The Effect of Oxidative Enzymes in Foods - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 301

isoperoxidases with different susceptibilities to heat, this has not yet been clearly
established as the main cause of the deviation from linearity.
Regeneration of enzymic activity after heat denaturation is unusual for enzymes
generally, although it is a well-recognized property of peroxidases. The ability of
peroxidases to regenerate after heat denaturation varies not only between different
plant species but also between the isoenzymes that occur within a single variety.
Restoration of peroxidase activity is generally observed over a period of a few hours
after heat treatment of either test solutions of enzyme or whole vegetables. For a
purified apple isoperoxidase, A2, up to 80% of the original activity was restored
when the heat-treated isoenzyme was held at 30°C. 100 Cationic isoperoxidases in
crude extracts from Brassica spp. and apples 89,96,100 have been shown to be incapable
of regeneration, which implies that the molecular structure of the protein may
primarily determine this property. Naveh et al. 109 elucidated an interesting linear
relationship between residual enzymic activity immediately after heat treatment and
subsequent regenerated peroxidase activity. A large slope of approximately 3.0 for
the graph implied that more drastic heat processing would not only increase inacti-
vation but also markedly decrease the extent of regeneration. Also, the proportion-
ality between increasing thermostability and the enzyme's ability to regenerate
supports the general observation that the more thermostable enzymes tend to exhibit
a greater ability to regenerate. At the molecular level, the mechanism of regeneration
of peroxidases is not fully understood. However, in general terms it is considered
that regeneration must require a reversal of denaturation with concomitant changes
in the conformational structure of the protein moiety. As it has been shown that
added hematin increases the amount of restored enzymic activity, 107 it has been
generally accepted that the hem moiety can recombine with the apoprotein to form
active enzyme. Tamura and Morita 110 observed that, after heat treatment, cooling for
20 h at room temperature resulted in a recovery in the absorbance at 404 nm which
indicates that the native molecule with bound hem can be reassembled under such
conditions.
Commercial methods, other than direct conductive heating, such as microwave
and gamma-irradiation, can justifiably be expected to affect enzymic activity and
has been reviewed by Thomas. 111 Microwave heating causes rapid changes in tem-
perature, not dependent on conduction, and may therefore be particularly useful for
heterogeneous foods. Nevertheless, the potential for rapid heating and control that
microwave processing offers may not yet have been sufficiently exploited for the
blanching of plant foods. Irradiation of pure solutions of horseradish peroxidase
using 60 Co gamma rays caused a loss of enzymic activity which was highest in the
presence of oxygen-saturated water where it is suggested that hydroxyl and peroxy
radicals were responsible for the inactivation. 112 It is reasonable to suppose that such
inactivation was the result of a secondary attack on peroxidase by free radicals
produced from water during the irradiation process. However, the species involved
are likely to include free radicals which will have a similar effect to those radicals
generated by the oxidative enzymes for which the process is designed to destroy.
The use of peroxidase activity as an indicator of blanching treatment is contin-
uously being appraised in view of the higher temperatures required to inactivate