Postharvest Biology and Technology 38 (2005) 66–79
Quality of oranges as influenced by potential radio frequency heat
treatments against Mediterranean fruit flies
S.L. Birla a , S. Wang a , J. Tang a,∗ , J.K. Fellman b , D.S. Mattinson b , S. Lurie c
a
Department of Biological Systems Engineering, Washington State University 213, L.J. Smith Hall, Pullman, WA 99164-6120, USA
b Department of Horticulture and Landscape Architecture, Washington State University, Pullman, WA 99164-6414, USA
c Department of Postharvest Science, ARO, The Volcani Center, Bet-Dagan 50250, Israel
Received 12 December 2004; accepted 15 May 2005
Abstract
There has been an increased interest in developing alternative quarantine treatment methods for control of fruit flies under
growing international pressure to replace the remaining use of methyl bromide fumigation because of concerns over its role in
ozone depletion. The present work explored the possibility of using radio frequency (RF) heating 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 provide quarantine security against fruit flies. The main objective of this research
was to study the influence of those RF heat treatments on the quality of treated fruit. Treated ‘Navel’ and ‘Valencia’ oranges
were evaluated for post-harvest quality after 10 days of 4 ◦ C storage. The quality parameters included: weight loss, loss in
firmness, color change, total soluble solids, acidity, and change in volatiles. The volatile analysis was done by the SPME-GC/MS
technique. The results 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 in comparison with conventional hot water heating. The treatment that raises fruit temperature from
19 to 48 ◦ C by RF heating in saline water and held then for 15 min in 48 ◦ C hot water would meet the quarantine security without
impairing the quality of the treated oranges. However, sensory evaluation for market acceptability of treated oranges should be
carried out for complete treatment protocol development.
© 2005 Elsevier B.V. All rights reserved.
Keywords: RF heating; Post-harvest quality; Citrus; Fruit flies; Quarantine; Hot water; Orange volatiles
1. Introduction
∗ Corresponding author. Tel.: +1 509 3352140;
fax: +1 509 3352722.
E-mail address: jtang@mail.wsu.edu (J. Tang).
0925-5214/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.postharvbio.2005.06.001
The Mexican fruit fly, Anastrepha ludens (Loew),
the Mediterranean fruit fly, Ceratitis capitata (Wiedemann), and the Caribbean fruit fly, Anastrepha suspensa (Loew), are quarantine pests of citrus fruit. Due
S.L. Birla et al. / Postharvest Biology and Technology 38 (2005) 66–79
to their broad host ranges, presence of these pests in
fresh produce can restrict domestic and international
commerce. Citrus shipments destined for states such as
California, Arizona, and Florida, as well as for export
markets including Japan and other Pacific Rim countries require methyl bromide fumigation to meet import
quarantine security requirements. Methyl bromide can
damage some citrus, e.g. mandarin, oranges (Williams
et al., 2000). Moreover, it has been identified as an
ozone-depleting chemical. Therefore, its use is being
restricted in accordance with an international agreement, the Montreal Protocol (USEPA, 1998). Currently
critical use exemptions may make the fumigant available for pre-shipment and quarantine purposes, but
future use of methyl bromide is at great risk due to
reduced production, increased price and future restrictions imposed on its uses under international agreements. Increasing legislative pressure on the use of
chemicals for postharvest quarantine disinfestation has
resulted in great interest in the development of nonchemical treatment methods.
Several alternative methods have been explored by
many researchers including ionizing radiation, cold
storage, and conventional hot air and water heating.
All of these methods have drawbacks. For example,
a common difficulty with hot air or water heat treatments for large fruit such as citrus and apple is the
slow rate of heat transfer resulting in hours of treatment time (Wang et al., 2001b). Shellie and Mangan
(1994, 1998), Sharp and MacGuire (1996), Schirra et
al. (2005) and Lurie et al. (2004) have extensively studied citrus heat treatments using hot water and moist hot
air. A long exposure time requirement and alterations
to flavor compounds (Obenland et al., 1999) were the
main difficulty in the development of quarantine treatment protocols. Radio frequency (RF) heating has been
studied for selected commodities as a rapid disinfestation treatment (Headlee and Burdette, 1929; Frings,
1952; Nelson and Payne, 1982; Wang et al., 2002). RF
heating has relative advantages over microwave heating
because it provides larger penetration depths, possible
differential heating of insects in commodities (Wang
et al., 2003), and simple field patterns (Zhao et al.,
2000). However, a number of potential problems need
to be addressed before RF heat treatments can be successfully used in commercial applications. One potential problem associated with RF heating is possible lack
of uniform heating in heterogeneous media (Tang et al.,
67
2000). A large temperature variation among and within
fresh fruit reduces the effectiveness of a treatment and
may cause severe thermal damage to the fruit. Recently,
efforts have been made to overcome non-uniform RF
heating of fruit. Birla et al. (2004) have shown improvements in RF heating uniformity of oranges and apples
when fruit were submerged in water and kept in motion
by water jets during RF heating.
Treatment protocols have been developed using RF
energy that can effectively control codling moth (Wang
et al., 2001a) and navel orangeworm (Wang et al., 2002)
in unshelled walnuts without causing quality losses.
These studies focus on dry nuts that have higher heat
tolerance than fresh fruit. Exposure to high temperature
can alter many fruit ripening processes, such as ethylene production, respiration, fruit softening, and cell
wall metabolism, pigment, carbohydrate and volatile
metabolism (Lurie, 1998). Many researchers (Shellie
et al., 1993; Shellie and Mangan, 1998; Obenland et al.,
1999) have reported the negative effects of heat treatment on postharvest quality of citrus fruit. Changes
in flavor quality were reported in these studies even
with no significant differences in soluble solids or titratable acidity after heat treatment. The instability of fresh
orange juice aroma during processing (e.g. heat treatment) and subsequent storage has been studied (Shaw,
1991). Decreased levels of characteristic fresh orange
juice aroma compounds on one hand, and off-flavor
formation on the other, lead to the distinct aroma differences between fresh and processed juice (Obenland
et al., 1999).
Similar to development of any new process or technology, many issues have to be addressed before the
RF heating method can be adopted as a commercial process. As RF heating is fast, very short treatment times can be developed. The development of a
treatment protocol requires establishment of process
parameters and verification and validation of treatment efficacy. The knowledge of thermal death kinetics
can be used in selecting time–temperature combinations for RF heat treatments to kill Mediterranean
fruit fly (Medfly) in citrus fruit. The objective of this
research was to evaluate different treatment conditions
that can control Medfly on orange quality. The quality
parameters such as weight loss, loss in firmness, color
change, total soluble solids, acidity, and the change
in volatile compounds from oranges were evaluated
after treatments. This study determined suitable treat-
68
S.L. Birla et al. / Postharvest Biology and Technology 38 (2005) 66–79
ment conditions to support further efficacy studies with
infested fruit.
2. Materials and methods
2.1. Thermal treatment
Freshly harvested ‘Navel’ and ‘Valencia’ oranges
(Citrus sinensis L. Osbeck) were procured from
Fillmore-Peru Citrus Association, California. The
freshly harvested, untreated, unwashed, and unwaxed
oranges were delivered overnight to Washington
State University, Pullman, WA. The average fruit
weight (mean ± S.D.) was 264 ± 21 and 255 ± 18 g
for ‘Navel’ and ‘Valencia’, respectively. The supplied
oranges were stored at 4 ◦ C until used for thermal treatments. The oranges were removed from the cold storage and left overnight at ambient temperature (∼20 ◦ C)
to ensure uniform initial fruit temperature. Based on the
thermal death kinetics reported by Gazit et al. (2004),
100% mortality of Medfly can be achieved by exposing infested fruit to 48 ◦ C for 15 min, 50 ◦ C for 4 min,
or 52 ◦ C for 1 min. Therefore, we chose three temperatures, 48, 50, and 52 ◦ C, and different holding times
(Table 1) corresponding to one level above and one
level below 100% mortality.
Table 1
Experimental design of heat treatments
Treatment
name
Heat treatment description
Cultivar
RF48 + 10
RF48 + 15
RF48 + 20
RF heating in saline water to 48 ◦ C
and holding at 48 ◦ C for 10, 15,
and 20 min
Valencia/Navel
RF50 + 2
RF50 + 4
RF50 + 6
RF heating in saline water to 50 ◦ C
and holding at 50 ◦ C for 2, 4, and
6 min
Valencia/Navel
RF52 + 0
RF52 + 1
RF52 + 2
RF heating in saline water to 52 ◦ C
and holding for 0, 1, and 2 min
Navel
HW48
Hot water heating at 48 ◦ C for
2.5 h
Pre-heating in 35 ◦ C hot water for
45 min and followed by RF heating in tap water to 48 ◦ C and holding 15 min
No heat treatment
Valencia
RFA35
Control
Valencia
Valencia/Navel
Initial experiments suggested that exposure at 52 ◦ C
caused irreversible and undesirable changes in the quality (flavor, and firmness) of treated ‘Navel’ oranges.
Therefore, for ‘Valencia’ oranges the 52 ◦ C RF treatment was removed and, instead, an experiment was
performed to evaluate the effect of pre-heating of fruit
(conventional hot water heating) before RF treatment.
Pre-heating fruit to a moderate temperature prior to
RF treatment could increase throughput and reduce the
cost of RF equipment and RF energy on a per unit
commodity basis in commercial applications. In principle, RF energy should be used sparingly as a means
to overcome the problems associated with conventional
heating methods so that it remains economically viable.
‘Valencia’ oranges were pre-heated in 35 ◦ C hot water
for 45 min. The pre-heated fruit were subjected to RF
heating in 35 ◦ C tap water to raise the fruit core temperature to 48 ◦ C, and then held at 48 ◦ C in hot water
for 15 min.
The RF heating of oranges was conducted in a
12 kW batch type RF heating system (Strayfield Fastran with E-200, Strayfield International Limited, Wokingham, UK). The movement and rotation of oranges
in water during RF heating for uniform heating was
carried out in a fruit mover. The details of the fruit
mover and operating procedure can be found elsewhere (Birla et al., 2004). In preliminary RF heating trials, it was found that 0.006 and 0.004% NaCl
salt in tap water were adequate for ‘Valencia’ oranges
and ‘Navel’ oranges, respectively, in order to minimize differential heating of fruit and water. Prior to
starting the RF treatment for oranges, an experiment
was conducted to obtain temperature profiles of the
fruit subjected to different thermal treatments. At the
end of each treatment stage (pre-heating, RF heating,
holding, and cooling), two oranges were removed and
thermal images were recorded by an infrared imaging
camera (ThermaCAMTM Researcher 2001, accuracy
±2 ◦ C, 5 picture recordings per seconds, FLIR Systems, Portland, OR). The temperature of the core and
subsurface (5 mm below the surface) was measured by
a pre-calibrated thermocouple (Type-T, 0.8-mm diameter and 0.8 s response time, Omega Engineering Ltd.,
CT) and temperature data during pre-heating and cooling time were recorded every 5 s by a data logger
(DL2e, Delta-T Devices Ltd., Cambridge, UK).
Eight oranges were placed in the fruit mover and
water of preset salt concentration was filled to the top
S.L. Birla et al. / Postharvest Biology and Technology 38 (2005) 66–79
of the covering plate placed over the oranges. Oranges
were kept in motion by means of water jet nozzles
mounted on the periphery of the fruit mover. The RF
input power (10 kW) was switched off when the water
temperature reached the treatment temperature, i.e. 48,
50, or 52 ◦ C. Oranges were then kept on hold at the
treatment temperature for 0–20 min depending upon
the treatment design, followed by hydro-cooling for
30 min in 3–4 ◦ C chilled water. The core temperature
of one randomly selected fruit was measured immediately after RF heating, holding time, and cooling.
For a comparative study, conventional hot water heating of ‘Valencia’ oranges was also carried out in a
water bath (Model ZD, Grant, Cambridge, UK) set
at 48 ◦ C. Hydro-cooling started after the core temperature reached 47.2 ◦ C. Treated and control oranges
were placed in cold storage (∼5 ◦ C and ∼95% RH)
for 10 days. Quality analyses of ‘Navel’ oranges suggested that water loss from both treated and control
fruit was considerable and resulted in a loss of firmness. Therefore, treated and control ‘Valencia’ oranges
were waxed (by Carnauba natural wax) before placing
them in storage to reduce moisture loss. Each treatment combination listed in Table 1 was replicated three
times.
2.2. Quality measurement
Weight, firmness, and peel color of each orange were
measured before and 10 days after treatment. Each fruit
within a treatment group was numbered on its stem
end. Color and firmness were measured at three marked
spots along the equatorial fruit surface. The firmness
was measured by a Texture analyzer (Model TA-XT2,
Stable Micro Systems, YL, UK) which was attached
with an aluminum disk (50 mm × 20 mm) on cross
head. The fruit was kept in position on a concave shaped
Nylon disk which was secured in place on the Texture
analyzer base during firmness measurement. The firmness was expressed in mm deformation by a 1 kg force
on the equatorial fruit surface for 10 s. The change in
firmness (difference in post-storage and pre-treatment
firmness of individual fruit) was expressed as a percentage of pre-treatment firmness. A positive value
suggests that there is a loss of firmness, whereas a negative value suggests that oranges became firmer after the
treatments. Peel color was measured at three marked
spots on an individual orange by a colorimeter (Model
69
CM-2002, Minolta Corp., Ramsey, NJ) calibrated to
a standard white reflective plate. The change in peel
color was analyzed as percent change in value of L* ,
C* , and h◦ color system (L* = darkness, C* = chroma,
h◦ = hue angle). The post-storage/treatment color was
expressed as a percentage of pre-treatment value of
color indices, which were calculated from ‘L’, ‘a’,
and ‘b’ values obtained before and after the treatments
using the colorimeter. Total soluble solids and percent titratable acidity were measured after 10 days of
storage on six oranges for each treatment. Juice was
expressed and titratable acidity (TA) was determined
by end-point titration of 5 ml juice to pH 8.2 with 0.1N
NaOH solution and expressed in terms of the equivalent anhydrous citric acid per 100 ml of juice. Total
soluble solids (◦ Brix) was measured by a hand-held
refractrometer (Model N-1␣, ATAGO Co. Ltd., Tokyo)
and expressed as percent soluble solids in juice. The
treated and unteated oranges were visually inspected
for external appearance, treatment damage, and decay
and assessed organolepticaily for any off-flavor development in the peel.
The measurements of individual quality attributes
were subjected to an analysis of variance (ANOVA)
and means were separated by L.S.D. (p < 0.05) and as
multiple pair by Tukey’s method (SAS Institute, 1990,
Cary, NC).
2.3. Volatile compounds analysis
2.3.1. Sample preparation
After 10 days of storage, juice from six oranges
(from each treatment group) was hand squeezed, filtered through cheese cloth to remove pulp and filled in
a 20 ml plastic (scintillation vial with a cone cap lid).
The sample vials were immediately sealed and stored
in a freezer until the samples were used for SPMEGC analysis. A solid phase micro-extraction (SPME)
technique was used to prepare samples for analysis
by gas chromatography (GC) (Steffen and Pawliszyn,
1996). The orange juice samples were removed from
the freezer just before volatile component analysis. The
sample vial was immersed in tap water for thawing. In
a 4 ml SPME vial 1 ml of juice was diluted in 1 ml
of de-ionized water containing 0.65 g NaCl, according
to Steffen and Pawliszyn (1996), and a 6 mm magnetic stirring bar. The vial was mounted on a SPME
stand and a fiber (0.65 m thick PDMS/DVB stationary
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S.L. Birla et al. / Postharvest Biology and Technology 38 (2005) 66–79
phase, Supelco Inc., Bellefonte, PA) was inserted in the
headspace (Yang and Peppard, 1994; Boyd-Bland et al.,
1994). The fiber was kept in the headspace for 30 min to
absorb the volatile compounds and attain equilibrium
(Arthur et al., 1992).
2.3.2. GC/MS analysis
The headspace sample adsorbed on the SPME fiber
was injected into a Hewlett-Packard (Agilent, Avondale, PA), 5890II gas chromatograph interfaced with a
5970 mass selective detector system. The volatiles were
desorbed into the injection port for 5 min set at 200 ◦ C
using a 0.75 mm SPME liner. The injection mode was
splitless for 2 min. The MS transfer line was held at
250 ◦ C and the GC programmed according to Mattheis
et al. (1991). The carrier gas (Helium) velocity was
set at 30.1 cm/s through the fused silica capillary column, a DB-1 (J&W, Folsom, CA) (60 m × 0.32 mm,
thickness 0.32 m). The various flavor compounds
present in the orange juice were identified based on
comparison of GC retention indices and mass spectra of those contained in the Wiley/NBS library and
with those of authentic compounds under the identical experimental conditions. The data were collected
and analyzed using the HP Chemstation G 1034C data
processing package. The reproducibility of flavor compounds analyzed by the SPME-GC/MS was assessed
by analyzing diluted identical samples in replicates
and reporting the percent relative standard deviation
(% R.S.D.).
2.3.3. Determination of response factors for major
flavor compounds
A standard aqueous solution was prepared to determine the response factors for major volatile flavor
compounds (ethanol, ethyl acetate, ethyl butanoate,
hexenal, ␣-pinene, -myrcene, sabinene, limonene, ␥terpinene, l-octanol, decanal, dodecanal, citral, transgeraniol, l-phellandrene, and valencene). The concentration of these components in the standard solution
was compared to the results of Shaw (1991) and our
preliminary experiment. The response factors of these
components were obtained by dividing GC peak areas
by concentrations of each standard component in the
standard solution.
3. Results and discussion
3.1. Temperature profiles
Fig. 1 shows the temperature–time history at the core
and sub-surface of ‘Valencia’ oranges (18.9 ◦ C initial
temperature) subjected to RF heating in saline water
(0.004%), followed by holding at 48 ◦ C for 15 min
before hydro-cooling for 30 min. The core temperature of the oranges after 5.5 min of RF heating at
10 kW power input was 46.4 ◦ C whereas surface temperature was 48 ◦ C. After 15 min of holding in hot
water at 48 ◦ C, the core temperature was 47.6 ◦ C and it
remained above 47 ◦ C for more than 15 min. The addi-
Fig. 1. Temperature–time history of subsurface (10 mm beneath surface) and core of the ‘Valencia’ oranges recorded during RF48 + 15 heat
treatment. The oranges were subjected to RF heating in 0.004% saline water for 5.5 min followed by holding at 48 ◦ C for 15 min before being
cooled by 4 ◦ C water for 30 min.
S.L. Birla et al. / Postharvest Biology and Technology 38 (2005) 66–79
71
Fig. 2. Temperature–time history of’Valencia’ orange subsurface (10 mm beneath surface) and core subjected to HW48 and RFA35 (pre-heating
in 35 ◦ C water for 45 min followed by RF heating in tap water for 2 min and holding at 48 ◦ C for 15 min) treatments.
tion of salt ensured that the core temperature was not
more than that of the treatment target temperature during RF heating to avoid prolonged exposure of the core
to high temperatures.
Fig. 2 shows the temperature–time profile of the
oranges subjected to pre-heating followed by RF heating. After 45 min of pre-heating at 35 ◦ C, core and subsurface (5 mm below the surface) temperatures were
29.6 and 34.6 ◦ C. Upon 2 min of RF heating in 35 ◦ C tap
water with 10 kW input RF power, the core and subsur-
face temperatures were 46.4 and 48 ◦ C. The holding of
oranges in hot water for 15 min at 48 ◦ C was carried out
to ensure the accumulation of thermal lethality at the
core by heat transfer from hot spots to cold spots. Fig. 2
also shows the time–temperature profile of an orange
subjected to 48 ◦ C hot water immersion for 2.5 h. Even
after such a long exposure time the core temperature of
fruit was not higher than 47.2 ◦ C.
Fig. 3 shows the thermal images of oranges taken
during the RFA35 experiment (see Table 1), at the end
Fig. 3. Thermal images of oranges taken by the infrared imaging camera during RF assisted hot water heat treatment at the end of pre-heating,
RF heating, and holding time.
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S.L. Birla et al. / Postharvest Biology and Technology 38 (2005) 66–79
of pre-heating, after RF heating and after holding for
15 min at 48 ◦ C. The pre-heating in hot water established a temperature gradient from surface to core. The
pre-heating of the oranges ensured that the core temperature remains below the treatment temperature (48 ◦ C)
at the end of RF heating. A thermal image in Fig. 3
showed that 15 min holding at 48 ◦ C eliminated the
temperature gradient and ensured a uniform distribution of temperature in the orange. The mean temperature over the orange cross section was 47.4 ± 1.2 and
47.8 ± 0.3 ◦ C before and after holding, respectively.
8.46% weight loss from untreated oranges. The weight
loss in the ‘Navel’ oranges subjected to RF treatments
corresponding to 48 ◦ C (0.77–0.97%) was significantly
less (1.06–1.65%) than that of treatments, to which the
oranges were subjected at 50 or 52 ◦ C (Table 2).
The application of wax coating on ‘Valencia’
oranges before storage improved moisture retention
(Table 2). There was no significant weight loss in the
‘Valencia’ oranges subjected to RF heating in comparison with the control group. A significantly lower weight
loss (0.16–0.24%) from ‘Valencia’ oranges subjected
to HW48 or RFA35 treatments in contrast to the control group (0.38%) was likely due to hydration of the
orange cells during treatment, and later on retention of
absorbed moisture by the wax coating. Therefore, for
oranges with a wax coating, weight loss could not be
used as a criterion for quality assessment as the negative effect of heat is masked by the wax coating.
3.2. Quality analysis
3.2.1. Weight loss
The percent weight loss after 10 days of cold humid
storage (5 ◦ C and 95% RH) was significantly higher in
‘Navel’ oranges (0.6–1.65%) than ‘Valencia’ oranges
(0.2–0.4%) (Table 2). The higher weight loss in ‘Navel’
oranges was likely caused by no wax coating applied to
oranges. Statistical analysis showed a significant effect
of temperature on weight loss of the treated ‘Navel’
oranges. The weight loss was significantly higher in
all the treatments in comparison with control oranges
(Table 2). Shellie and Mangan (1998) have reported
that a hot water treatment (46 ◦ C for 3 h, storage for
4 weeks at 7 ◦ C and 1 week at 23 ◦ C) of oranges
caused 10.45% loss of moisture in comparison to
3.2.2. Firmness
The loss/gain in firmness was expressed by the percentage change in firmness over the period of 10 days
storage. Positive values suggest a loss in firmness and
negative values show a gain in firmness upon treatment and storage. In ‘Navel’ oranges, the effect of heat
was pronounced because of excessive weight loss during storage (Table 2). Except treatment RF48 + 10, all
the other treatments caused significant loss in firmness
Table 2
Change in the postharvest physical quality traits of ‘Navel’ and ‘Valencia’ oranges upon 10 days of storage of oranges subjected to different
thermal treatments
Cultivar
Treatment
Weight loss (%)
Firmness change (%)
Peel color change (%)
L*
C*
h◦
Navel
Control
48 ◦ C + 10 min
48 ◦ C + 15 min
50 ◦ C + 2 min
50 ◦ C + 4 min
52 ◦ C + 1 min
52 ◦ C + 2 min
0.60
0.77
0.93
1.63
1.65
1.27
1.32
±
±
±
±
±
±
±
0.22a
0.12b
0.15c
1.53e
0.34e
0.44d,e
0.34e
−8.98
0.55
19.44
35.46
42.24
23.97
16.16
±
±
±
±
±
±
±
9.86a
27.93a,b
35.44b,c,d
41.90c,d
49.87d
42.85b,c
23.32b
99.07
98.85
98.38
98.47
97.41
97.40
97.41
±
±
±
±
±
±
±
1.61a
2.04a
1.88a,b
1.88a,b
2.34b,c
2.36b,c
2.56b,c
99.34
101.09
102.02
101.98
102.09
101.1l
101.37
±
±
±
±
±
±
±
2.00b
3.02a,b
2.94a
3.11a
2.98a
2.01a
3.30a
100.41
1.0014
100.68
97.73
97.15
98.44
99.05
±
±
±
±
±
±
±
2.36b
3.11b,c
2.94b
4.22c,d
3.74d
4.10b,c,d
3.78b,c,d
Valencia
Control
48 ◦ C + 10 min
48 ◦ C + 15 min
50 ◦ C + 2 min
50 ◦ C + 4 min
HW48 ◦ C + 155 min
RFA35 + 48 ◦ C + 15 min
0.38
0.36
0.39
0.32
0.36
0.16
0.24
±
±
±
±
±
±
±
0.15a
0.13a
0.11a
0.12a,c
0.10a,c
0.10b
0.06b,c
−10.25
−12.49
−11.16
−13.26
−14.56
−6.10
−14.45
±
±
±
±
±
±
±
8.60a
14.09a
14.18a
9.99a
16.22a
10.82a
8.82a
97.28
98.08
97.78
97.25
97.71
98.63
97.06
±
±
±
±
±
±
±
2.97a
2.35a
3.36a
3.81a
2.41a
1.30a
2.71a
98.72
99.30
98.04
101.37
100.27
99.08
99.68
±
±
±
±
±
±
±
2.89a
2.62a
7.04a
6.36a,c
1.80a
1.84a
3.56a
97.99
92.76
93.71
95.90
93.77
94.08
100.62
±
±
±
±
±
±
±
5.95b,c
4.64d
4.93c,d
3.79c
1.43d
3.94c,d
5.82a,b
Entries with different superscripts letters (a–c) in the same column of each cultivar are significantly different (p < 0.05).
S.L. Birla et al. / Postharvest Biology and Technology 38 (2005) 66–79
in comparison to the control oranges. In the case of
‘Valencia’ oranges, control as well as treated oranges
became firmer after 10 days of storage. There was
no significant difference in firmness between control
and treated oranges. This gain in firmness might be
attributed to the wax coating that prevented moisture
loss during storage. Even the oranges subjected to hot
water treatment (HW48) did not lose firmness. Therefore, we conclude that prevention of moisture loss by
the wax coating can maintain firmness.
3.2.3. Skin color
Change in skin color was calculated from the color
measurement before and after 10 days of treatment. In
‘Navel’ oranges, L* value of the peel was significantly
lower in the oranges subjected to a temperature of 50 ◦ C
for more than 4 min or 52 ◦ C for more than 1 min in
comparison to controls (Table 2). The lower value of L*
indicates a darker shade of peel color. Physical observation also suggested that ‘Navel’ oranges subjected to
RF heat treatment at 50 ◦ C and 4 min or more holding
time or exposure to 52 ◦ C lost the luster of peel surface. The loss of luster might be due to diffusion of
peel essential oil from peel to hot water. The change
in hue angle that signifies the shift toward yellow or
red within the yellow to red quadrant was significantly
higher in all treatments except for the heat treatment
RF48 + 10 (Table 2). The trend of chroma value or color
intensity change was not clear but in most of the treatments the color intensity did not change significantly
in comparison with the control oranges.
In ‘Valencia’ oranges, peel color in terms of L*
did not change significantly for all treatments. The
values of hue for all treatments were not statistically
different from control oranges except for the treatment
RF50 + 6 min. The color intensity of oranges was again
found to vary from treatment to treatment, but in comparison with control oranges most of the treatments
were not significantly different. The lowest color intensity was recorded for the treatment RF48 + 10 (Table 2).
3.2.4. Total soluble solids/titratable acidity
There was no significant difference in the value
of total soluble solids (TSS) for all treatments (data
not shown). The mean value of TSS was 10.78 ± 0.5
and 10.24 ± 0.5% for ‘Navel’ and ‘Valencia’ oranges,
respectively. A sharp decrease in TA in heat-treated
fruit may be an indication of heat damage (Schirra et
73
al., 2005). But in the present study we did not find
a significant difference in values of TA for all heat
treatments in comparison with the control (data not
shown). The mean value of acidity in treated oranges
were 0.98 ± 0.04 and 1.04 ± 0.05 g/100 ml for ‘Navel’
and ‘Valencia’ oranges, respectively.
3.2.5. Visual observations
In the present study, we observed incidences of
decay and off-flavor development in the peel of oranges
subjected to HW48 treatment. Shellie and Mangan
(1998) reported that hot water heating (46 ◦ C for 4 h)
of oranges inflicted deleterious effects on fruit flavor,
and decay incidence. McGuire (1991) also reported a
higher incidence of decay and off-flavor development
in grapefruit that were subjected to hot water (48 ◦ C
for 3 h) than in grapefruit treated by forced hot air at
48 ◦ C. The RF heat treatments did not cause any visible
peel damage except for treatments at 52 ◦ C. Control
oranges showed the onset of stem rot after 10 days
of cold-humid storage. Upon 10 days of storage 12%
of control oranges, 22% of HW48 and 8% of RF52
treated oranges were found with some decay. The onset
of stem rot in untreated control oranges was likely
due to a large load of active pathogens, whereas an
increased incidence of decay in heat-treated oranges,
HW48 and RF52 was likely due to pathogens invading areas on the fruit injured by heat treatment. Mulas
et al. (2001) studied the response of ‘Tarocco’, ‘Moro’,
‘Sanguinello’ and ‘Doppio sanguigno’ blood oranges
to hot water heat treatment. They observed that a fruit
core temperature of 44 ◦ C for 100 min or 46 ◦ C for
50 min did not induce visible damage to the fruit, but
inflicted deleterious effects on quality attributes such as
the development of off-flavors and off-taste, decreased
fruit firmness and reduced fruit resistance to decay.
A distinct oily odor from the peel of ‘Valencia’ oranges was detectable by the nose in all heat
treatments, however, such odor was not detected
from ‘Navel’ oranges. The impairment of water-gas
exchange by the wax coating has been studied by Cohen
et al. (1990) and Baldwin et al. (1995), who reported the
notable increase in the synthesis of volatiles associated
with anaerobic conditions, such as ethanol, methanol,
and acetaldehyde which might be the reason behind
off-flavor development. Secondly, the movement and
rotation of oranges during RF heating in the fruit mover
might have caused peel bruising that could lead to
74
S.L. Birla et al. / Postharvest Biology and Technology 38 (2005) 66–79
the % R.S.D., acetaldehyde and ethanol were excluded
because we could not accurately quantify these components in every sample since the peaks representing these components were often too small and too
poorly resolved for accurate determinations. Therefore, the concentrations of acetaldehyde and ethanol
may not be accurate. However, the areas were averaged for the replicates whose peaks had large enough
values so we might view the effect of different heat
treatments on these compounds. The values of R.S.D.
(<10%) indicated excellent reproducibility of SPMEGC/MS analysis under the analytical conditions used.
This level of reproducibility was adequate to separate
the effect of different heat treatment regimes on the
flavor compounds. The quantitative values determined
for the terpenes, esters, alcohols, and aldehydes listed
in Table 4 are in agreement with reported literature values for most of the compounds found in hand squeezed
unheated orange juice (Shaw, 1986; Nisperos-Carriedo
and Shaw, 1990). The data in Table 4 also lists the odor
threshold (OT, ppm, in water) of the major compounds
as compiled by Rychlik et al. (1998).
The GC/MS analysis of untreated orange juice
showed that typical citrus-like flavor was contributed
by more than 31 volatile components present in varying quantities. Among those 31 flavor compounds, 16
the phenomenon called Oleocellosis, or oil spotting on
orange peel. This is a common peel injury of citrus
fruit that is usually caused by mechanical damage that
forces the toxic oil out of the oil glands. This oil kills
nearby parenchyma, epidermal and subepidermal cells
of the flavedo. Cells killed by oil are readily invaded
by fungi resulting in increased decay (Wardowski et al.,
2004). In the present study, the distinct flavor development in ‘Valencia’ orange peel might be attributed to
the combined effect of the wax coating and bruised
peel oil glands. This speculation was based on the
observation that no distinct off flavors from the peel of
untreated, waxed ‘Valencia’ oranges were detected. A
consideration of mechanical damage is very important
in designing a system for the handling and movement
of citrus fruit.
3.3. Flavor analysis
Tables 3 and 4 show the concentrations of the
major volatile compounds identified in the ‘Navel’
and ‘Valencia’ orange juice samples using SPME and
GC/MS techniques after treatment and 10 days of cold
storage. The last row in the tables shows the percent
relative standard deviation (% R.S.D.) in each treatment for all the volatile components. In calculation of
Table 3
Concentration (g/ml) of the major volatiles compounds quantified using the SPME-GC/MS technique in ‘Navel’ oranges subjected to different
RF heat treatment regimes and 10 days of cold storage
Volatile compound
Acetaldehyde
Ethanol
Ethyl acetate
Hexanal
Ethyl butanoate
␣-Pinene
Sabinene
-Myrcene
Limonene
␥-Terpinene
Linalool
l-␣-Terpineol
Decanal
Dodecanal
Valencene
R.S.D. (%)
LV (g/ml)
3–8.5 a
64–900 a
0.01–0.58 b
0.02–0.65 a
0.26–1.02 b
0–0.22 b
0–0.15 b
1.54 c
1–278 a
0.04–0.46 b
0.15–4.6 b
0.09–1.1 a
0.01–0.15 a
NA
0.8–15 b
Control
52 ◦ C
50 ◦ C
48 ◦ C
1 min
2 min
2 min
4 min
10 min
15 min
38.6 ± 3.8
57.0 ± 19.2
0.11 ± 0.03
0.06 ± 0.01
0.66 ± 0.14
0.75 ± 0.10
0.64 ± 0.04
17.5 ± 1.10
61.2 ± 4.10
0.01 ± 0.02
0.58 ± 0.18
0.30 ± 0.05
0.83 ± 0.26
0.11 ± 0.05
1.57 ± 0.39
39.8 ± 36.7
67.9 ± 13.6
0.05 ± 0.01
0.01 ± 0.00
0.02 ± 0.01
0.26 ± 0.06
0.93 ± 0.15
10.5 ± 1.70
39.6 ± 3.90
0.02 ± 0.03
0.95 ± 0.19
0.69 ± 0.07
1.50 ± 0.95
0.29 ± 0.07
1.16 ± 0.45
41.7 ± 12.6
79.9 ± 43.8
0.02 ± 0.02
0.00 ± 0.00
0.01 ± 0.02
0.24 ± 0.15
0.84 ± 0.21
11.4 ± 0.50
38.0 ± 3.00
0.05 ± 0.01
1.23 ± 0.42
0.70 ± 0.19
1.83 ± 0.88
0.27 ± 0.20
1.32 ± 0.76
40.5 ± 41.6
81.9 ± 48.5
ND
0.02 ± 0.00
0.13 ± 0.18
0.27 ± 0.05
1.17 ± 0.31
15.5 ± 2.60
54.4 ± 2.90
0.12 ± 0.04
1.11 ± 0.15
0.73 ± 0.12
l.25 ± 0.27
0.68 ± 0.75
1.73 ± 0.10
37.5 ± 20.3
102.8 ± 15.6
0.05 ± 0.00
0.02 ± 0.01
0.10 ± 0.06
0.21 ± 0.06
0.88 ± 0.41
12.2 ± 1.70
42.8 ± 2.10
0.08 ± 0.10
1.14 ± 0.25
0.66 ± 0.14
1.78 ± 0.71
0.20 ± 0.14
1.30 ± 0.52
35.3 ± 5.4
107.3 ± 12.0
0.01 ± 0.01
0.02 ± 0.02
0.12 ± 0.04
0.61 ± 0.01
0.69 ± 0.20
12.8 ± 0.80
58.0 ± 9.40
0.05 ± 0.02
0.95 ± 0.05
0.64 ± 0.08
1.11 ± 0.39
0.22 ± 0.01
1.26 ± 0.29
45.8 ± 8.3
79.0 ± 24.1
0.04 ± 0.00
0.02 ± 0.00
0.09 ± 0.05
0.70 ± 0.02
0.62 ± 0.21
12.8 ± 1.8
53.2 ± 3.2
0.04 ± 0.03
1.23 ± 0.09
0.67 ± 0.05
2.09 ± 0.62
0.35 ± 0.02
1.16 ± 0.18
11.5
13.5
11.5
9.7
9.9
14.8
8.5
LV: literature values cited from (a) Shaw (1986), (b) Nisperos-Carriedo and Shaw (1990), (c) Steffen and Pawliszyn (1996); linear range
(0.03–0.00013); ND: not detected; NA: not available.
Volatile
compounds
Retention
time (min)
LV (g/ml)
Acetaldehyde
Ethanol
Ethyl acetate
Hexanal
Ethyl butanoate
␣-Pinene
Sabinene
-Myrcene
Limonene
1-Octanol
␥-Terpinene
Linalool
l-␣-Terpineol
Decanal
Dodecanal
Valencene
3.83
4.30
6.03
13.4
14.0
22.1
23.8
24.7
26.7
27.5
27.7
28.9
32.0
32.33
38.57
41.51
3–8.5 a
64–900 a
0.01–0.58 b
0.02–0.65 b
0.26–1.02 b
0–0.22 b
0–0.15 b
1.54 c
1–278 a
NA
0.04–0.46 b
0.15–4.6 a
0.09–1.1 a
0.01–0.15 a
NA
0.8–15 b
R.S.D. (%)
–
–
OT (ppm)
0.01 a
NA
0.5
0.015
0.001
0.030
NA
0.016
0.034
NA
NA
0.001
NA
0.007
NA
NA
Control (g/ml)
29.1
19.7
0.05
0.13
0.52
0.19
0.25
6.51
29.5
0.06
0.03
0.61
0.17
0.33
0.12
2.11
13.9
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
13.6
8.8
0.01
0.08
0.06
0.05
0.10
0.80
3.1
0.00
0.02
0.04
0.01
0.07
0.02
0.89
HW 48 ◦ C
RFA35
RF 50 ◦ C
2.5 h
48 ◦ C + 15 min
RF 48 ◦ C
2 min
4 min
10 min
15 min
53.1 ± 13.7
64.9 ± 13.5
ND
0.01 ± 0.02
0.12 ± 0.08
0.04 ± 0.03
0.16 ± 0.06
1.56 ± 0.84
13.7 ± 1.1
0.08 ± 0.09
0.02 ± 0.00
1.12 ± 0.60
0.57 ± 0.36
0.13 ± 0.07
0.01 ± 0.00
0.20 ± 0.06
32.1 ± 14.2
54.8 ± 7.2
ND
0.02 ± 0.02
0.21 ± 0.02
0.22 ± 0.04
0.45 ± 0.07
6.50 ± 0.08
25.3 ± 0.1
0.05 ± 0.01
0.04 ± 0.00
0.64 ± 0.04
0.26 ± 0.01
0.35 ± 0.10
0.06 ± 0.00
0.76 ± 0.22
30.7 ± 12.8
53.1 ± 2.5
ND
0.01 ± 0.00
0.03 ± 0.00
0.12 ± 0.00
0.43 ± 0.04
3.36 ± 0.20
26.3 ± 0.6
0.11 ± 0.02
0.03 ± 0.04
0.86 ± 0.10
0.35 ± 0.00
0.49 ± 0.10
0.11 ± 0.01
0.70 ± 0.03
22.9 ± 17.3
82.2 ± 77.0
ND
0.00 ± 0.00
0.02 ± 0.02
0.16 ± 0.09
0.48 ± 0.18
5.18 ± 1.35
25.2 ± 3.3
0.06 ± 0.04
0.02 ± 0.02
0.98 ± 0.24
0.39 ± 0.08
0.27 ± 0.09
0.11 ± 0.02
1.03 ± 0.41
11.0 ± 6.9
26.3 ± 5.0
0.04 ± 0.00
0.03 ± 0.00
0.27 ± 0.12
0.12 ± 0.04
0.18 ± 0.05
4.33 ± 0.80
21.5 ± 2.9
0.06 ± 0.00
0.01 ± 0.02
0.51 ± 0.02
0.26 ± 0.03
0.21 ± 0.07
0.11 ± 0.01
1.19 ± 0.31
24.2 ± 7.0
35.7 ± 4.9
0.02 ± 0.00
0.02 ± 0.00
0.18 ± 0.00
0.14 ± 0.03
0.52 ± 0.01
4.74 ± 0.25
22.8 ± 2.5
0.09 ± 0.00
0.05 ± 0.01
0.66 ± 0.04
0.38 ± 0.02
0.44 ± 0.10
0.06 ± 0.01
0.67 ± 0.11
21.6
2.8
20.8
4.0
15.7
9.8
OT, odor threshold; data complied by Rychlik et al. (1998), ND, not detected; NA, not available; LV, literature values cited from: (a) Shaw (1986), (b) Nisperos-Carriedo and Shaw
(1990), (c) Steffen and Pawliszyn (1996); linear range (0.03–0.00013).
S.L. Birla et al. / Postharvest Biology and Technology 38 (2005) 66–79
Table 4
Concentration (g/ml) of the major volatile compounds quantified using the SPME-GC/MS technique in ‘Valencia’ oranges subjected to different heat treatment regimes and 10
days of cold storage
75
76
S.L. Birla et al. / Postharvest Biology and Technology 38 (2005) 66–79
major volatiles were selected and quantified based on
their abundance in the juice and contribution in overall citrus flavor. Sizer et al. (1988) broadly categorized volatile orange flavor compounds and reported
that 75–98% of flavor compounds are hydrocarbons,
0.6–1.7% aldehydes, 1% esters, 1% ketones, and 1–5%
alcohols. In the present study, the volatile compounds
identified in the juice of both ‘Navel’ and ‘Valencia’
orange varieties were similar, but abundance of many
volatiles was higher in ‘Navel’ oranges (Tables 3 and 4).
The volatile compounds responsible for the delicate, fruity flavor of orange, including ethyl butanoate,
ethyl hexanoanate, octanal (Ahmed et al., 1978),
were present in relatively low quantities in untreated
oranges. These compounds have very low odor thresholds (ppb range) thus making them indispensable for
the fresh orange flavor (Table 4). Upon heat treatments
all of these components were diminished to a level at
which detection and quantitation by the present method
was not reproducible (data not shown). Ethyl butanoate
is the major volatile ester in orange juice and it is an
important contributor to desirable top-notes in orange
flavor (Ahmed et al., 1978). A general decrease in
the amount of this ester is associated with decreased
fresh orange flavor quality (Nisperos-Carriedo and
Shaw, 1990). In both varieties, ethyl butanoate levels were somewhat reduced by the heat treatments
(Tables 3 and 4). Heat is known to inactivate enzyme
systems responsible for the synthesis of esters, but a
study by Fallik et al. (1997) on apple suggested that
heat treatment only temporarily inhibits aroma volatile
emission, mainly esters. Therefore, we would expect to
see renewed biosynthesis of ethyl butanoate upon long
term storage of treated oranges.
Heat treatments led to significant changes in
acetaldehyde and ethanol concentrations. In the most
severe heat treatment, i.e. HW48 ethanol and acetaldehyde increased two to three-fold (Table 4). An increase
in the concentrations of the ethanol and acetaldehyde
were least in the oranges subjected to heat treatment
RF48 + 10 and RF48 + 15 in comparison with other
heat treatments. Ethanol build-up after heat treatment
is a well-documented trend shown in the literature. A
study by Schirra and D’hallewin (1997) showed a twofold increase in ethanol levels in oranges after a heat
treatment at 58 ◦ C for 3 min. In the present study, we
also observed the same trend of increased ethanol with
increasing holding time and temperature particularly
in ‘Navel’ oranges (Table 4). Ethanol build-up might
be attributed to long exposure at high temperature,
which increases the respiration rate leading to onset of
the anaerobic pathway. Ethanol concentration was also
considerably higher in the oranges subjected to RFA35
and HW48 heat treatments (Table 4). Obenland et al.
(1999) reported that oranges exposed to 48.5 ◦ C forced
air heating for more than 200 min (47.2 ◦ C core temperature) caused a large increase in ethanol build-up
in the range of 1200 g/ml. Though ethanol enhances
other flavors, its build-up, along with acetaldehyde will
lead to an off-flavor in oranges (Cohen et al., 1990).
Limonene was the most abundant volatile component in orange juice. ‘Navel’ oranges contained
two times more limonene than ‘Valencia’ oranges
(Tables 3 and 4). The loss of limonene in thermal treatments was observed, but losses were very
high (30–50%) for heat treatments HW48, RF52, and
RF48 + 20 min (Tables 3 and 4 and Fig. 4).
The compound ␣-terpineol is a thermal degradation product of limonene and it is a known contributor
to the off-flavor in orange juice at levels of 2 ppm or
higher (Tatum et al., 1975). It is evident from a trend
shown in Fig. 4 that decrease in limonene is associated with spiking in volatiles such as linalool and
l-␣-terpineol. The quantity of these components dramatically increased by more than two times in the
oranges subjected to either HW48 or RF heating at
52 ◦ C (Fig. 4 and Table 3). A reduction in important
flavor components and increases of linalool and l␣-terpineol concentrations upon heat treatment could
be a possible explanation for poor orange juice flavor quality (Nisperos-Carriedeo and Shaw, 1990). The
oranges subjected to RFA35 and RF48 + 10 and15 heat
treatments showed the minimum increase in these two
components (Fig. 4).
Other significant volatile hydrocarbons influenced
by heat treatment include ␣-pinene, sabinene, myrcene and valencene. The flavor compound ␣pinene has a positive contribution to flavor, whereas
valencene has a citrus-like aroma and sabinene contributes a warm, spicy aroma and flavor (Arctander,
1969). The flavor of -myrcene has a musty geranium
odor (Högnadòttir and Rouselff, 2003). In the present
study, the heat treatments RF50, RF52, and HW48
reduced ␣-pinene amount by half and -myrcene by
more than 25% (Tables 3 and 4). The effect of heat
treatment on sabinene was not consistent; therefore, a
S.L. Birla et al. / Postharvest Biology and Technology 38 (2005) 66–79
77
Fig. 4. Volatile compounds concentration (g/ml, ppm) in ‘Valencia’ oranges subjected to different heat treatments (temperature + holding, min)
and 10 days of cold storage.
definite conclusion could not be drawn. The amount of
valencene was found to slightly decrease with severity of heat treatments and maximum loss was observed
in oranges treated by HW48 (Table 4). Obenland et
al. (1999) reported a substantial loss of ␣-pinene, myrcene and limonene in oranges subjected to 48.5 ◦ C
high-temperature forced-air over 200 min.
The aldehyde hexenal, believed to contribute green,
grassy orange notes, was found to decrease substantially upon heat treatments (Tables 3 and 4 and Fig. 4).
Another aldehyde, decanal, contributes to the green
soapy flavors in oranges (Buettner and Schieberle,
2001) was found more in ‘Navel’ orange (0.8 ppm) than
that of ‘Valencia’ orange (0.33 ppm). However, Ahmed
et al. (1978) found that 0.72 ppm of decanal made a neg-
ative contribution to orange juice flavor. In the present
study, we observed a large increase in decanal after heat
treatments of ‘Navel’ oranges whereas heat treatments
of ‘Valencia’ oranges did not show a consistent trend
(Tables 3 and 4). Obenland et al. (1999) reported an
abrupt increase in the amount of decanal in oranges
exposed to humid hot air of 48.5 ◦ C up to 3 h, but this
started decreasing with further increase in exposure
time.
The effect of heat treatments on some volatiles that
are abundant in peel oil such as decanal, myrcene,
sabinene, linalool, and limonene, should be interpreted
with caution because during sample preparation some
portion of the peel oil might have mixed with juice. This
is due to the fact that heat-treated oranges required more
78
S.L. Birla et al. / Postharvest Biology and Technology 38 (2005) 66–79
hand pressure to squeeze juice from the vesicles and
so peel oil might be squeezed out too. The heat might
change cell wall structure and results in less extractability of juice from the sacs.
The flavor analysis enabled us to choose RF48 + 15
and RFA35 as potential RF heat treatments that merit
further investigations for complete treatment protocol
development. In the present study, the advanced SPME
GC/MS technique was used for detection and quantification of the volatile components in orange juice.
However, due to the extremely low concentrations of
potent orange juice odorants such as methyl and ethyl
esters, hexenal, octanal, etc., direct identification and
quantitation in the headspace by means of instrumental methods such as GC/MS is sometimes difficult
(Buettner and Schieberle, 2001). Therefore, the high
sensitivity of the human nose and taste buds should be
employed in a confirmatory test using sensory evaluation techniques to judge consumer acceptability of RF
heat-treated oranges.
With the reported thermal death kinetic information
for Medfly (Gazit et al., 2004) and the treatment conditions determined in this study to minimize quality
changes in oranges, our next logical step will be validation by conducting in-situ efficacy studies. We plan
to conduct experiments with a 12 kW 27 MHz radio frequency system at Hilo (HI), USA on infested orange
fruit with the treatment conditions determined in this
study.
4. Summary
RF heating was explored as a tool to expedite the
internal heating rate of oranges in the hot water treatment in order to decrease exposure time. The selection
of time and temperature combinations for different RF
heat treatments was based on the thermal death kinetics
study of the Medfly. We hypothesized that a reduction
in exposure time at elevated temperature would retain
the postharvest quality of the treated oranges. But slow
cooling after RF heating resulted in more quality damage at 52 ◦ C even for very short time exposures than
at 48 ◦ C for 20 min. Considering the overall analysis
of quality attributes such as color, firmness, weight
loss and change in flavor components, RF treatment
corresponding to a target temperature of 48 ◦ C and
holding for 15 min seems to be the best case scenario.
If we consider the overall prospect for developing an
RF heating process, hot water pre-heating followed by
RF heating treatment seems to be the best option for
practical implementation. To validate optimal RF heat
treatments, further confirmative studies are required on
sensory evaluation, simulated marketing period storage, and consumer acceptability. Based on results of
our previous and present studies, a next logical step
will be an efficacy test. This test is an essential step for
validation of the RF heat treatment protocol.
Acknowledgements
This research was conducted at Washington State
University (WSU) Agricultural Research centre and
WSU Impact centre. The project was supported by
grants from BARD (US-3276-01) and USDA-IFAFS
(00-52103-9656). We sincerely thank Fillmore-Peru
Citrus Association, Peru, California, for supplying
oranges for this research.
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