The removal of water from juices using reverse osmosis (RO) has the advantage of not needing state passages; it is not necessary to consume energy to transform the water rate that one wants to remove in steam (thermal evaporation) or in ice (freeze-concentration). It is necessary just to apply on the juice a pressure higher than the osmotic one while it passes on suitable semi-permeable membranes. Unfortunately, with regard to this, citrus juices, as well as having a rather high content of dissolved solids, to be commercially valid as concentrates, must have a concentrate degree of 4—6 times. To reach these values very high pressures must be applied; orange juice at 60° Brix has an osmotic pressure of about 200 bar, not easily maintainable in industrial equipments also for membranes collapse.
Therefore, RO concentration is usually applied when much lower concentrations must be reached, 20-25° Brix for orange juice.
This technology is usually applied in particular cases, like when one must retrograde an orange juice 65° Brix, thermally concentrated, to 42° Brix as cutback. This kind of frozen concentrated is usually prepared adding to the thermally obtained concentrate single strength juice to improve its taste and aroma. In consequence of the industrial introduction of reverse osmosis, several producers use pre-concentrated juice at about 24° Brix, osmotically obtained with pressures of about 60 bar, instead of single strength juice. The favourite configuration, dealing with pulpy juice, is the tubular one. The proposals of using RO as a pre-concentration stage for semi-processed product, to be completed with the other above-described technologies, seem not to have been followed.
Higher concentrations are reachable recurring to multi-stages systems, combining a first stage of high rejection membranes with a second of low rejection membranes. In this last stage a part of the soluto crosses the membrane increasing the osmotic pressure of the permeate (Gostoli etal., 1995). The permeate, containing a not negligible quantity of soluble solids, is reintroduced in the first stage at high rejection.
The system studied by Watanabe etal. (1990) is based on three stages, of which the first and the second equipped with high rejection membranes while the third brings low rejection membranes. A spiral module is used. The applied pressures are: 7.5 MPa at the first stage and 9.5 at the second and third stages. The concentration degree reachable with clear juice is of about 45° Brix.
On the same principle is based the 'FreshNote', set up by SeparaSystem with FMC and Du Pont. The squeezed juice is soon cooled at 10°C and placed under nitrogen; these conditions are maintained for the whole process.
As first step, the juice is completely de-pulped in an ultra-filtration system with membranes at flat configuration, with plate and frame modules. Two phases are obtained, the clear permeate passing to RO and the pulpy retentate constituting about 1/10—1/20 of the starting volume and containing the macro-molecules and all the insoluble solids, enzymes, pectins, micro-organisms. The retentate is pasteurised and cooled to be subsequently mixed to the RO concentrate.
The clear permeate, which contains sugars, acids, salts, amino acids, vitamins and aromatic oxygenated substances, goes to the RO concentrating section, constituted by a series of hollow fine fiber permeators. The membranes, made of aromatic polyamide, have an inner diameter of 42 micron and an outer one of 93 micron, not to collapse under the high pressure they are subjected to. The first concentration section contains high rejection membranes, followed by a second one with low rejection; this last permeate, which contains a quantity of dissolved solid substances, returns at the beginning of the first section together with the UF permeate to be concentrated. In order to recover the small quantities of dissolved solids and aromas, which can escape together with the water going out from the first section of the concentrating system, there is the possibility of inserting a polishing section.
The number of the ultra-filtration stages is function of the plant potentiality and of the concentration ratio of insoluble solids. The number of the permeators is function of the plant dimensions and of the concentration ratio of insoluble solids.
The reverse osmosis systems works at a pressure included between 105 and 140 bar. Adding the pasteurized UF retentate to the RO concentrate, the final juice at 45° Brix is obtained, when orange juice with aromatic richness corresponding to 80 per cent of the initial juice is processed (Goettsch etal., 1991).
Alternatively to reverse osmosis, the Cogia SA of Palaiseau, France, proposes the Osmotic Evaporation, which consists in removing water from the juice with a brine solution through microporous hydrophobic membranes. High concentration degrees can be reached (Anonymous, 1995).
Water is extracted from the juice by means of an extractor before the hydrophobic porous membrane. The passage of the water from the juice to the extractor occurs through the membrane pores under steam form without any penetration of juice or brine solution in the membrane, whose pores remain full of gaseous phase.
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