By careful selection of pressure, temperature, and treatment time and use of the adiabatic temperature rise, it is possible to sterilize with high pressure. Hayakawa, Kanno, Tomita, and Fujio (1994) have previously described how Bacillus stearothermophilus spores can be inactivated with an oscillatory pressure treatment at 600 MPa and 70°C (Matser et al., 2004).
Sterilization with high pressure is interesting due to the combined effect of pressure and temperature resulting in adiabatic heating, the uniform temperature distribution, and the relatively short treatment times.
Adiabatic heating is the uniform temperature rise within the product, which is solely caused by pressurization. The magnitude of the temperature rise is determined by the initial product temperature and the material properties of the product, using the following equation (Hoogland et al., 2001):
T temperature (K) p pressure (Pa)
a volumetric expansion coefficient (1/K) p density (kg/m3)
Cp specific heat (J/kgK)
Upon decompression, the product will usually expand back to its initial volume (Farkas and Hoover, 2000). The compression and decompression can result in a transient temperature change in the product during treatment. As shown in Figure 126.96.36.199, the temperature of foods increases (T1~T2) as a result of physical compression (P1~P2). Product temperature (T2~T3) at process pressure (P2~P3) is independent of compression rate as long as heat exchange between the product and the surroundings is negligible. In a perfectly insulated (adiabatic) system, the product will return to its initial temperature upon decompression (P3-P4). In practice, however, the product will return to a temperature (T4) slightly lower than its initial temperature (T1) as a result of heat losses during the compression (elevated temperature) phase(5).
The rapid heating and cooling resulting from HPP treatment offer a unique way to increase the temperature of the product only during the treatment, and to cool it rapidly thereafter. The temperature increase of food materials under pressure is dependent on
Figure 188.8.131.52 Time/Temperature-Pressure of HPP process151.
factors such as final pressure, product composition, and initial temperature. The temperature of water increases about 3°C for every 100 MPa pressure increase at room temperature (25°C). On the other hand, fats and oils have a heat of compression value of 8°-9°C/100 MPa, and proteins and carbohydrates have intermediate heat of compression values (Balasubramaniam, Farkas, Turek, et al., 2015).
Like any other processing method, HPP cannot be universally applied to all types of foods. HPP can be used to process both liquid and solid foods. Foods with a high acid content are particularly good candidates for HPP technology. At the moment, HPP is being used in the United States, Europe, and Japan on a selected variety of high-value foods either to extend shelf life or to improve food safety. Some products that are commercially produced using HPP are cooked ready-to-eat meats, avocado products (gua- camole), tomato salsa, applesauce, orange juice, and oysters.
HPP cannot yet be used to make shelf-stable versions of low-acid products such as vegetables, milk, or soups because of the inability of this process to destroy spores without added heat. However, it can be used to extend the refrigerated shelf life of these products and to eliminate the risk of various foodborne pathogens such as Escherichia coli, Salmonella, and Listeria. Another limitation is that the food must contain water and not have internal air pockets. Food materials containing entrapped air such as strawberries or marshmallows would be crushed under high-pressure treatment, and dry solids do not have sufficient moisture to make HPP effective for microbial destruction.