Novel Processing Technologies for Fruits - Tropical and Subtropical Fruits: Postharvest Physiology, Processing and Packaging - page 80

Icier et al. (2008) studied thermal kinetics of polyphe-
noloxidase enzyme (PPO) using OH at different voltage
gradients (20, 30, and 40 V/cm) from 20 ◦ to 90 ◦ C. The
critical deactivation temperatures were found to be 60 ◦ C
or lower for 40 V/cm and 70 ◦ C for 20 and 30 V/cm. Var-
ious kinetic models for the deactivation of PPO by OH at
30 V/cm were fitted to the experimental data. The one
step first-order kinetic model was found to adequately de-
scribe the deactivation kinetics of PPO for the temperature
range of 70 ◦ -90 ◦ C. The simplest kinetic model involving
one step first-order deactivation was better than more com-
plex models. The activation energy of the PPO deactivation
for the temperature range of 70 ◦ -90 ◦ C was found to be
83.5 kJ/mol.

juice than in pasteurized juice during storage. Myrcene and
limonene concentrations in the OH-heated and convention-
ally pasteurized orange juice did not show any detectable
trend during storage. Sensory evaluation tests showed no
difference between fresh and ohmic heated orange juice,
suggesting that high-temperature OH treatment can be ef-
fectively used to pasteurize fresh orange juice with minimal
sensory deterioration.
A complete inactivation of bacteria, yeast, and mold
was observed during ohmic and conventional treatments
(Leizerson and Shimoni, 2005). Considering a critical ac-
ceptable microbial load of 10 3 CFU/ml, the shelf life of
both OH and conventionally pasteurized orange juice was
105 days; after 105 days, both juices showed microbial
counts greater than 10 3 CFU/ml.
OH has been shown to increase exudation of intracellular
liquids from cellular food materials; for example, vegeta-
bles. This has led to the prospect that it could be used as
a pretreatment for drying and juice expression (Wang and
Sastry, 2000, 2002). Other potential uses include blanch-
ing (Sensoy and Sastry, 2004a), and extraction (Sensoy and
Sastry, 2004b).

Quality parameters
Sastry and Palaniappan (1991) reported that

(torque, nec-
essary to keep the cylinder moving during viscosity mea-
surement) of orange juice increased as function of tem-
perature and decreased with solids content; reducing the
particle size of juice solids increased the effective conduc-
tivity of the juice. Effects of OH on the quality of orange
juice were evaluated and compared to HTST pasteuriza-
tion at 90 ◦ C for 50 sec (Leizerson and Shimoni, 2005).
Orange juice was processed at temperatures of 90 ◦ , 120 ◦ ,
and 150 ◦ C for 1.13, 0.85, and 0.68 sec in a 50 kW pilot
scale electroheating system (Fig. 5.7).
Leizerson and Shimoni (2005) reported the effect of OH
on quality parameters. The vitamin C was reduced by 15%
by the OH treatment. The concentrations of five represen-
tative flavor compounds (limonene, pinene, myrcene, oc-
tanal, and decanal) exhibited better retention in OH orange

σ

MICROWAVE HEATING
Microwaves are a part of the electromagnetic spectrum that
comprises frequencies between 300 and 3,000 MHz. Mi-
crowave heating refers to dielectric heating due to polariza-
tion effects at a selected frequency band in a nonconductor.
Microwave heating has a long history with its household
use in the form of microwave ovens. Microwave is also
used in the industry, especially in tempering frozen foods
and cooking of solid foods, and recently continuous flow
microwave heating of fluids has emerged as a viable alter-
native for thermal processing.
Microwaves find their most widespread use in communi-
cations, radar and medical devices, thus in the United States
the frequencies in which these microwave ovens operate
have been regulated by the Federal Communication Com-
mission. The allocated frequencies for industrial, medical,
and household applications are 915

1 2

± 13 MHz, 2,450 ±

50 MHz, 5,800 125 MHz

for industrial, scientific, and medical applications
(47CFR18.301, 2004). The 2,450 MHz frequency is used
mostly in household microwave ovens, while 915 MHz is
used in industrial applications. The typical bands permitted
for MW heating are 915

± 75 MHz, and 24,125 ±

3

4

50 MHz
with penetration depths ranging from 8 to 22 cm at 915 MHz
and 3-8 cm at 2,450 MHz, depending on moisture content
(Decareau, 1985).
The heating of foods using microwaves relies on gen-
eration of heat inside the food by the conversion of the

± 25 MHz and 2,450 ±

5

6

Figure 5.7. Ohmic heating for juice processing
(1: salt solution tank; 2: product tank; 3: preheating;
4: electroheating; 5: rapid cooling; and 6: secondary
cooling) (source: Leizerson and Shimoni, 2005).