Thermal Food Processing

Pressure-Assisted Thermal Processing 537
depends on the decomposition of pectic substances by ß-elimination, 24,25 and the
ß-elimination is repressed by HHP.
17.4 EFFECT OF PRESSURE ON BROWNING
Browning due to the Maillard reaction between sugars and amino acids induces
favorable color and flavor for processed foods and dishes, whereas it also deteriorates
food quality in some cases. It is known that processing conditions like
temperature, pH, and constituents of the ingredients affect the Maillard reaction.
The reaction progresses in two stages: the condensation between carbonyl and
amino compounds followed by the browning reaction. Tamaoka et al. 28 showed
that the Maillard reaction is inhibited by high pressure, which controls the browning
reaction considerably more than the condensation reaction. The HHP is
therefore considered to be one of the important factors controlling the Maillard
reaction. However, the effect of HHP treatment on the Maillard reaction has not
yet been determined in the temperature range of around 100°C or higher used in
practical circumstances, such as in the food industry. The effect of pressure on
browning at high temperatures (100 to 115°C) is introduced in this section, using
a glucose–glycine solution as a model of browning, and the applicability of the
result is confirmed using a white sauce as one of the model foods.
17.4.1 BROWNING BY THERMAL TREATMENT COMBINED
WITH PRESSURE
Browning of a glucose–glycine solution is measured in the temperature range of 100
to 115°C and in a pressure range of 0.1 to 400 MPa, and the result is shown in
Figure 17.8. The browning value increased considerably with an increase in temperature
at the same pressure. For example, at the atmospheric pressure (0.1 MPa),
the value of 0.28 at 100°C for 30 min increased to 6.8 at 115°C for 30 min. When
pressure was added to the thermal treatment, the browning was inhibited at each
temperature depending on the pressure. This inhibition of browning by pressure
coincides with a previous study, 28 though the heating temperature in the study was
much lower than those used here. For 30 min at 100°C, the degree of browning at
400 MPa was about one tenth of that at 0.1 MPa, whereas in the same treatment
time at 115°C, the degree of browning at 400 MPa was one third of that at 0.1 MPa,
because the progress curves of browning at over 105°C did not obey a first-order
reaction equation, as shown below.
The relationship between the treatment time and logarithm of the browning
values is also shown in Figure 17.9. Only the curves at 100°C are straight (r =
0.997 ~ 0.982), as shown in Figure 17.9a, indicating that the browning at each
pressure obeys a first-order reaction equation at 100°C. The slope of the curves
decreased with an increase in pressure. However, in experiments conducted at temperatures
other than at 100°C, none of the browning curves were straight but became
concave upward; thus, the browning did not obey first-order kinetics. The browning
rates can therefore be calculated from the experimental results at 100°C by linear