In the field of citrus processing the clear juices have a minor relevance. The bigger part is represented by lemon juices and lime juices, often used as nectars and syrups acidifying and in kitchen products. Juices of other species, as orange juice, are used in cordials and squashes above all in the northern markets.
For many years, clear citrus juices production has been based on the action of pecto-lytic enzymes naturally present in the juice. It was placed in tanks and left there until the end of the destructive action of pectinase. When the upper phase had become clear, it was taken away by means of decanting, filtrated on kieselgur and concentrated. Because the attack time of pectic enzymes naturally present in the juice is very long, even of several weeks, and depends not only on their concentration but also on pH and temperature, it was necessary to add also some sulphur dioxide to prevent juice fermentation. During concentration, a part of the S09 was removed; but a part remained in the concentrate, above all that chemically linked with the monosaccharides of the juice.
Subsequently, in order to accelerate times and eliminate the use of preservative substances, added industrial enzymes are employed. The juice to be transformed in clear is pasteurized, cooled, added with industrial enzymes and placed in tanks. Proceeding to the inactivation of 'native' enzymes causes the increase of the quantity of necessary industrial enzymes, but the standardization is better both for the enzymatic concentration in the juice, and for what concerns operative conditions.
Opportunely dosing the enzymatic concentration, the clarification of the juice occurs in much shorter time than that necessary with traditional technology. The test with alcohol (or, better, with acetone) is useful to verify if pectin breakdown has occurred.
The upper opalescent phase is taken away, added with clarifying such as silica sol, and filtrated on kieselgur. If the juice has been de-pulped before being treated with enzymes, the lower pulpy phase is not quantitatively relevant. It is passed through centrifuge, added with clarifying substances and filtrated on kieselgur.
A simplified technology, used for lemon juice, excludes pasteurization. After having added industrial pectinase, the juice is left at rest for 6—8 hours; it is then added with silica sol and placed for decantation. The upper phase, practically clear, is filtrated with kieselgur on pressure filter while the sedimentation is filtrated, with kieselgur too, on rotary filter in vacuum. For the whole process no more than 24 hours at room temperature are necessary.
Complete pectin breakdown can occur only when all the pectolitic enzymes that act on the pectic molecule are present in a correct ratio also enzymes attacking the 'hairy region' (Urlaub, 1999).
The silica sol acts as inhibitor towards pectolitic enzymes; it is thus necessary to avoid its addition in the juice before the pectic enzymes have completed their work.
During the storage opalescence phenomena can occur, because of flavonoids precipitation, when they are present in a very high quantity in the juice. For this problem the solution is strongly cooling the juice so that its crystallization is accelerated, and facilitated by hesperidin crystals (Fisher-Ayloff-Cook etal., 1991).
The most recent technology implies the use of crossflow filtration, in which pecto-lytic enzymes are employed to decrease juice viscosity and improve permeate flow. The complete pectin breakdown implies an increase of galacturonic acid content and of its oligomers in the clear juice. Also in clear juices obtained with crossflow filtration it is possible that the cold stored concentrate becomes opalescent. The permeate treatment with absorbing resins before concentration avoids the formation of opalescences and increases juice stability (Lenggenhager, 1998).
The membranes used can be of the polymeric kind in polyvinylidene fluoride (PVDF) or in polysulphone (PS), with plain, tubular, or hollow fiber configuration, or of the ceramic kind.
When the citrus fruit juice passes through a membrane system, the permeate is free from insoluble solids, practically free from pectin and enzymes. Sugars, acids, vitamins and minerals do not undergo any change, except the decrease of phosphorous content, clearly due to the rate of this element contained in the juice under organic form.
The slight lowering of the Brix is related to the quantity of pectin removed (Capannelli etal., 1990).
The permeate does not possess pectin-esterasic activity, even when the membrane cut-off is higher than the molecular weight of the enzyme. The same happens with pectin. This phenomenon can be due to the fact that pectin in juice is associated with cellulosic material, while pectin-methyl-esterase is associated with suspended solids. The gel layer formed on the membrane wall can reduce its pore size (Hernandez etal., 1992).
Comparing polymeric membranes of different kinds (PS and PVDF) and different configurations, fed with orange and lemon juice with constant composition, it has been noticed that the permeate fluxes are largely independent from the kind of membrane and from the nominal molecular weigh cut-off. Cross sections of the membranes at the end of the process, observed using a scanning electron microscopy, show a fibrous deposit on the surface of the membrane, which appears as a dynamic membrane which influences the system behaviour (Capannelli etal., 1992).
In cross filtration systems using polymeric and ceramic membranes with tubular configuration, the permeate fluxes strongly depend on the tangential feed velocity at the membrane and are almost independent from the driving force at average pressure over a membrane greater than 0.2 MPa.
Because of its different superficial structure and roughness, the pectin-pulp deposit formed on the membranes has different properties in the two cases. The ceramic membranes show higher fluxes at a lower Reynold number than the polymeric membranes (Capannelli etal., 1994).
In the industrial production of clear citrus juices two systems of crossflow filtration are used: batch and continuous processing.
In the first case the retentate returns to the feeding tank and joins the new juice to be treated, until the mix viscosity in the feeding tank is so high (and the permeate flux so low) as to break the operation and proceed to cleaning. Therefore, the system works with increasing viscosity and pulposity.
The continuous processing consists in a series of elements of ultra-filtration or microfiltration in more stages. The retentate going out from the first stage feeds the second and so on.
Because the juice circulating in the crossflow filtration system becomes warmer by friction, exchangers for cooling must be installed in the plants.
At the end of each cycle the membranes must be washed to remove the substances left on the membranes surfaces and to restore flux capacities. Alkaline solutions are used, followed by hypochlorite solution or by nitric acid and then by warm water. To help cleaning, pectolytic enzymes can be used: hydrolizing pectin, they help their removal (Jansen, 1997).
The lime processing shows some peculiarities, and therefore European Community had to include in its rules on fruit juices a derogation allowing the production of lime juice from the whole fruit, while for other citrus fruit the juice must derive only from endocarp.
The older system of lime processing in Mexico is based on fruit pressing, after removal of unsuitable ones and after washing, in screw-press. The juice-essential oil emulsion thus obtained is steam distilled and essential oil is separated from condensate by decantation.
The drastic thermal treatment in presence of an acid medium such as lime juice brings deep changes in oil essential components. Lime distilled oil has thus chemical, physical and sensory characteristics very different from essential oil extracted without thermal treatment. But this flavour is appreciated for some uses. Moreover, having undergone a drastic acid-thermal action, its flavour does not change when it is used in acid liquids (lemonades, cordials, and so on), even for long commercial times.
Because of the drastic treatment, the juice has a bad quality, while pectin can be produced from the peel. To obtain a good quality juice and thermally degraded essential oil, the greatest part of primary emulsion juice-oil produced by screw-presses is refined in finishers and sent to disc centrifuges, where an oil richer emulsion is separated. The juice is cleared, filtrated and concentrated while rich emulsion is mixed to the rest of primary emulsion (corresponding to about 10-15 per cent of the whole quantity of processed lime) and steam distilled. The acidity given by this limited juice quantity (which goes lost) is enough to give the pH necessary to the desired degrading of oil aromatic compounds.
The juice, placed in settling tanks, is added with pectolytic enzymes and left at rest. When native enzymes together with added ones have completed their work, the upper layer is decanted, added with bentonite and silica, filtrated on kieselgur and concentrated. Thus, sacrificing a limited juice quantity, the principal two products required by the marked are obtained: human alimentation juice and distilled oil. From the pulp left in tanks further distilled oil is produced.
Because of higher acidity of lime juice, the time necessary for enzymatic clarification is very long, even of more than four weeks, and the addition of preservatives is necessary. The operation becomes much faster using membrane filtration.
Using suitable rasping equipments followed by juice extractors (or with screw press and centrifugation systems for essential oil) not thermally degraded juice and oil can be obtained. With normal processes cloudy concentrate juice can be produced.
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