A typical temperature-pressure history of food material subject to high pressure processing is shown in Figure 1. Knowledge of both pressure & thermal history of the product is important. Some of the important process parameters during high pressure processing of foods are summarized below.
Initial Product Temperature (i.e., precondition the product prior to pressure treatment)
Product at T1 is chilled or preheated to an initial temperature T2 (just before pressurization). For food pasteurization applications products are often chilled. For sterilization of low-acid foods, products are often preheated to around 50-80 ºC.
Process temperature
The temperature which a product under processing attains after the target pressure is reached is termed as process temperature. Process temperature is governed by initial temperature of product, pressure transmitting fluid and their respective heat of compression values and the initial temperature of the pressure vessel. It is recommended to equilibrate the product, pressure transmitting fluid and pressure vessel within ±0.5% of the target initial temperature (Balasubramaniam et al., 2004). Good thermal lagging needs to be provided to the pressure vessel in order to prevent the decrease in process temperature due to loss of heat to the surroundings.
Pressure Come-up-time
Time required for the pressure to increase from 0.1MPa (atmospheric pressure) to the process pressure. This time primarily depends on the horsepower of the high pressure pump used and the target process pressure. For many industrial or pilot scale high pressure equipment, pressure-come-up time ranged from 1-3 min.
Process pressure
The pressure at which the sample to be treated is held in the pressure vessel is termed as process pressure. Strain gauges that function on the principle of change in resistance under pressure strain (compression-decompression) and displacement transducers mounted on the external frame could be effectively used to measure pressure (Balasubramaniam et al., 2004). Mechanical Bourdon tube type gauges lack good reliability under heavy use at the upper end of high pressures associated with high pressure processing. Regular calibration of pressure sensors needs to be done. As of now, the US National Institute of Standards and Technology (NIST) does not have calibration service for high pressure levels associated with high pressure processing (Balasubramaniam et al., 2004).
Pressure holding time
Time interval for which the product needs to be held at the process pressure. It is the time between the end of compression and beginning of decompression. Often 3-10 min pressure holding time is used for economically viable commercial process.
Just after pressurization (P1), often the temperature of the product slightly increases from T3 to T3‘ possibly due to molecular rearrangement in response to pressure.
For isothermal conditions, there will not be any heat loss during pressure holding time. Otherwise, the temperature drops to T4 due to heat exchange with the surroundings.
Decompression time
The time required to bring the food sample from process pressure back to the atmospheric pressure (0.1 MPa). Generally, depressurization time is in the order of seconds.
As a consequence of depressurization, the product temperature drops to a final value Tf (close to the initial temperature T2 ) or slightly lower than T2 if heat is lost from the product to the surroundings.
Figure 1. A typical temperature-pressure history graph for high pressure processing.