Thermal Food Processing

Pressure-Assisted Thermal Processing 529
processing. Therefore, in this chapter, thermal processing assisted by pressure is
described, based on our experiments using a compact and simple pressure vessel 1
capable of easily controlling the sample temperature and recording the temperature.
Using the pressure vessel, the thermal treatment combined with pressure was
studied in order to seek the potentiality of pressure as a new technique of
thermal processing. The effects of pressure on the softening of vegetables, the
browning of glucose–glycine solution, the inactivation behavior of microorganisms
due to HHP, the effect on bacterial spores by thermal treatment combined
with pressure, and some practical studies of pressure combined with mild
heating are also discussed in this chapter.
17.2 PRESSURIZING TREATMENT
17.2.1 COMPACT PRESSURE VESSEL
The thermal treatments combined with pressure are carried out using the pressure
vessel shown in Figure 17.1. Hydrostatic pressure is applied to the pressure vessel
from the right side, and a sheath thermocouple (1.5 mm in diameter) is inserted
from the left side. A pressure gauge is placed between the pressure vessel and
the pressurizing pump.
In the case of liquid samples, two milliliters of sample solution is poured
into a sample container (4 × 100 mm silicone rubber tube sealed at one end)
without air bubbles, and the open end of the container is sealed up with a silicon
rubber plug. The container filled with sample solution is inserted into the pressure
vessel. In the case of solid samples, it is directly inserted into the vessel. After
each sample has been put in, the pressure is applied to the sample, and then the
pressure vessel is maintained in a water bath at 25°C for 5 min to ensure the
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FIGURE 17.1 Schematic diagram of experimental apparatus. 1, pressure vessel; 2, thermocouple;
3, silicon tube; 4, sample; 5, pressure gauge; 6, pressure pump.
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