Novel Processing Technologies for Fruits - Tropical and Subtropical Fruits: Postharvest Physiology, Processing and Packaging - page 71

Pectin methylesterase
Pectin methylesterase (PME) is an enzyme that generally
tends to lower the viscosity of fruit products and induces
cloud destabilization in orange juices. Inactivation of PME
is a prerequisite for the preservation of orange or related
juices. The pressure range applied for inactivation of PME
varies widely from 150 to 1,200 MPa (Ludikhuyze et al.,
2002). The inactivation of PME in orange juice was shown
to be a function of pressure level and holding time, pH,
and total soluble solids (Basak and Ramaswamy, 1996;
Van den Broeck et al., 2000). An instantaneous pressure
kill was, however, dependent only on pressure level, and
a secondary inactivation effect was dependent on holding
time at each pressure level.
The kinetics of pectin methyl esterase (PME) inactiva-
tion in orange juice over a pressure range of 400-600 MPa
(temperature 25 ◦ -50 ◦ C) for different holding times fol-
lowed a first-order reaction kinetics, with a residual ac-
tivity of pressure-resistant enzyme (Nienaber and Shell-
hammer, 2001). Calculated D -values for the inactivation
process were 4.6 and 117.5 min at 600 MPa/50 ◦ C and 400
MPa/25 ◦ C, respectively. Goodner et al. (1998) studied PME
inactivation using isostatic HP in the range of 500-900 MPa
in orange and grapefruit juices. The higher pressures (

where A 0 and A t are enzyme activities at times zero and t
respectively; k is the rate constant (time − 1 ).

Peroxidase and polyphenol oxidase
Peroxidase (POD) is a ubiquitous enzyme in plant cells. It
is related to food quality in processing and can contribute
to adverse changes in the flavor and color of both raw fruits
and processed products (L opez et al., 1994). The effects of
HPP (200-600 MPa) and temperature (10 ◦ -50 ◦ C) on POD
activity in kiwifruit were investigated by Fang et al. (2008).
The synergistic effect of temperature and pressure on POD
deactivation was enhanced by the elevation of temperature.
At 50 ◦ C, a significant inactivation of POD activity was
observed at all pressure levels. The residual POD activity
in kiwifruit after pressure treatment at 600 MPa and 50 ◦ C
for 30 min was found to be 29.40%, which indicated that
the studied pressure level was not adequate to inactivate
POD completely. Prolongation of the holding time had no
great effect after the first 15 min.
Polyphenoloxidase (PPO) is responsible for enzymatic
browning of many edible plant products, especially fruits
and vegetables during postharvest handling and processing.
MacDonald and Schaschke (2000) evaluated inactivation
of PPO and POD enzyme following combined pressure,
temperature, and holding time treatment in banana ( Musa
acuminata ). Using pressures of up to 110 MPa, tempera-
tures of up to 70 ◦ C, and holding times of up to 25 min,
based on a central composite design, the interactive ef-
fects were found to significantly influence the activity of
both enzymes in prepared banana pulp. Temperature and
pressure were found to influence the inactivation of PPO
separately, while temperature, pressure, and holding time
were found to influence the loss of POD in the banana,
although no significant interactive effects were found. The
reduction in PPO activity was found to be less influenced
by the combined treatment than POD activity.
Phunchaisri and Apichartsrangkoon (2005) studied pres-
sure effect (200-600 MPa; 20 ◦ -60 ◦ C; 10 or 20 min) on
POD and PPO activities of fresh lychee and samples pre-
served in syrup. Pressure treatment at 200 MPa signifi-
cantly increased POD activity, and this effect was higher
at 40 ◦ Cthanat20 ◦ and 60 ◦ C. A combination of pres-
sure (400-600 MPa) and temperature (20 ◦ -40 ◦ C) treatment
did not affect the activity of POD, but some inactivation
at 60 ◦ C was observed. The combined effect of pressure
and temperature on PPO activity was significant while the
treatment time was prolonged (20 min). A pressure 600
MPa, at 60 ◦ C for 20 min caused extensive inactivation of
POD and PPO in fresh lychee, over 50% and 90%, re-
spectively, but for those processed in syrup; the effects

600
MPa) caused instantaneous inactivation of the heat labile
form of the enzyme but did not inactivate the heat stable
form of PME. Treatment times caused significantly differ-
ent total PME activity losses in orange but not in grapefruit
juices, and PME inactivation at different pressures was sig-
nificantly different in both juices. Heat labile grapefruit
PME was also more sensitive than orange to pressure. The
D -values for orange PME inactivation at 500 and 600 MPa
were 83.3 and 2.4 min, respectively, while the z -value was
65 MPa between 500 and 600 MPa. A pressure-resistant
fraction of PME was reported by Van den Broeck et al.
(2000), and authors advocated that a first-order fractional
conversion kinetic model (equations 5.1 and 5.2) is more
appropriate for describing the kinetics of inactivation of
PME in orange juice. Fractional conversion, f, takes into
account a nonzero activity after prolonged heating and/or
pressurizing ( A ∞ ). A fractional conversion model was also
used for pressure (600-700 MPa at 10 ◦ C) inactivation of
the banana PME enzyme (Nguyen et al., 2002):

>

ln A t − A ∞
A 0 −

− = =− kt

ln(1 f ) (5.1)

A ∞

where

= ln A 0 −

A t
A 0 − A ∞

f (5.2)