Some technologies proposed in recent years for the process of citrus fruit juices will be now treated. Some of them are already used in several plants, even if they are still substantially being modified for a better setting-up; other are still in a phase of advanced study and we can reasonably think that their application can be usefully employed in citrus juices production. In particular, the technologies related to non-traditional pasteurisation systems, debittering and osmotic concentration will be treated.
In substitution of the above-described pasteurization processes, alternative systems are available for citrus processing industry. Some of them use heat to inactivate microbiologic population and enzymes, but with different technologies, others operate with methods which do not foresee significant thermal variations.
To reach in a very short time the pasteurization temperature and avoid the overheating risks present in traditional technique, several heating technologies are available, such as direct injection of steam and microwave use.
In the first case, food-grade steam is introduced in the juice to be pasteurized. When ready-to-serve juice must be confectioned from concentrate, the condensed steam can remain in the juice as part of the water necessary for dilution. The cooling is obtained injecting sterile-filtred-deaerated water (Muller etal., 1988). If the purpose of the thermal treatment is the production of NFC juice, a quantity of water corresponding to the introduced steam must be removed subjecting the juice to flash evaporation. Thus the removal of air but also that of part of the aroma is obtained (Knorr, 1994). But if the introduced steam comes from meteoric water, the characteristic isotopic rates of the so treated juice can undergo some variations because a corresponding quantity of 'total' water is removed from the juice, not only the meteoric water introduced under steam form. The isotopic rates would not undergo notable variations if the steam needed for pasteurization is obtained from condensate of juice separated from the same kind of juice during thermal concentration and opportunely purified. In the use of condensate the necessary cautions must be taken to avoid pollutions. The steam evaporated from the juice is not constituted by simple distilled water; juice sprays are present and are transported in the steam phase; the organic substances, even if diluted, permit the growth of a significant microbial population. Moreover, if thermal energy present in the condensate is lowered in the refrigerating tower, the pollution possibilities increase. Contaminations from thermo-resistant spores of acidophilic microorganisms of Alicyclo-bacillus genus can occur.
The pasteurization technique by means of microwave energy has been studied above all at the Florida Department of Citrus, Scientific Research Department, Lake Alfred, Florida, USA, in the production of orange juice. Using as thermal source microwave energy at 2,450MHz, the juice reaches more rapidly than the traditional conduction/ convection method the temperature of 96 °C. Maintaining these conditions for 15—25 seconds, the 99-9 per cent of pectin-methyl-esterase is inactivated and the total microbial charge is reduced to values lower than 20 Colony Forming Units per millilitre (Nikdel etal., 1993). Operating at 70 °C for a time comprised between 12 and 26 seconds, the pectin-esterase inactivation is less marked while micro-organisms are practically destroyed also in these conditions. Since using microwave energy as heat source, the heat forms inside the heated body itself, the risks of off-flavours due to localized overheating are avoided.
From the point of view of thermal balance, it was noticed that at least 90 per cent of given heat is absorbed by the juice, with limited heat losses in the environment (Fox, 1994).
The electromagnetic radiations in the field of radio frequency can be used in a different way: when electromagnetic waves are passed through the juice, this behaves as a dielectric component of a capacitor (with microwave energy the heating occurs through irradiation). The electric field established between the two flat electrodes generates a disordered movement among the bipolar molecules subject to the action of inverted polarity million times a second. A rapid and even increase of temperature then occurs (Fox, 1994; Di Giacomo, 1995).
For pasteurization of alimentary products with particulates up to 24 mm 'Ohmic heating' can be used, with alternating current in the low frequency (50-60Hz). During the process, both solid parts and the liquid phase are heated in the same way (Ladwig, 1988).
It is not thus necessary to overheat the liquid phase to ensure the reach of the desired temperature inside even large particulates, as it is required with traditional heat transfer equipment.
The inactivation of the enzymes present in citrus fruit juice and the destruction of the microbiologic population are, as it has been already said, necessary operations to increase the shelf life of the finished product. The use of heat implies always a more or less marked alteration of the characteristics of the treated juice. Citrus processing industry tends to minimize these alterations reducing processing times and temperatures and using new heat transmission systems. In any case, to reach the purpose it is fundamental that used citrus fruits are as much as possible intact and the plants are built and managed in a suitable way for an accurate sanitation.
The destruction of microbial charge and the enzymatic inactivation (or, at least their reduction) is obtainable with technologies, which do not foresee heat use but only alternative systems. Above all when the purpose is not to reach a long shelf life but to ensure to the produced juice a commercial life sufficient to the normal supplying to the consumer, under predetermined cooling conditions.
High hydrostatic pressure (pascalization)
It is the non-thermally pasteurization system more used in citrus processing in alternative to the traditional one. It is based on the principle that irreversible modifications of microorganisms morphology can be caused by the application of high pressures. Generally, the microbial vegetative cells are destroyed, while the spores, if present, appear to be stronger.
The Bacillus stearothermophilus spores are destroyed when they are subjected to six cycles of oscillatory pressurization at 70 °C and 600 MPa (5 min/cycle). The destruction is not complete with a continuous pressurization system for the time of 60 minutes at the same conditions (Hayachawa etal., 1994).
The destruction speed depends on several factors such as product concentration, sugars concentration, pH and temperature, as well as applied pressure and processing time. At equal conditions, enzymes and micro-organisms in the concentrate orange juice show a higher resistance than single strength juice (Knorr, 1994).
The application of high hydrostatic pressure implies irreversible alteration to the protein structure of enzymes too. These modifications also depend on several factors such as pH, quantity of dissolved substances, temperature, and kind of enzyme. The inactivation of pectin-methyl-esterase in orange and grapefruit juices is obtainable with pressures of 500-900 MPa.
Pressures higher than 600 MPa inactivate thermolabile but not thermostabile pectin-esterase (Goodner etal., 1998). If the pH juice is lowered through C02, the inactivation of thermolabile pectin-esterase can be obtained with pressures lower than 100 MPa (Balaban etal., 1995).
Aromatic substances do not undergo substantial modifications, as well as vitamins, colourings and aminoacids.
High-pressure technology has used at industrial level from the 90s. In Japan, Wakayama Prefectural Agricultural Processing has developed a system used for the production of tangerine juice. The treatment occurs at 4,000—6,000 bar and permits the production of single strength juice without original taste modifications. Also Wakayama Nokyo Food Industry has perfected a process for the production of pasteurized citrus fruit juices by means of high pressure, while Pokka Corp. obtains not bitter grapefruit juice using a technology realized in cooperation with Mitsubishi Heavy Industry (Di Giacomo, 1995).
The industrial plants using high hydrostatic pressure to increase the shelf life of citrus fruit juices have a different structure according to the state in which the product to be worked is. The Alstom ACB, Nantes, France, provides two different equipments, one suitable to the treatment of the juice already confectioned in the final container and another for bulk processing.
In the first case the high-pressure chamber is filled with containers. These must be built with flexible materials for the transmission of pressure variations and have suitable form for the best room exploitation. Moreover, the headspace inside the container must be reduced to minimize oxygen and optimise effects. When the high-pressure chamber has been charged and closed, empty spaces are filled with water or with water containing small oil quantities, carefully removing all the air. Instead of water, another pressure transmitting medium can be used. High pressure is generated through direct compression and uniformly transmitted until predetermined values.
For bulk processing a semi-continuous system can be used with three dephased pressure cells, one for the filling, one for the treatment and one for discharge. Because the treatment is conduced on the not confectioned juice, in the upper of each cell a floating piston separates the juice from the pressure medium (Bignon, 1996).
Freshly squeezed orange juice, treated at 3,500 bar for one minute at the temperature of 30 °C acquires, if maintained in conditions of opportune refrigeration, a shelf life of 60 days keeping the organoleptic characteristics of the fresh product (Donsi etal., 1996).
Treatments of pressurization/depressurization between 5 0 and 400 MPa for 15 minutes with mild thermal processes between 20 and 60 °C on freshly squeezed orange juice can bring significant reductions of the activity of peroxidase and of pectin-methyl-esterase. At a temperature of 35 °C these activities reduce of 50 per cent (Cano etal., 1997).
The fresh orange juice subjected to high pressure treatments maintains similar characteristics to those of referring juice for 30 days at 4°C (Pelletier, 2000).
In order to reduce the micro-biologic charge in fruit juices, the Coca Cola Company has patented in 1993 a process using ultra high pressure, about 15,000psi, obtained with a homogenizator APV Model 30CD cell disrupter. The high-pressure difference between the inlet in the homogenizator and the outlet implies also pulp smashing.
Treated with this technique, the freshly squeezed orange juice has a shelf life of 40 days at 4 °C, while the corresponding not treated juice shows a shelf life not longer than 10 days. Flavour and other chemical, physical and organoleptic characteristics do not undergo any variations (Fox, 1994).
When microorganisms are subjected to strong electric fields, they undergo such alterations to cause their death. Because the cellular membrane is not conductive, when an electric field is applied to a cell, a trans-membrane potential is generated. If the critical point is overcome, the phenomenon becomes irreversible with the formation of spores on the membrane because of the repulsion between the molecules and thus causing the cell death. With the application of stronger forces, also the spores can be inactivated.
This technology, applied to freshly squeezed orange juice, according to proofs conducted by FMC and Krupp, brings a strong reduction of the total microbial charge but it does not inactivate pectin-esterase. The organoleptic characteristics, as well as vitamins, colour and flavour do not undergo any alterations (Knorr, 1994).
The Pure Pulse Technologies Inc., San Diego, California, USA, in order to not thermally pasteurize juices, proposes a system based on the application of multiple, short-duration, high-intensity electric field pulses. The product to be treated passes between the two electrochemically inert electrodes and is subjected to electrical forces for a time of 1—10 microseconds with the destruction of microorganisms and without damages for the treated juice (Lander, 1996).
The bitter taste sometimes present in citrus fruit juices is fundamentally due to the excessive presence of two substances: naringin and limonin. As it is well known, limonin is present in orange juice, limonin and naringin in grapefruit juice, but also in other citrus fruit juices.
When the concentration of the former overcomes 500mg/L and that of the latter 5 mg/L, the bitter taste becomes perceptible and, at a higher concentration, the juice acquires a clearly bitter taste (Maier etal., 1977).
The high content of bitter substances can be linked to several factors: the variety (for example, Navel oranges also in the USA and Biondo Comune oranges in Italy), climatic factors such as for Temporona oranges (Hofsommer etal., 1991). Also the rootstock, independently from variety, can concur to determine a high content of limonin in oranges (Di Giacomo etal., 1977).
Because, as it is known, limonin and naringin, but also other flavonoids, are contained in the maximum part in the peel and in membrane materials, their concentration in the juice depends also on the processing system. A hard squeezing in juice extractors and in finishers increases the juice yield but also increases the quantity of bitter compounds passing from solid parts into liquid.
Many technologies have been proposed to reduce bitterness in citric products and this abundance of attempts shows the importance that this problem has for processors and for the market.
At present, two absorbing substances are generally employed for this purpose by citric industry: activated carbon and non-ionic resins.
The use of activated carbon in citrus processing is usually linked to the production of clear juices. In these cases, the treatment occurs in bulk; after the enzymes action, both naturally present in the juice and added, necessary for pectin demolition, the juice is added with flocculation agents and, if necessary, with activated carbon in powder form. For carbon removal the juice is filtrated on kieselguhr.
When cloudy citric products must be debittered with activated carbon, this must be used in granular form. Carbons with absorbing inner surface bigger than 1,000 square meter per gram and with granules having an average diameter higher than 1 mm must be chosen. The pore volume must be higher than 0.8 ml per gram.
The debittering operation is conduced at 'fixed bed', passing the juice on a carbon layer high at least a meter and placed in stainless steel vessels. Every treatment unit includes two dephased vessels, working alternatively, one in debittering phase and one in washing and regenerating phase, so as to realize a semi-continuous process. The employed technique is basically equal to that used for absorbing resins. The pulp must not be brought through centrifugation at values higher than 1 per cent, but it is not necessary to pay much attention to the content of essential oil present in the liquid to be treated because the absorbent capacity of the carbon is not damaged by it.
Since carbon has a specific gravity, which is double of the absorbing resins used in citrus industry (2.0-2.2 instead of slightly more than 1), the removal of the pulp stuck on the carbon granules is not a difficult operation. It is just necessary to have enough water pressure in the backwashing; carbon losses for transportation together with the pulp are limited. In this stage it is important to care that every carbon particle, eventually formed for granules breakdown, is removed.
To have a correct debittering of citric products and to exploit completely the capacities of absorbing substances in a vessel system with fixed bed, it is important that the product to be treated is uniformly distributed on the entire mass surface.
The advancing front of the liquid inside the mass must be as much as possible parallel to the absorbing surface. If privileged advancing directions form, a part of the absorbing contained in the vessel will not carry out its function.
During the backwashing the absorbing must be well mixed. If pockets not reached by water (and subsequently by alkali) are formed, areas of microbiologic contamination will occur.
For regeneration, solutions of alkaline hydroxides are used; potassium hydroxide is preferred to sodium hydroxide even if it is more expensive. Generally, after one or two campaigns it is opportune to proceed to high-temperature reactivation, which permits to restore the initial absorbing potentiality.
The carbon action is not only due to the absorbing activity of Van der Waals forces. The complexes carbon-oxygen, which make the surface of the carbon slightly polar, have at the same time an oxidative action polyphenols and anthocyanins. In fact, carbon has also a decolouring action.
The debittering with activated carbon is used both in juice treatment and in core-wash and citrus extracts treatments. These products, which are employed in the processing industry of drinks and squashes, are characterized by high viscosity and a higher cloud than the juice. Cloud stability is the most appreciated factor in these productions.
The alternative to carbon to debitter and upgrade citrus products is constituted by resins. Styrene-divynil-benzene hydrophilic absorbents are used. They are macro-porous resins, FDA approved for the treatment of alimentary products, with surface area greater than 700 square meters per gram (on dried resin). In the United States the use of absorbing resins for the debittering of Californian Navel oranges and Florida grapefruit juices is legal (Lenggenhager etal., 1997), for the type for manufacturing without obligation of declaration on label. Since 1997 also in Italy the debittering with absorbing resins is permitted as physical treatment of fruit juices.
Many studies have been carried out on the activity of this kind of resins as limonoids and flavonoids absorbents from citrus products. All the researches confirm that the so treated juice loses the excessive bitter and maintains its characteristics improving the total quality. Chemical and physical data substantially remain the same, except the content of limonin, hesperidin, naringin and of other flavonoids. Not only orange and grapefruit juices have been tested in relation to debittering on absorbent resin, but also bergamot and sour orange juices. In this case the taste can be improved reducing acidity by means of the contemporary use of absorbing resins and ions-exchanging resins. Also the debittering of pulp-wash and by-products is possible, even if with some more difficulties (Grohmann etal., 1999).
The absorbing mechanism is linked to Van der Waals forces between the resin surface and the phenolic compounds. This link is broken by alkaline solutions.
To realize industrially the debittering of citrus products by means of resins, it is necessary first to remove or at least strongly reduce the pulp. For this operation two different ways can be followed: centrifugation or ultra-filtration. The removal of the greatest part of the pulp through centrifugation reduces the quantity of solid particles retained inside the resin layer, but it does not eliminate the difficulties for their total removal in the backwashing phase. Since the not high relative density of the used resins, big losses of resin spheres can occur together with the pulp. On the contrary, it can happen that the smaller particles of pulp and cloud are not removed with loss of absorbing activity. To avoid these risks, in some plants the removal of the resin bed from the vessel is carried out at the end of each cycle and also the treatment in a scrubbing system (Lyndon, 1996).
The essential oil content in the juice must be very low because the resins absorb the oil, which is not removed by alkaline solutions. Moreover, the specific gravity of the resin is lowered, and it can float. Therefore, after pasteurization, the juice is de-oiled. For this purpose the first stage of the evaporator can be used.
After debittering, the juice is mixed with the pulp separated during centrifugation to restore the characteristics of the initial juice.
In the other alternative, the crossflow filtration, the clear permeate passes on the resin bed (Akin etal., 1991)- The absence of solid particles and terpenes, which constitute the main part of essential oils, makes the treatment easier, without the risks linked to the pulp presence.
The clear debittered juice is thus mixed with the retentate; the pulp retained by ultra-filtration membranes maintains all its starting limonin and naringin and thus its rate of bitter taste. If the bitterness of the final mix is still excessive, it can further be lowered subjecting the retentate to dialysis. Instead of meteoric deionised water it is better to use condensate obtained during thermal concentration of the juice opportunely purified and, if necessary, sterilised. The liquid at low Brix coming from dialysis is used as final phase of debittering.
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