Novel Processing Technologies for Fruits - Tropical and Subtropical Fruits: Postharvest Physiology, Processing and Packaging - page 83

compounds were quantified during MW heating: zeaxan-
thin and

are available on RF heating and its applications (Zhao et al.,
2000; Marra et al., 2009). RF-pasteurized fruit juices had
better microbiological and sensory qualities as compared to
conventional thermal treatments (Geveke and Brunkhorst,
2004). Since most of the liquid foods are now processed
aseptically, development and optimization of RF heating
will be attractive for processing and preservation of liquid
foods.

-carotene content decreased by about 26%, while
no differences ( p

β

-cryptoxanthin
in the same trial. Vitamin C retention was decreased by
increasing MW heating time, even at 60 ◦ and 70 ◦ C. For the
mentioned temperatures, vitamin C retention ranged from
about 92% (after 2.5 min at 70 ◦ C) to about 82% (after
5minat70 ◦ C), while at 60 ◦ C the values decreased from
97% (after 2.5 min) to about 88% after 5 min.
Commercial processing of sweet potatoes with contin-
uous microwave heaters (Yamco LLC, Snow Hill, North
Carolina, United States) have been reported (Parrott,
2010). Sensory evaluation of sweet potato purees processed
through cylindrical heating of Industrial Microwave Sys-
tems (IMS) at North Carolina State University in 2006
demonstrated that the products were on par with freshly
made purees and were clearly preferred by consumers over
conventional tubular aseptic, frozen, and canned purees
(Parrott, 2010). Brody (2011) compared and summarized
the microwave heating with continuous and in-pack pro-
cessing systems elsewhere.

< .05) were found for β

Fruit processing
There is much literature available on RF heating and its po-
tential applications in food processing. RF heating toward
fruit processing is mostly limited to infestation control and
quarantine purposes. Limited research has been carried out
on juice pasteurization.
Birla et al. (2005) explored the possibility of using RF
heating (12 kW batch type heater) as a means to increase the
internal fruit heating rate in water to control pests. Based
on the thermal death kinetics of the Mediterranean fruit fly
(Medfly), thermal treatments were designed that could pro-
vide quarantine security against fruit flies. Treated 'Navel'
and 'Valencia' oranges were evaluated for postharvest qual-
ity after 10 days of 4 ◦ C storage. The quality parameters
included weight loss, loss in firmness, color change, to-
tal soluble solids, acidity, and change in volatiles. The re-
sults indicated a significant change in volatile flavor profiles
upon RF heat treatments even when there was no significant
difference in the other quality parameters. The reduction in
process time due to RF heating helped in retention of many
volatile compounds. The treatment that raises fruit tem-
perature from 19 ◦ to 48 ◦ C by RF heating in saline water
andthenheldfor15minin48 ◦ C hot water would meet
the quarantine security without impairing the quality of the
treated oranges.
Casals et al. (2010) studied RF heating (27.12 MHz,
with 17 mm distance between fruit and upper electrode and
18 min exposure time) to control brown rot in peaches and
nectarines artificially inoculated with Monilinia fructicola
or with natural Monilinia spp. inoculum. RF treatment was
investigated to control M. fructicola inoculated 0, 24, and
48 hours before RF treatment and using inoculum concen-
trations of 10 3 ,10 4 , and 10 5 conidia/ml. RF treatment sig-
nificantly reduced the incidence of brown rot in 'Summer
Rich' peaches inoculated at 10 3 ,10 4 , and 10 5 conidia/ml,
whereas in 'Placido' peaches, brown rot was only reduced
when fruit were inoculated at 10 3 conidia/ml. No brown
rot control was observed in nectarine fruit artificially inoc-
ulated or with natural inoculum.
To understand the interaction between the fruit and elec-
tromagnetic energy, dielectric properties (DPs) of various

RADIO FREQUENCY HEATING
RF heating is an innovative technique among the several re-
cent electro-technologies. It allows rapid, uniform heating
throughout a medium. The RF technology generates heat
energy within the product and throughout its mass simulta-
neously due to the frictional interactions of polar dielectric
molecules rotating in response to an externally applied AC
electric field (Kinn, 1947; Koral, 1990). The amount of
heat generated in the product is a function of frequency, the
square of the applied voltage, dimensions of the product,
and the dielectric properties (mostly the loss factor) of the
material.
RF heating (13.56, 27.12, and 40 MHz) differs from the
higher frequency microwave heating (915 and 2450 MHz)
in which the wavelength is comparable or smaller than the
dimensions of the sample and heating occurs in a metal
chamber with resonant electromagnetic standing waves.
Since RF waves have a longer wavelength than MW, the
electromagnetic waves in the RF system can penetrate
deeper into the product so there is less surface overheat-
ing and fewer hot or colds spots—a common problem with
MW heating. The RF heating also offers simple uniform
field patterns as opposed to the more complex and char-
acteristically nonuniform standing wave patterns in a mi-
crowave oven. One of the earliest applications of RF heating
was in the baking of bread, and thereafter the technology
has been extensively exploited by the bakery industry for
postbake drying/finishing purposes. Several review articles