT sterilisation. It takes less than 1.5 minutes for a cow milk (conductivity around 0.5 S/m) to be heated from 20°C to 140°C. The cooling is also very fast, temperature decreases from 140°C to less than 100°C within about 2 seconds. The good reproducibility of the thermal profiles allows us predicting the thermal treatment undergone by a product. Finally, the ohmic prototype was proven to have a good reproducibility of the heat load delivered to the product. The ability to take samples during a thermodynamically stable holding phase should allow producing reliable experimental data for kinetic studies.