High-pressure processing (HPP) is a nonthermal food preservation technique that inactivates harmful pathogens and vegetative spoilage microorganisms by using pressure rather than heat to effect pasteurization (see Figure 126.96.36.199).
Figure 188.8.131.52 Process diagram of an HPP plant. Image credit: ThyssenKrupp.
HPP utilizes intense pressure (about 400-600 MPa, or 58,000-87,000 psi) at chilled or mild process temperatures (<45°C), allowing most foods to be preserved with minimal effects on taste, texture, appearance, or nutritional value (Balasubramaniam, Farkas, Turek, et al., 2015).
HPP is primarily practiced as a batch process where prepackaged food products are treated in a chamber surrounded by water or another pressure-transmitting fluid. Semi-continuous systems have been developed for pumpable foods, where the product is compressed without a container and subsequently packaged aseptically.
The primary components of an HPP system include a pressure vessel; closure(s) for sealing the vessel; a device for holding the closure(s) in place while the vessel is under pressure (e.g., yoke); high-pressure intensifier pump(s); a system for controlling and monitoring the pressure (P) and (optionally) temperature (T) (see Figure 184.108.40.206).
A typical HPP process uses food products packaged in a high-barrier, flexible pouch or a plastic container. The packages are loaded into the high-pressure chamber (Figure 220.127.116.11). The vessel is sealed and filled with a pressure-transmitting fluid (normally water) and pressurized by the use of a high-pressure pump, which injects additional quantities of fluid. The packages of food, surrounded by the pressure-transmitting fluid, are subjected to the same pressure as exists in the vessel itself. After holding the
Figure 18.104.22.168 Isostatic pressure chamber. Image Credit: Avure Technologies, Inc.
product for the desired time at the target pressure, the vessel is decompressed by releasing the pressure-transmitting fluid (Farkas and Hoover, 2000). For most applications, products are held for 3 to 5 minutes at 600 MPa.
After pressure treatment, the processed product is removed from the vessel and stored/distributed in a conventional manner. Liquid foods (such as a fruit puree) can be processed in a batch or semicontinuous mode. In the batch mode, the liquid product is pre-packaged and pressure-treated as described for packaged foods. Semicontinuous operation requires two or more pressure vessels, each equipped with a free-floating piston that allows each vessel to be divided into two chambers. One chamber is used for the liquid food; the other for the pressure-transmitting fluid.
The basic operation involves filling one chamber with the liquid food to be treated. The fill valve is closed and then pressure-transmitting fluid is pumped into the second chamber of the vessel on the opposite side of the floating piston. Pressurization of the fluid in this second chamber results in compression of the liquid food in the first. After an appropriate holding time, the pressure is released from the second chamber. The product discharge valve is opened to discharge the contents of the first chamber, and a low-pressure pump injects pressure-transmitting fluid into the second chamber, which pushes on the piston and expels the contents of the product chamber through the discharge valve. The treated liquid food is directed to a sterile tank from which sterile containers can be filled aseptically.
Typically, three pressure vessels are used to create a semicontinuous system capable of delivering a continuous product output. This is accomplished by operating the three vessels such that one is loading, one is compressing, and one is discharging at any point in time (Farkas and Hoover, 2000). High-pressure processing can be used for sterilization of food products if applied at elevated temperatures and using the temperature increase due to adiabatic compression. By choosing the appropriate process conditions, it is possible to completely inactivate both vegetative cells and microbial spores, resulting in food products that are shelf stable. The quality of high-pressure sterilized products is usually superior to conventionally heat sterilized products. This applies particularly to texture, flavor and retention of nutrients. The effect of high-pressure sterilization on color is product dependent. This varies between a full retention of the fresh color and the same color change as obtained by conventional techniques.
At ambient temperatures, application of pressures in the range of 400-600 MPa inactivate vegetative microorganisms and reduce the activity of enzymes resulting in a pasteurized product, which can be stored for a considerable time at 4°-6°C (Cheftel, 1995). High-pressure inactivation of vegetative microorganisms is caused by membrane damage, protein denaturation, and decrease of intracellular pH, suggesting that pressure results in deactivation of membrane-bound enzymes associated with efflux of protons (Smelt, 1998). Inactivation of vegetative microorganisms and enzymes, combined with retention of small molecules responsible for taste and color and many vitamins, results in high-pressure pasteurized products with a prolonged shelf life and fresh characteristics (Balasubramaniam, Farkas, Turek, et al., 2015).