Fundamental Food Microbiology, Third Edition - Fuad Fathir

522 FUNDAMENTAL FOOD MICROBIOLOGY
B
E
D
Figure 39.1 Schematic diagram of a hydrostatic pressure vessel (A). The packaged food (B)
is put in the liquid from the top inside the chamber (C) and the top is closed by
the movable cylinder (D) that has an opening valve (E) to removed air and excess
liquid before pressurization. Once the chamber is closed, excess liquid is pumped
in through the tube (F) that has a valve (G) that closes when the desired pressure
is reached. The pressure is maintained constant during pressurization time, at the
end of which the inlet valve (G) opens and the excess liquid is removed by gravity
to bring the pressure inside the chamber to atmospheric pressure. Then the top
cylinder is moved and the package is removed from the chamber. The liquid in
the chamber can be heated to the desired temperature by installing a heating coil
around the vessel (A). The temperature of the liquid inside the chamber can be
monitored by placing a thermocouple in the top movable cylinder (D). The process
is clean, energy efficient, and requires relatively less space to operate compared
with other food-processing methods.
Hydrostatic pressure processing of food requires a very short time, less energy,
and less space, and is safer than thermal processing. It does not act on covalent
bonds of molecules; thus, vitamins, color pigments, and other small molecules
are not adversely affected. Although it is basically a batch process, because of
the short time requirements, it can be made continuous by installing three or
more units in which while one is being filled, the second one is pressurized or
depressurized and the third one is emptied. With the availability of units that can
handle larger volumes, this becomes a commercial reality. The process has three
parameters: pressure, temperature of pressurization, and time of pressurization.
By adding suitable antimicrobials during pressurization, it can be made a fourdimensional
unique process.
G
F
C
A