Thermal Food Processing

530 Thermal Food Processing: New Technologies and Quality Issues
initial temperature. Then, thermal treatments combined with pressure are started;
the pressure vessel is heated in a water bath (25 to 90°C) or an oil bath (90 to
110°C) for a given time. After the treatment is finished, the vessel is cooled
down in a water bath, then the pressure is released, and the sample is taken out
of the vessel. For example, in the initial period of pressurization at 400 MPa,
the sample temperature rises to about 8°C from the initial temperature because
of adiabatic compression. In addition, in an initial period of heating, the pressure
increases from 20 MPa ∼ 30 MPa due to the thermal expansion of the water in
the vessel. To avoid the additional increase in pressure, the pressure is adjusted
by controlling the valve. It takes about 15 to 30 sec to reach 100 to 400 MPa
and about 15 sec to return to atmospheric pressure. The temperature and pressure
history curves of the liquid sample during the pressurizing treatments are shown
in Figure 17.2. It takes about 5 min to reach the given temperature. Thus, the
treatment time contains the time needed to reach the given temperature, and
does not contain the time needed for the temperature to cool down. Furthermore,
the time needed to reach and release the given pressure is not contained in the
treatment time.
17.2.2 PRESSURE TREATMENT MACHINE
For treating a large volume of samples, a pressure treatment machine, capable of
simultaneously pressurizing up to 400 MPa and heating up to 120°C is used
(Figure 17.3). The pressure chamber is 100 mm in diameter by 300 mm in height,
and the pressurized medium (water) in the chamber is heated by an inner heater
(200 W) and an outer heater (400 W). This apparatus is used for the testing of
white sauce in Section 17.4, and oysters shucked by pressure and autolysis under
pressure combined with mild heating in Section 17.7.
Temperature (°C)
100
75
50
25
0
0
5 10
Time (min)
FIGURE 17.2 An example of temperature and pressure history curves of a sample solution
during heating at 100°C for 10 min under 400 MPa.
400
0. 1
Pressure (MPa)