Pressure-Temperature Effect
To understand and foresee the effect of HPP on foods it is
necessary to take in attention the net combined pressuretemperature
effect on the treated foods.
During the compression phase (t1-t2) of pressure treatment
food products undergo a decrease in volume as a function
of the pressure (Fig. 3). The product is held under pressure
for a certain time (t2-t3) before decompression (t3-t4). Upon
decompression, the product will usually expand back to its
initial volume (18). The compression and decompression can
result in a transient temperature change in the product during
treatment. The temperature of food (T1-T2) increases 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
phase. 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 of water increases about 3oC for
every 100 MPa of increased pressure at room temperature.
On the other hand, fats and oils have a heat of compression
value of 8-9oC/100 MPa, and proteins and carbohydrates have
intermediate heat of compression values (41, 44).