284 Part II: Water, Enzymology, Biotechnology, and Protein Cross-linking
inactivation is less sensitive to temperature changes,
compared with PME inactivation (as can be seen by
comparing the slopes of the corresponding curves in
Fig. 12.6). Values for the activation energies (Ea, see
Eq. 12.3) equal to 134.5 15.7 kJ/mol for the case
of PG and 350.1 6.0 kJ/mol for the case of PME
thermal inactivation have been reported (Crelier et
al. 2001). It must be noted that the literature values
presented here are restricted to the system used in
the particular study and are mainly reported here for
illustrative purposes. Thus, for example, the origin
and the environment (e.g., pH) of the enzyme can
influence the heat resistance (k or D—the decimal
reduction time—values) of the enzyme as well as
the temperature sensitivity (Eaor z—the temper-
ature difference required for 90% change in D—
values) of the enzyme thermal inactivation rates.
HIGH-PRESSUREINACTIVATION
High hydrostatic pressure processing of foods [i.e.,
processing at elevated pressures (up to 1000 MPa)
and low to moderate temperatures (usually less than
100°C)] has been introduced as an alternative non-
thermal technology that causes inactivation of
microorganisms and denaturation of several en-
zymes with minimal destructive effects on the quali-
ty and the organoleptic characteristics of the prod-
uct. The improved product quality attained during
high-pressure processing of foods, and the potential
for production of a variety of novel foods with par-
ticular, desirable characteristics, have made the
high-pressure technology attractive (Farr 1990,
Knorr 1993).
As far as high-pressure enzyme inactivation goes,
PG is easily inactivated at moderate pressures and
temperatures (Crelier et al. 2001, Shook et al. 2001,
Fachin et al. 2003), while PME inactivation at ele-
vated pressures reveals an antagonistic (protective)
effect between pressure and temperature (Crelier et
al. 2001, Shook et al. 2001, Fachin et al. 2002,
Stoforos et al. 2002). Depending on the processing
temperature, PME inactivation rate at ambient pres-
sure (0.1 MPa) is high, rapidly decreases as pressure
increases, practically vanishes at pressures of 100–
500 MPa, and thereafter starts increasing again, as
illustrated in Figure 12.7.
High-pressure inactivation kinetics for both PG
and PME follow first-order kinetics (Crelier et al.
2001; Stoforos et al. 2002; Fachin et al. 2002, 2003).
Values for the reaction rate constants, k, for high-
pressure inactivation of PME and PG as a function
of processing temperature, at selected conditions,
are given in Table 12.1 (Crelier et al. 2001).
Through the activation volume concept (Johnson
and Eyring 1970), the pressure effects on the reac-
tion rate constants can be expressed as(12.5)where kPrefis the reaction rate constant at a constant
reference pressure, Pref, and Vais the activation vol-
ume.
Models to describe the combined effect of pres-
sure and temperature on tomato PME or PG inacti-
vation have been presented in the literature (Crelier
et al. 2001, Stoforos et al. 2002, Fachin et al. 2003).
Based on literature data (Crelier et al. 2001), a
schematic representation of high-pressure inactiva-
tion of tomato PG and PME is presented in Figure
12.7. From data like these, one can see the possibili-
ties of selective inactivation of the one or the other
enzyme by appropriately optimizing the processing
conditions (pressure, temperature, and time).FUTURE PERSPECTIVES
Texture in ripe tomato fruit is largely dictated by cell-
wall disassembly during the ripening process. Cell-
wall polysaccharides are depolymerized, and their
composition is changed as ripening progresses. The
coordinated and synergistic activities of many pro-
teins are responsible for fruit softening, and althoughkkV
RPP
PP Taref
=−ref
⎡ −
⎣⎢⎤
⎦⎥exp()Figure 12.7.Schematic representation of the effect of
processing pressure on PG and PME inactivation rates
during high-pressure treatment (at 60°C).