Résumé
The clarification of tropical fruit juices by tangential microfiltration makes it possible to assemble a large range of new products. However, in spite of this potential, it is very difficult to clarify fruit juices with high pulp contents. The combining of filtration and enzymatic liquefaction is an interesting alternative for facilitating the operation. In this context, the joint effect of filtration conditions and enzymatic treatment has been studied mainly in passion fruit juice. An economic strategy was devised to enable the continuous production of clarified juice. The procedure can be used for pulpy fruit juices without generating wastes or by-products. The advantages of immobilisation on a support in a bioreactor in order to reduce enzyme consumption have also been evaluated. Fruit juices can also be concentrated in order to reduce transport and storage costs. Concentration is frequently used for tropical fruit juices because of the distance between production and consumption zones. Consumer demand is currently tending towards fruit juices that conserve the sensorial and nutritional qualities of fresh fruits. However, conventional concentration techniques cause substantial losses of aroma and vitamins. In this context, the study is aimed at evaluating the industrial advantages of osmotic evaporation, a new cold concentration process. The issue is that of proposing to professionals a technique that better conserves the quality of their product. Osmotic evaporation is a new membrane process, consisting of interposing a porous hydrophobic membrane between the fruit juice to be processed and a concentrated saline solution. The difference in water activity between the two solutions creates a vapour pressure gradient in the membrane pores, that remain filled with air. This spontaneous phenomenon causes a transfer of water from the fruit juice to the brine without the need to heat the product. Tests performed on passion fruit juice at 30°C under industrial conditions showed that the technique can be used to achieve a soluble dry extract (SDE or Brix) of at least 60%. The evaporation flow depends on the treatment conditions. A flow of 0.65 kg.h-1.m-2 was measured during juice concentration from 14 to 40% SDE. It decreases to 0.50 kg.h-1.m-2 between 40 and 60% SDE. The use of two osmotic evaporators in series improves the throughput. This configuration makes it possible to achieve an average evaporation flow of 0.62 kg.h-1.m-2 to produce 60% SDE concentrate. Under these conditions, continuous concentration can be performed for more than 20 hours with a constant evaporation flow. The results of sensorial analyses revealed improved colour, taste and aroma of the juice in comparison with thermal concentration. Furthermore, the osmotic evaporation technique has the advantage of conserving almost all the vitamin C in the product thanks to the low process temperature. (Résumé d'auteur)