Agriculture Reference
In-Depth Information
texture of these foods (Nunes et al., 2006). Hou and Chang
(1996) showed that PME also catalyzed a transacylation
reaction of the galacturonic acyl groups from methanol to
other hydroxyl groups of pectin, resulting in the formation
of new ester linkages between pectin molecules and thus
contributing to tissue firming. However, softening of the de-
methyl-esterified HGA could also occur when divalent ions
or transacylation are not present and other pectin-degrading
enzymes are present (Pelloux et al., 2007).
Several methods have been used to control the activity
of PME in fruits. Thermal treatment has been the most
commonly used method to inactivate PME (Eagerman and
Rouse, 1976). Pasteurization of fruit juices is necessary
both to prevent microbiological spoilage and to inactivate
the native enzymes, principally PME, polygalacturonase,
and polyphenoloxidase. PME isozymes in orange juice are
quite thermostable, surviving temperatures of up to 80 C
and requiring processing at about 90 C for sufficient in-
activation (Braddock, 1999). The thermo-resistant PME
isozymes are responsible for most processing problems in
the orange juice industry, though they only account for
about 10% of total PME activity, varying with cultivar and
degree of ripening (Braddock, 1999; Vercet et al., 1999).
Heat inactivation of PME has also been studied in guava,
papaya, and mango (Siddalingu et al., 1985).
Combination processes have been explored to optimize
PME inactivation in fruit and fruit products. Manoth-
ermosonication, or the simultaneous application of heat
(72 C) and ultrasound under moderate pressure (200 kPa),
was investigated as a milder mode of inactivating heat-
resistant PME in orange (Vercet et al., 1999). In ba-
nana, Ly-Nguyen et al. (2003) were able to inactivate cer-
tain PME isoforms using a combination of moderate heat
treatment 30 -76 C, in combination with high pressure,
0.1-900 MPa. A combination of high pressure (600 MPa)
processing and carbon dioxide treatment demonstrated sig-
nificant PME inactivation in orange juice to yield the great-
est cloud stability and highest ascorbic acid retention in
orange juice (Boff et al., 2003) compared to high pressure
and thermal processing when used alone. In their study,
Irwe and Olsson (1994) concluded that processing of or-
ange juice at 400 MPa required additional heating, whereas
600 MPa was effective at 20 C in reducing PME activity
to less than 10% and that the degree of PME inactivation
was dependent on the orange variety used (Nienaber and
Shellhammer, 2001).
Control of PME activity in tropical and
subtropical fruits
PME may cause desirable and undesirable effects before,
during, or after processing of fruit and/or fruit products.
PME (1) enhances the texture of fruit and vegetable prod-
ucts (Fuchigami et al., 1995), (2) increases the extracting
yield of juices by conventional methods (Anastasakis et al.,
1987), (3) promotes water removal from the tissues on dry-
ing (Manabe, 1982), and (4) is also used to obtain natural
clouding agents from orange peels (Espachs-Barroso et al.,
2003). On the other hand, activation of PME is accom-
panied by the hydrolysis of the methyl ester groups from
pectin, leading to the formation of a calcium pectate gel
(Ben-Shalom et al., 1985) causing phase separation and
cloud loss in fruit juice manufacturing (Krop and Pilnik,
1974). Together with PG and cellulase, PME enzymes are
implicated in the softening of plant tissues (Paull and Chen,
1983). Fruit softening is associated with cell wall disassem-
bly, and modifications to the pectin fraction due to PME ac-
tivity are some of the most apparent changes that take place
in the cell wall during ripening (Marin-Rodriguez et al.,
2002). Hence, the control of PME activity has been a sub-
ject of interest because of its activity in fruits. Table 3.6
summarizes the impact of PME on fruit quality and
the control processes in some tropical and subtropical
fruits.
Table 3.6. Role of pectin methylestrase (PME) in tropical and subtropical fruits quality and its control
strategies.
Role in Fresh Fruit and
Processed Products
Fruit
Inactivation or Control Options
Reference
Banana
Textural softening, cloud loss,
and phase separation
Heat treatment, high-intensity pulsed
electric field treatments
Espachs-Barroso et al. (2006)
Heat treatment (85 -90 C, 10 sec),
high pressure (above 500 MPa), and
heat treatment (50 C), high pressure
and carbon dioxide
Orange
Cloud loss and phase
separation
Braddock (1999); Sentandreu
et al. (2005); Boff et al.
(2003)
 
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