Enzymes in Quality and Processing of Tropical and Subtropical Fruits - Tropical and Subtropical Fruits: Postharvest Physiology, Processing and Packaging - page 41

heat to induce oil separation). The volatile compounds en-
zymatically produced from polyunsaturated fatty acids are
incorporated into the oil to confer its characteristic aroma
(Angerosa et al., 2004). Therefore, the aroma of virgin olive
oil is determined by the activity level and properties of the
enzymes involved in the LOX pathway, which in turn de-
pend on the olive cultivar and ripening stage. In addition,
malaxation conditions also affect the final aroma volatile
profile, especially the time and temperature of the process
(Georgalaki et al., 1998).

LOX in plant-derived foods have been investigated. In ap-
ples, a mild heat treatment at 38 ◦ C for 4 days was found to
temporarily inhibited but not fully destroy enzyme activ-
ity, as resynthesis of enzyme systems occurred over time,
causing severe off-flavor (Fallik et al., 1997). Pulsed elec-
tric field (PEF) processing has been shown to be effective
in enzyme and microorganism inactivation while retaining
taste, color, texture, nutrients, and heat-labile functional
components (Elez-Martınez et al., 2007). High-pressure
treatment has also been demonstrated to inactivate LOX
at either mildly elevated or subzero temperatures in certain
foods (Ludikhuyze et al., 1998; Indrawati et al., 2000) and
could potentially be used to control LOX in tropical and
subtropical fruits.

Control of LOX activity in tropical
and subtropical fruits
LOX activity has a significant detrimental effect on the
organoleptic and nutritional qualities of minimally pro-
cessed fruits and vegetables (Robinson et al., 1995). LOX
has been associated with the destruction of essential fatty
acids and may also catalyze the co-oxidation of carotenoids,
including carotene, resulting in the loss of essential nutri-
ents and the development of off-flavors and color degrada-
tion (Ludikhuyze et al., 1998).
LOX is of particular importance in fruit ripening, where
fresh fruit taste and quality can be influenced by its activity.
In fruits in general, there is a close association of both LOX
genes and enzyme activity with fruit ripening and associ-
ated food quality properties such as aroma development
(Griffiths et al., 1999). Hence it is important that the activ-
ity of LOX is controlled to preserve the sensory attributes
of fruits.
Both animal and plant LOX enzymes are inhibited by
phenolic compounds, mainly phenolic acids and their re-
lated esters and flavonoids, which are naturally occurring
antioxidants that scavenge oxygen (Kohyama et al., 1997).
Avocado LOX has been shown to be inhibited using pheno-
lic compounds such as epicatechin (Prunsky et al., 1985).
However, this approach to inactivate LOX may also be detri-
mental for avocado products since epicatequin is a substrate
for PPO, increasing the rate of browning (Jacobo-Velazquez
et al., 2010).
Similar to other enzymes, heat treatment has been used
to control or inactivate LOX. Water-heat treatment of olive
fruit, for example, has been used to significantly reduce the
bitter taste associated with high levels of LOX by-products
(Luaces et al., 2007). Blanching has also been shown to be
an effective method of inactivating LOX (Bah¸eci et al.,
2005) but must be optimized for the specific fruit or fruit
product to minimize loss of certain flavors, colors, vitamins,
and nutrients.
To counter the undesirable effects associated with ther-
mal treatment, alternative methods for the inactivation of

PEROXIDASE (POD)

Nomenclature and reactions catalyzed
Peroxidases (POD, EC 1.11.1.7) are a heme-containing
group of oxidoreductases that utilize hydrogen peroxide
to oxidize a wide variety of organic and inorganic com-
pounds. The term peroxidase (POD) represents a group
of specific enzymes, such as NADH PODs (EC 1.11.1.1),
glutathione peroxidase (EC 1.11.1.9), and iodide peroxi-
dase (EC 1.11.1.8), as well as a group of nonspecific en-
zymes that are simply known as PODs (Hamid and Rehman,
2009). The reaction catalyzed by POD is represented as
follows:

E + H 2 O 2 → Cpd I + H 2 O (3.1)

Cpd I + AH 2 → Cpd II + AH · (3.2)

Cpd II + AH 2 → E + AH · + H 2 O (3.3)

In the above mechanism, E is the ferric POD enzyme; Cpd I
and Cpd II are the oxidized POD intermediates, Compound
I and Compound II; and AH 2 and AH · are the electron
donor substrate and the radical product of its one elec-
tron oxidation, respectively (Dunford, 1999). PODs con-
tain iron (III) protoporphyrin IX (ferriprotoporphyrin IX)
as the prosthetic group. As an example, the catalytic cycle
of POD with ferulate as a reducing substrate is illustrated
in Fig. 3.2.

Occurrence and function of POD in plants
PODs are ubiquitous in the plant kingdom and POD
isozymes are known to occur in a variety of plant tis-
sues. The pattern of expression of each isoform varies in
the different tissues of healthy plants and is developmen-
tally regulated and influenced by environmental factors
(Lagrimini and Rothstein, 1987). For example, POD