Novel Processing Technologies for Fruits - Tropical and Subtropical Fruits: Postharvest Physiology, Processing and Packaging - page 79

resistant to PEF than S. enteritidis and E. coli in both juices
when treated at the same processing conditions.

of temperature, which is also another benefit of OH. Fur-
ther incorporating electrolytes such as salt can enhance the
electrical conductivity.
In continuous processes, the product flows continuously
throughout the heating, holding, and cooling sections simi-
lar to pasteurization of liquid foods, except the process can
handle particulates. Viscous slurry is pumped to the contin-
uous flow OH system via a feed pump. The slurry is passed
through a series of electrodes in the OH column followed
by the holding section where the product retains for a def-
inite residence time to achieve commercial sterility. The
product then passes through cooling section and packed
aseptically.
The major factors influencing particle velocity in a
stream of carrier fluids are viscosity, relative density (par-
ticle to fluid), relative size (particle to tube), particle shape,
and concentration of the solid phase in the fluid (Chan-
darana and Unverferth, 1996). Therefore, it is essential to
have a good knowledge of the physical, mechanical, ther-
mal, and electrical properties of the particles and the carrier
fluid to develop continuous thermal processing. The uses of
food compatible electrodes, which are currently available,
produce correct electrical current density.
A 75 kW system is in operation at the Wildfruit Products
Division of Nissei Co. Ltd. in Japan, where it is running
a variety of fruit products. These products are filled into
10 liter bags for the food service and dairy markets.

OHMIC HEATING
Ohmic heating is referred to as Joule heating, electrical re-
sistance heating, or electroconductive heating. Ohmic heat-
ing (OH) is defined as a process where electric currents are
passed through foods or other materials using a variety of
voltage and current combinations with the primary purpose
of heating. Heat is generated volumetrically, resulting in
rapid and uniform heating of the product, and the product
does not experience a large temperature gradient within it-
self. The major advantage of the OH process is that all of
the components of the product flowing through the two-
phase food system, including fluid and particulates, which
are heated virtually simultaneously. Even it is reported that
it is possible to heat the center of the solid particle faster
than the liquid (Sastry and Palanippan, 1992; Tulsiyan et al.,
2008). Thus it is unnecessary to overcook the liquid phase
in order to sterilize the particulates.
OH has potential in a large number of food processing
applications and the technique is currently being used for
the processing of whole fruits in Japan and the UK (Sastry
and Barach, 2000). The US Food and Drug Administration
(FDA) is currently considering the OH process for com-
mercialization in the United States.
The OH is based on the passage of electrical alternat-
ing current (AC) through a body like a liquid-particulate
system, which serves as an electrical resistance. The nov-
elty of such electric heating system is on direct transfer of
energy from the electromagnetic source to the food mate-
rial without heating the heat transfer surface. AC voltage
is applied to the electrodes at both ends of the product's
body (Fig. 5.6). The rate of heating is directly proportional
to the square of the electric field strength and the electri-
cal conductivity. The electrical conductivity may be varied
by appropriate selection of electrode distance or voltage
applied. The electrical conductivity increases as function

OH treatment of fruits
Much research has been carried out on the electrical con-
ductivity of fruit products (Palaniappan and Sastry, 1991;
Castro et al., 2004; Icier and Ilicali, 2005). Measurements
of σ of different fruits (peach, pear, and pineapple) are
available in the literature (Mitchell and de Alwis, 1989;
Castro et al., 2003; Sarang et al., 2008). The electrical con-
ductivities of fresh fruits (peach, pear, and pineapple) were
found to increase linearly with temperature in the wide
temperature range between 25 ◦ and 140 ◦ C (Sarang et al.,
2008). Within fruits, peach was more conductive than pear
and pineapple.

AC power

Inactivation of enzyme
Inactivation of enzyme is a necessary step for fruits process-
ing, however, application of OH for inactivation of enzymes
is limited. Castro et al. (2004) compared the deactivation of
different enzymes samples heated with ohmic or conven-
tional heating. They showed that the electrical field applied
during OH caused the faster deactivation than the conven-
tional heating. Leizerson and Shimoni (2005) observed that
OH reduced pectinmethylesterase activity by 98%.

Food

Figure 5.6. Schematic for a typical ohmic heating
unit.