The main components of a high pressure system are:
• a pressure vessel and its closure
• a pressure generation system
• a temperature control device
• a materials handling system (Mertens, 1995).
Most pressure vessels are made from a high tensile steel alloy 'monoblocs' (forged from a single piece of material), which can withstand pressures of 400-600 MPa. For higher pressures, prestressed multilayer or wire-wound vessels are used (Mertens, 1995). Vessels are sealed by a threaded steel closure, a closure having an interrupted thread (which can be removed more quickly), or by a sealed frame that is positioned over the vessel. In operation, after all air has been removed, a pressure transmitting medium (either water or oil) is pumped from a reservoir into the pressure vessel using a pressure intensifier until the desired pressure is reached. This is termed 'indirect compression' and requires static pressure seals. Another method, termed 'direct compression' uses a piston to compress the vessel, but this requires dynamic pressure seals between the piston and internal vessel surface, which are subject to wear and are not used in commercial applications.
Temperature control in commercial operations can be achieved by pumping a heating/cooling medium through a jacket that surrounds the pressure vessel. This is satisfactory in most applications as a constant temperature is required, but if it is necessary to change the temperature regularly, the large thermal inertia of the vessel and relatively small heat transfer area make this type of temperature control very slow to respond to changes. In such situations, an internal heat exchanger is fitted.
There are two methods of processing foods in high pressure vessels: in-
container processing and bulk processing. Because foods are reduced in volume at the very high pressures used in processing (for example, water is reduced in volume by about 15% at 600MPa), there is considerable stress and distortion of the package and the seal when in-container processing is used. Plastic and foil pouches are the best candidates for HP processing, and research is continuing on the optimum design of the package, seal integrity and other suitable packaging materials. Materials handling for in-container processing is achieved using automatic equipment, similar to that used to load/unload batch retorts. Bulk handling is simpler, requiring only pumps, pipes and valves.
HP equipment has long been in use in commercial production of quartz crystals and ceramics. This equipment is also suitable for food processing with some modification. Among the many equipment manufacturers, the following may be mentioned: Mitsubishi Heavy Industries and Kobe Steel Ltd in Japan, Flow International Corporation, GEC Alstom-ACB Pressure Systems, Stansted Fluid Power and Engineered Pressure Systems International, in Europe and the USA.
Pressure chambers for food processing are available of up to 5001 in volume and for pressures up to 800MPa. For cost reasons, there is a practical limitation at 600 MPa, which will be sufficient for most applications. For technical reasons, all available units are batch systems, even if development work is being undertaken to develop truly continuous systems. By combining a number of units in a staggered fashion, semi-continuous production can be achieved. The pressurising medium is usually water and foods are packed in flexible packaging with little or no headspace in order to be able to withstand and evenly distribute the pressure. Most systems are vertical, some with an external high pressure intensifier to minimise the number of sensitive high-pressure components in the hydraulic system. The ACB company has developed a semi-continuous horizontal pressure vessel with a double set of pistons for loading and unloading in a straight line. Commercial lines are designed to be automated to streamline production and minimise time for loading, pressurisation, holding, depressurising, unloading and drying.
Semi-continuous processing of fruit juices at 4000-6000lh-1, using pressures of 400-500MPa for 1-5min at ambient temperature, is used by one company in Japan, whereas another uses a similar process operating at 120-400MPa followed by a short heat treatment before the juice is packaged. The process is highly energy efficient although at present the capital costs of equipment remain high. It is possible that such liquid foods could also be used as the pressurising fluid by direct pumping with high pressure pumps. Such systems would reduce the capital cost of a pressure vessel and simplify materials handling. If liquids were also rapidly decompressed through a small orifice, the high velocity and turbulent flow would increase the shearing forces on microorganisms and thus increase their rate of destruction (Earnshaw, 1992). Developments in high pressure processing reported by Knorr (1995a) include combined freeze concentration, pressure freezing and high pressure blanching. Initial results suggest that pressure blanched fruits are dried more rapidly than those treated by conventional hot water blanching.
Examples of semi-continuous systems have been developed by, for example, the companies Alstom and Flow Pressure Systems. In the Flow Pressure semi-continuous system, the liquid to be processed is pumped into one or several so-called isolators (pressure vessels in which a separator partitions the food liquid from the ultra high pressure (UHP) water source). After pressure treatment, the liquid is pumped into a holding tank and aseptic filling station. In the Alstom system, the pressure chamber is filled with the liquid to be treated and compressed directly by a mobile piston (pushed by pressurised water) up to a maximum pressure of 500MPa. After a predetermined holding time, pressure is released and the liquid pumped by the piston to a holding vessel. Several pressure chambers can be served in parallel by the same main pressure generator so that a continuous downstream flow can be maintained. Since the pressure chamber is completely filled with product, the capacity per cycle is considerably increased compared to the processing of already packaged products in a conventional batch system and cycle time is reduced by about 30%.
Hoogland (2001) has described the development, within a Dutch consortium, of a more cost efficient new generation of HP processing equipment. By using composite materials instead of steel the cost of the pressure vessel is reduced. The use of internal pressure intensifiers, pressurised by external pumps, further reduces cost. With the new system, now at pilot plant stage, cycle times are being reduced to 2-5min. Another advantage of using composite materials for the pressure chamber is that the chamber wall, which dissipates some of the adiabatic heat generated when pressurising the food load, will not cool the product surface. Since pressure and product temperature have a synergistic inactivation effect, cooling at the chamber wall could compromise the inactivation process. Many efforts are being made to substitute batch processing with a truly continuous HP process. Unilever, for example, have patented a continuous system in which the material to be treated is passed down an open narrow tube in a steady flow. A pressure differential of 100MPa or more is maintained between the entrance and exit ends of this tube.
Overall estimates by several equipment manufacturers point towards investment costs for a commercial system in the range 0.5-2 million Euro and production costs at 400 MPa of 0.1-0.2Eurokg-1 of processed goods. A high pressure plant for fruit juice pasteurisation is about 20 times the cost of an equivalent heat exchanger system (Manvell, 1996). Actual costs will depend on chamber capacity, fill density, time-pressure-temperature combinations in processing and the degree of utilisation of the line. Investment cost will be about 75% of total production costs.
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