BLBS102-c12 BLBS102-Simpson March 21, 2012 13:11 Trim: 276mm X 219mm Printer Name: Yet to Come
242 Part 2: Biotechnology and EnzymologyT (°C)
105100959085 80 75 70 65 60 55-9.0-8.0-7.0-6.0-5.0-4.0-3.00.0026 0.0027 0.0028 0.0029 0.0030 0.0031
1/T (K-1)lnk (k in s-1)PG
PMEFigure 12.6.Effect of processing temperature on PG and PME
thermal inactivation rates at ambient pressure. (Based on data
presented by Crelier et al. 2001.)PG thermal inactivation follows first-order kinetics (Crelier
et al. 2001, Fachin et al. 2003) as suggested by Equation 2
above, or a fractional conversion model (Equation 4), which
suggests a residual enzyme activity at the end of the treatment.A=A∞+(Ao−A∞)ekT (4)whereA∞is the residual enzyme activity after prolonged
heating.
From the data presented by Crelier et al. (2001), the effect
of processing temperature on crude tomato juice PG and PME
thermal inactivation rates at ambient pressure is illustrated in
Figure 12.6. The higher resistance of PG, compared with PME,
to thermal inactivation is evident (Fig. 12.6). Furthermore, PG
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 Equation 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 (korD—the decimal reduction time—values) of the
enzyme as well as the temperature sensitivity (Eaorz—the
temperature difference required for 90% change inD—values)
of the enzyme thermal inactivation rates.High Pressure InactivationHigh 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 nonthermal technology that causes inactivation
of microorganisms and denaturation of several enzymes with
minimal destructive effects on the quality and the organolep-0.000.010.020.030.040.050 100 200 300 400 500 600 700 800
P (MPa)k (min-1)PG
PMEFigure 12.7.Schematic representation of the effect of processing
pressure on PG and PME inactivation rates during high-pressure
treatment (at 60◦C).tic characteristics of the product. The improved product quality
attained during high-pressure processing of foods, and the po-
tential for production of a variety of novel foods, in particular,
desirable characteristics, have made the high pressure technol-
ogy attractive (Farr 1990, Knorr 1993).
As far as high pressure enzyme inactivation goes, PG is
easily inactivated at moderate pressure and temperatures
(Crelier et al. 2001, Shook et al. 2001, Fachin et al. 2003), while
PME inactivation at elevated 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 pressure (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 on Figure 12.7.
High pressure inactivation kinetics for both PG and PME
follow first-order kinetics (Crelier et al. 2001, Fachin et al. 2002,
Stoforos 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 reaction rate constants can be
expressed as:kp=kPrefexp[
−Va
R(p−Pref)
T]
(5)wherekPrefis the reaction rate constant at a constant reference
pressure,Pref,andVais the activation volume.
Models to describe the combined effect of pressure and tem-
perature on tomato PME or PG inactivation have been presented
in the literature (Crelier et al. 2001, Stoforos et al. 2002, Fachin
et al. 2003). On the basis of the literature data (Crelier et al.
2001), a schematic representation of high pressure inactivation
of tomato PG and PME is presented in Figure 12.7. From data
like these, one can see the possibilities of selective inactivation