508 Produce Degradation: Reaction Pathways and their Prevention
As a defense against the plant oxidative burst the microbial invader uses a
combination of catalase and superoxide dismutase to inactivate H 2 O 2 and O 2 – , respec-
tively. Glutathione-associated enzymes and small proteins, such as thioredoxin and
glutaredoxin, work in concert within the cell as antioxidants, maintaining a reducing
environment. Therefore, in terms of produce degradation the role of SOD, catalase
and glutathione is indirect insofar as it enables the plant pathogens to go forward
and cause disease in the host plant. However, SOD, catalase, and glutathione are
common features of all aerobes, so there remains uncertainty about the significance
of their activity during plant interactions (Lebeda et al., 2001). This is especially
true considering that many fungal pathogens produce greater concentrations of
hydrogen peroxide during invasion compared to that released by the plant’s hyper-
sensitive response.
The antimicrobial properties of hydrogen peroxide could potentially be har-
nessed to extend the shelf life of vegetables and fruits through suppression of
microbial activity. One approach is based on using lactic acid bacteria to produce
hydrogen peroxide in situ and hence retard the growth of spoilage bacteria such as
Pseudomonas spp. Although lactic acid bacteria are facultative anaerobes they are
devoid of catalase activity, even though hydrogen peroxide is typically produced
during aerobic growth. Instead, a flavoprotein, NADH peroxidase, functions to
remove hydrogen peroxide, but this tends to be slower than its production and the
net result is the accumulation of H 2 O 2. This is one reason why the growth of lactic
acid bacteria is often greater under anaerobic conditions, where hydrogen peroxide
production is negligible, compared to when they are cultivated aerobically. The
potential of excess hydrogen peroxide production as a preservation method has been
evaluated using Lactobacillus delbrueckii subsp. lactis (Harp and Gilliland, 2003).
The model systems studied were broccoli, cabbage, carrots, and lettuce inoculated
with Escherichia coli and Listeria monocytogenes. The workers found that popula-
tions of L. delbrueckii subsp. lactis remained at high levels during storage but there
was no noticeable antagonistic action against the pathogens. The reason for the
limited efficacy of the hydrogen peroxide-producing strain was probably caused by
the action of endogenous catalase activity from the vegetables.
17.2.2 LACCASESE, POLYPHENOL OXIDASES, AND LIPOXYGENASE
Laccasese catalyzes the reduction of O 2 to H 2 O using a range of phenolic compounds
as hydrogen donors. The enzyme shares a number of suitable substrates with
monophenol monooxygenase, catachol oxidase, and polyphenol oxidase. The com-
mon substrates shared by all these enzymes can make direct identification of activity
problematic. It is well established that laccasese and polyphenol oxidases are widely
distributed in plants, playing a significant part in enzymic browning and flavor. The
same enzymes are also widely distributed in fungi, including the human pathogen
Cryptococcus neoformans. It is thought the oxidation of aromatic substances within
the body by C. neoformans plays a central role in protecting the organisms against
the immune system (Liu et al., 1999). How the enzymes contribute to vegetable and
fruit degradation remains obscure. It is known that fungal polyphenol oxidases have
an antioxidant effect in juice systems by removing residual oxygen present, thus