Résumé
Microfiltration is broadly used in fruit juices industries to ensure clarification and sterilization of many fruit juices or to concentrate their pulpy fraction. However, the industrial application of microfiltration in fruit juice industries remains hampered by the decrease of membrane permeability during the operation, due to the fouling mechanism. To increase technological feasibility and economical productivity, fruit juices microfiltration is generally performed using tubular membrane under high crossflow velocities, with the aim to limit particles deposition. Indeed, high shear rates close to the membrane surface can reduce membrane fouling by reducing particles deposition, but under these hydrodynamic conditions large compounds, present in the bulk suspension, can be fractionated into smaller particles. Since fruit juices contain cell fragments, colloidal aggregates, etc. this fractionation can release new compounds characterized by a high fouling propensity. The aim of this work was to study the impact of high crossflow velocity on orange juice characteristics, i.e. particles size distribution, insoluble particles cohesion energy and overall fouling propensity. A raw orange juice (produced locally) was subjected to a shear rate of 2.105 s-1 (without filtration) using a pilotscale filtration unit (TIA, Bollene, France). Laser diffraction was used to determine the impact of this shear rate on the particles size distributions of the orange juice. Dynamical rheological measurements, according to the strain amplitude sweep test, were realized on the insoluble compounds of the raw and the sheared juices. The obtained complex moduli and critical yield stresses were used to calculate the energy of cohesion of the network of this class of particles. Finally, the fouling propensities of the raw and the sheared juices were determined using a stirred and pressurized lab-scale filtration cell. The obtained results showed that the crossflow velocity changed significantly the properties of the orange juice. Indeed, large particles were not only removed from the membrane wall but also divided into smaller particles. Moreover, rheological measurements showed that higher network particles strength took place after subjecting the juice to a cross-flow velocity, suggesting that the fractionation of large particles released smaller compounds leading to higher particles-particles interactions. Furthermore, filterability tests confirmed that the fouling propensity of fractionated juice was different from that of the raw one. These results show that the subjecting orange juices to high crossflow velocities during microfiltration can generate small particles and subsequently enhance the membrane fouling. According to this conclusion, new filtration configurations such as using hollow fiber membrane (dead-end filtration mode) and softer operating conditions could be proposed as an interesting alternative to the conventional fruit juices microfiltration conditions. (Résumé d'auteur)