Reduction of Cycle Time.

Capacity of the system is closely related to the cycle time of the process. For batch systems, the cycle time may be broken down into a number of steps:

1. Fill vessel with the product.

2. Close the vessel.

3. Pressurize the system.

4. Holding time.

5. Release pressure.

6. Open the vessel.

7. Empty the vessel.

The time needed for each of these steps is related to mechanical limitations, except step 4, which is related to microbiological and food quality considerations. In order to reduce holding time, an effective setting of temperature and pressure must be applied. In practice, a combination of high temperature and high pressure may reduce holding time significantly. A disadvantage is that higher working pressures result in higher demands to the high pressure system, thus increasing costs. A second disadvantage is that at higher temperatures product quality is sacrificed. Therefore, for each product a specific combination should be determined that balances quality, product safety, and economics. Recently researchers have learned that holding time can be reduced significantly by an effective combination of pressure, temperature, and two consecutive treatments.

Filling, closing, opening, and emptying of the vessel can be speeded up by-automation of the system. Units such as the ACB Pressure System-Alstom Hyperbar system feature a quick closing system that can be opened and closed automatically within 20 seconds. Because the locking system displaces the lid aside from the vessel, it is possible to load and unload the vessel in a horizontal straight line (see Figure 1). Automated loading and discharging will decrease the overall cycle time but will also add investment costs (Lebas, personal communication, June 1998, February 2000).

The time needed for pressurizing the vessel is closely related to the power of the high pressure generator (typically, 100 to 200 kW is needed for a 200- to 300-liter system operating at 400 to 600 MPa). the final pressure required, and the type of hydraulics (internal/external intensifier). In practice, the time needed for this step may vary from seconds for laboratory-scale systems, up to 3 to 5 minutes for units of commercial use (200 to 400 liters), again depending on pressure. Increasing pumping power may increase capacity, it also increases investment costs. For processes with a relatively short holding time, it may be useful to reduce time of pressurization by applying high-power pressure pumps or accumulators. Depressurization can be quickened by using specially designed pressure valves capable of handling high discharge flow rates without erosion or cav-itation damage. Typical times for this process are 20 to 30 seconds for commercial units (Traff. personal communication, June 1998, February 2000: Lebas, personal communication, June 1998, February 2000).

Obviously, capacity is related to the time that the equipment is operational. Because of the need to maintain seals, equipment is out of operation about 1 day per month. For instance, the Avomex unit is operated in three shifts a day and down on Sunday. During the down time, maintenance takes place. Effectively, the unit is in use 86% of the time (Waugh, 2000).

Also related to capacity are the packing efficiency and the effective volume of the vessel. Because of the shape of the packages and the way these are designed and stacked effective volume may vary from 50% to 80%: the space between the packages is waste. Cubical or hexagonal packages yield a higher effective volume than cylindrical bottles. For the latter, volume utilization is most effective when the bottle diameter approaches the diameter of the vessel core, or when the diameter of the bottle is one third of the diameter of the vessel core, thus covering the cross-section of the vessel with seven bottles (78% efficiency). To achieve a maximum efficiency, ACB Pressure System-Alstom standardized its pressure vessels to two internal diameters, 90 and 280 mm. In the 90-mm vessel, cylindrical 1-liter bottles with a similar diameter are processed; the 280-mm unit contains packages of seven of these bottles. In considering the effective volume, one should take into account enclosed air and the volume of the packaging material as well (Lebas. personal communication, June 1998, February 2000).

During the past 10 years high pressure food processing has matured from laboratory-scale technology into a promising, economically feasible preservation method. High pressure technology has been introduced in the food industry, and the first products manufactured using these new techniques have emerged on the market. The costs of high pressure processing are decreasing, mainly because of new development in hardware and methods of treatment. Equipment manufacturers will further optimize their designs to reduce equipment costs and increase efficiency. The application of composites enables operation at higher pressures and lower costs. The method of treatment as introduced by Meyer (2000) may facilitate sterilization of products (activation of bacterial spores), which appeared very difficult using the conventional high pressure treatment. Thus, high pressure processing has the potential to develop into a preservation technique that is applied on a large scale in the food industry, in particular for products where retention of flavors and nutrients is desired.