Food Biochemistry and Food Processing

12 Pectic Enzymes in Tomatoes 283

selective inactivation a combination of heating and
high hydrostatic pressure can be used during pro-
cessing or alternatively, the expression of a particu-
lar enzyme can be suppressed using genetic engi-
neering in fruit.

THERMALINACTIVATION

Thermal processing (i.e., exposure of the food to
elevated temperatures for relatively short times) has
been used for almost 200 years to produce shelf-
stable products by inactivating microbial cells,
spores, and enzymes in a precisely defined and con-
trolled procedure. Kinetic description of the destruc-
tive effects of heat on both desirable and undesirable
attributes is essential for proper thermal process
design.
At constant temperature and pressure conditions
PME thermal inactivation follows first-order kinet-
ics (Crelier et al. 2001, Fachin et al. 2002, Stoforos
et al. 2002):

(12.1)

where A is the enzyme activity at time t, k the reac-
tion rate constant, and P and T the pressure and tem-
perature process conditions.
Ignoring the pressure dependence, Equation 12.1
leads to

−= =

dA

dt

kA k f T, ( , P,.. .)

(12.2)

where Aois the initial enzyme activity and the reac-

tion rate constant kT, a function of temperature, is

adequately described by Arrhenius kinetics through

Equation 12.3.

(12.3)

where kTrefis the reaction rate constant at a constant

reference temperature Tref, Eais the activation ener-

gy, and R is the universal gas constant (8.314 J/

(mol·K)).

PG thermal inactivation follows first-order kinet-

ics (Crelier et al. 2001, Fachin et al. 2003), as sug-

gested by Equation 12.2 above, or a fractional con-

version model (Eq. 12.4) that suggests a residual

enzyme activity at the end of the treatment.

(12.4)

where Ais the residual enzyme activity after pro-

longed 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 ambi-

ent 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

AA A Ae=+ −∞∞()o −ktT

kk

E

TT RT T

a

ref

=−−ref

⎛

⎝⎜

⎞

⎠⎟

⎡

⎣

⎢

⎤

⎦

exp ⎥

11

AAe= o−ktT

Figure 12.6.Effect of processing temperature on PG and PME thermal inactivation rates at ambient pressure.
(Redrawn from Crelier and others 2001.)