Clarification and concentration of citrus and carrot juices by integrated membrane processes

摘要

The use of integrated membrane processes for the clarification and the concentration of citrus (orange and lemon) and carrot juices is proposed as an alternative to the traditional techniques of the agro-food industry. The ultrafiltration (UF) process was studied on a pilot plant unit to clarify the raw juice. A limpid phase was produced in this step and it was used for concentration by successive membrane treatments. The reverse osmosis (RO) process, performed on a laboratory plant unit, was used to preconcentrate the permeate coming from the UF step up to 15-20 g TSS/100 g. A final osmotic distillation (OD) step yielded a concentration of the retentate coming from the RO up to 60-63 g TSS/100 g at an average throughput of about 1 kg h m~(-2). This laboratory unit was mainly used to develop operational parameters for the design of a full-scale plant and, secondly for the production of sample concentrates for their testing and evaluation. The results obtained in the experimental tests concerning the identification of membrane modules, the operating and fluid-dynamic conditions and cleaning procedures of membrane modules, are reported and discussed. The performance of the membrane modules was evaluated on the basis of productivity, quality of the product (evaluation of the antioxidant activity) and fouling characteristics. On the blood orange juice samples it was demonstrated that the total antioxidant activity (TAA) of juices concentrated by evaporation is lower than that of the fresh juice. During the UF process TAA was maintained, with respect to the fresh juice, both for the permeate and for the retentate. A little decrease of the TAA was observed in the RO treatment, probably on account of the high pressure employed during the process. After this step the subsequent concentration treatment by OD did not induce any significant changes to TAA independently from the final concentration obtained. Moreover, the juices concentrated by membrane technology retained their colour and large part of their aroma which is on the contrary lost during thermal concentration. According to the results obtained, it is possible to suggest an integrated membrane process that, starting from the raw juice, yields concentrated juices of high quality and high nutritional value. The residual fibrous phase coming from the UF process could be submitted to a stabilising treatment and reused for the preparation of beverages enriched in fibres. Advantages are in terms of: reduction of clarification times, simplification of the clarification process, increasing of clarified juice volumes, the possibility of operating at room temperature and preserving the juice freshness, aroma and nutritional value, improvement of the quality of the final product through the removal of extraneous substances and improvement of the production processes.