Thermal Food Processing

Ohmic Heating for Food Processing 449
selected enzymes, the media used for the enzyme activity assays, and the methods
for those assays. For both ohmic and conventional processes, and for similar samples’
thermal histories, the corresponding D values were determined, leading to the calculation
of the z value, the activation energy (E a), and the preexponential factor (k 0)
of the Arrhenius equation for each enzyme (Equations 14.36 to 14.38):
logCA − logCAo
=
t D
1
log D2 − log D1
1
=
T − T Z
2 1
kt () = k exp
0
⎛ E ⎞
a
−
⎝
⎜ RT ⎠
⎟
(14.36)
(14.37)
(14.38)
Lipoxygenases (LOX) are nonheme but iron-containing dioxygenases that
catalyze the dioxygenation of polyunsaturated fatty acids containing a cis,cis-
1,4-pentadiene-conjugated double-bond system, such as linoleate and linolenate,
to yield hydroperoxides. Garotte et al. 59 evaluated the sensory characteristics of
blanched vegetable purees to which isolated enzymes had been added and found
that LOX was the most active enzyme in aroma deterioration in English green
peas and green beans. LOX is distributed widely in vegetables, and evidence is
accumulating to support its involvement in off-flavor development and color
loss. 59 To optimize the blanching process of vegetables, it is essential to establish
the kinetic model for the inactivation of this indicator enzyme. LOX is also
important in the baking industry because it interacts with a gluten side chain in
dough, making the gluten more hydrophobic and subsequently stronger. With
stronger gluten, the dough has better gas retention properties and increased
tolerance to mixing.
Table 14.4 presents the kinetic parameters obtained for the thermal degradation
of LOX. 60 This demonstrates that the electric field has an additional effect
on the LOX inactivation with much lower D values. This means that for the same
inactivation degree, the time required for thermal treatment is much lower, thus
reducing negative thermal effects in the other food components.
Pectinase (PEC) hydrolyzes the linkages that bind the small building blocks of
galacturonic acid together in pectic substances, producing smaller molecules. Its
main functions in foods are (1) to clarify juices and wines, (2) to reduce viscosity,
(3) to accelerate the rate of filtration, (4) to prevent pectin gel formation of fruit
juice from the fruit, and (5) to improve color extraction, e.g., from grape skin. 51
The kinetic parameters for PEC inactivation are presented in Table 14.5, and
it is clear that the electric field does not have any influence in the inactivation
kinetics. Both conventional and ohmic processing D and z values are identical.
The enzyme responsible for enzymatic browning, ortho-diphenol oxygen
oxidoreductase, is also known as catecholase, tyrosinase, phenolase, and