The Effect of Oxidative Enzymes in Foods - Food Shelf Life Stability: Chemical, Biochemical, and Microbiological Changes - page 302

peroxidases, although it is frequently suggested that such temperatures are not
needed for the blanching of vegetables. Nevertheless, Halpin et al. 113 have recently
restated that other less stable enzymes are not more appropriate as blanching indi-
cators. Therefore, it still seems likely that peroxidase activity will continue to serve
this requirement. In this context it is important to realize that peroxidase catalyzes
oxidative reactions through the involvement of free radicals, which by their very
nature can initiate and propagate other chain reactions and thus allow a wide range
of substrates to be oxidized. For this reason peroxidases should be inactivated in
processed foods, even though some of their degradation products containing hem
may still initiate chemical reactions, but obviously at much slower rates.

O XIDATIC A CTIVITY

Peroxidases can also catalyze an oxidatic reaction without the need of hydrogen
peroxide. This type of reaction, accompanied by the uptake of oxygen, is most
frequently demonstrated by the oxidation of the synthetic substrate dihydroxyfu-
maric acid (DHF) by compounds I and II, 114 although other naturally occurring
oxidatic reactions may involve ascorbic acid, thiols, and hydroquinones. All of these
are characterized by the lack of need for added hydrogen peroxide as the oxidizing
substrate. Based on the powerful inhibition of the reaction by superoxide dismutase,
as opposed to hydroxyl radical scavengers, 114 an oxidative scheme involving the
superoxide radical and the continual oxidation of DHF has been widely accepted:

Per-Fe III + H 2 O 2 Per-Fe V

native peroxidase compound I

An initial amount of H 2 O 2 , if not naturally present, which is required for the formation
of compound I, is believed to be generated by the non-catalytic autoxidation of DHF

2DHF · + O 2 2-

2DHF + O 2

Peroxidase then as compounds I or II catalyzes more rapidly the production of
greater amounts of DHF ·

DHF · + PerFe IV
compound II

Per-Fe V + DHF

DHF · + PerFe III
native peroxidase

PerFe IV + DHF

Further reactions result in the formation of diketosuccinate (DKS) by dismutation
of DHF ·

2DHF · DHF + DKS

As shown in the above, other reactive oxygen species including H 2 O 2 and superoxide,
O 2 · and hence also compound III (Per-Fe II O · ) can be formed by autoxidation of DHF.