dewpoint temperature of the chamber 2°C below the coolest fruit surface. Humidity was introduced
into the chamber by computer-controlled injec- tion of water through no.12 Baumac™ nozzles.
An atmosphere of air or 1 kPa O
2
with 20 kPa CO
2
balance nitrogen was maintained inside the chamber during heating. Oxygen and CO
2
con- centration was quantified in an HP 5890 gas
chromatograph equipped with an Alltech CTR 1 column and thermal conductivity detector. The
altered atmosphere was established initially by flushing with nitrogen, and then the required
amount of air, CO
2
, or nitrogen was introduced using needle valve rotameters Dakota Instru-
ments, Monsey,
NY and
compressed gas
cylinders. The temperature of the heating medium and the
center of each heat-treated fruit was logged at 60 s intervals using 36-gauge 0.13 mm Type T
copper constantan thermocouples. The concentra- tion of O
2
and CO
2
inside each heat-treated fruit was monitored at 30 min intervals by manually
extracting gas from the fruit interior Shellie et al., 1997. A 22 gauge, 15.2 cm long Luer lock needle
was inserted into the center of each heat-treated fruit. The Luer lock end of the needle was fitted
with a Luer air lock. Needles extended through a rubber septum embedded in the outside wall of
the heat chamber, and the Luer air lock was attached to the chamber exterior. A gas-tight,
glass sampling syringe was attached to the air lock, and used to extract a 1 ml gas sample from
the fruit interior.
The grapefruit were heated in graduated tem- perature water baths, or the forced atmosphere
chamber for either 2 or 4.5 h. Equal numbers of artificially infested grapefruit in each replication
were either heated or left unheated. Larval mor- tality in heated fruit was adjusted for natural
mortality in unheated controls. Due to larvae and fruit availability, a different number of replicates
were completed for each time dose and heating media. The 2 and 4.5 h exposures to forced air
were replicated eight and 11 times with two heated fruit per replication, resulting in a total of
800 and 550 treated larvae 50 and 25 larvae per fruit. The 2 and 4.5 h exposures to forced, 1 kPa
O
2
with 20 kPa CO
2
were replicated nine and eight times with two heated fruit per replication,
resulting in a total of 450 and 400 treated larvae. The 2 and 4.5 h immersions in graduated temper-
ature water baths were replicated ten and four times with three and five fruit, respectively per
replication, resulting in a total of 1500 and 500 treated larvae 50 and 25 larvae per fruit.
Late third instar larvae were heated in rearing diet to evaluate the efficacy of atmospheric alter-
ations during heating independent of grapefruit. Twenty-five larvae were placed into cups contain-
ing 6 g of rearing diet. Ten cups were exposed for 2 h to forced, vapor-pressure-deficit air or forced,
1 kPa O
2
with 20 kPa CO
2
at 44°C. Air speed, temperature recording, and dewpoint control in-
side the treatment chamber were the same as described previously. An equal number of cups
were either heated or left unheated, and larval mortality in heated cups was adjusted for natural
mortality in unheated controls. Heating in air or 1 kPa O
2
with 20 kPa CO
2
was repeated twice, resulting in a total of 500 treated larvae in each
atmosphere. Larvae were removed from heat-treated and
non heat-treated fruit or diet 24 h after comple- tion of each heat treatment, and larvae were
observed until they either died brown and desic- cated or pupated. Adjusted mortality data was
expressed as a percentage by dividing the adjusted number of dead larvae by the number of larvae
that were treated and multiplying this by 100. Mortality after 2 or 4.5 h treatments was indepen-
dently analyzed in an analyses of variance with heating medium as the main effect GLM proce-
dure, SAS
®
Institute Inc., Cary, NC, USA.
3. Results
3
.
1
. Heat transfer
:
commodity and heating medium
The average temperature of each heating medium was 48.6 9 0.2, 48.1 9 0.1, and 47.5 9
0.3°C for VFA, HW, and MFA, respectively Fig. 1. The relative humidity inside the forced air
treatment chamber remained near 100 during VFA treatments chamber dewpoint temperature
maintained equal to the coolest fruit surface, whereas it increased from 50 to a maximum of
90 during MFA treatments chamber dewpoint temperature maintained 2°C cooler than the
coolest fruit surface. The surface of the fruit heated with VFA was wet after treatment,
whereas the fruit surface remained dry after heat- ing with MFA.
Heat was transferred into the fruit fastest dur- ing VFA heating Fig. 2. It took 62, 72, or 116
min, for the slowest heating type of fruit to reach an average temperature of 46°C, 1.6 cm deep into
the fruit. Papaya and mango fruit heated more rapidly than orange and grapefruit Fig. 2, even
though they weighed 50 g more than the citrus fruit and had a higher density than the other
fruits Table 1. Mango and papaya fruit had an average density of 1.03 and 0.91 g ml
− 1
, respec- tively, and heated about 20 min faster than or-
anges 0.88 g ml
− 1
and grapefruit 0.80 g ml
− 1
. The temperature of the fruit surface during
heating varied according to heating medium, but was similar among all fruit types. Fruit surface
temperature was coolest when fruit were heated in MFA Fig. 3. The surface temperature of fruit
heated with VFA or heated in HW reached 46°C, or 96 of the temperature of the heating medium,
within 5 min of exposure. In contrast, the surface temperature of fruit heated in MFA reached
39°C, or 81 of the temperature of the heating medium, within 5 min of exposure. During heat-
ing in VFA, the surface temperature of the fruit exceeded the temperature of the heating medium
after 20 minutes of exposure, and remained
1.0°C hotter throughout the entire 62 min treatment. However, the surface temperature of
fruit heated in MFA remained cooler than the temperature of the heating medium throughout
the entire 116 min treatment. The surface temper- ature of fruit immersed in HW was similar to the
temperature of the water when the treatment was terminated.
The relationship between temperatures at the fruit surface and 2 mm below the fruit surface
also varied by heating medium Fig. 3. A temper- ature differential between the hotter fruit surface
and 2 mm deep into the fruit was maintained throughout the entire time of exposure to VFA. A
similar, though less dramatic, temperature differ- ential was also maintained between the hotter
Fig. 1. Average temperature of forced water vapor-pressure-deficit air MFA solid circle, forced vapor-saturated air VFA solid triangle, and water HW solid square; and percentage relative humidity inside the hot air treatment chamber open circle and
triangle, respectively for MFA and VFA. Standard errors based upon mean of four treatment replications.
Fig. 2. Average temperature 16 mm deep into grapefruit circle, orange triangle, papaya square, or mango diamond during exposure to 48°C in forced vapor-pressure-deficit air MFA closed symbol, dotted line, forced vapor-saturated air VFA open
symbol, dotted line, or water HW closed symbol, solid line until the average temperature of the slowest heating type of fruit reached 46°C. Standard errors based upon mean of four replications with three of each fruit type per replication.
fruit surface and 2 mm below the fruit surface for the first 60 min of exposure to HW or MFA.
However, after about 60 min of exposure to HW or MFA, this temperature differential dissipated,
and the temperature of the fruit surface actually became cooler than 2 mm below the fruit surface.
3
.
2
. Heating medium
:
fruit internal atmosphere and lar6al mortality
Isothermal heat doses were delivered to the center of grapefruit using three different heating
media: MFA, MFCA 1 kPa O
2
with 20 kPa CO
2
or HW Fig. 4. The temperature at the center of the fruit reached 41°C and a maximum of 45°C,
respectively, after 2 and 4.5 h of heating. Upon termination of the heat treatments, the tempera-
ture of the fruit center was cooled to 30°C within 200 min. The range among center temperatures of
heated fruit reached a maximum after 30 min of exposure, and then declined. Variability among
fruit center temperatures was greatest during heat- ing in water.
The concentrations of O
2
and CO
2
inside the grapefruit were different after 1 h of exposure to
the isothermal heat dose Fig. 5. Prior to heating, the internal atmosphere of all grapefruit con-
tained 19.8 9 0.2 kPa O
2
and 1.5 9 0.3 kPa CO
2
. After 1 h of heating, when the fruit center tem-
perature was 35°C the concentration of O
2
inside the grapefruit immersed in water or heated in
MFCA declined to below 10 kPa and the concen-
Table 1 Average weight and density of papaya, mango, grapefruit, and
orange heated in 48°C water, or forced air that was or was not vapor-saturated until the center of the coolest type of fruit
reached 46°C
a
Fruit Weight g Fruit Type
Density g ml
− 1
Papaya 352.2
a
0.91
b
335.0
a
1.03
a
Mango 302.1
b
Grapefruit 0.80
c
Orange 0.88
b
287.8
b
a
Mean separation in columns by Duncan’s multiple range test, P50.05 superscript bold letters.
Fig. 3. Average temperature of the fruit surface solid symbol and 2 mm below the fruit surface open symbol during heating in vapor-pressure-deficit air MFA circle, vapor-saturated air VFA triangle, or water HW square. Values represent average of
12 fruit grapefruit, orange, papaya, and mango over four treatment replications.
tration of CO
2
increased to 7 or 11 kPa, respec- tively. In contrast, the internal concentration of
O
2
and CO
2
after 1 h of heating in MFA was 15.4 and 4.2 kPa, respectively. A maximum decrease in
O
2
and increase in CO
2
was reached after 3 h of heating, when the grapefruit center temperature
had reached 44°C. Fruit heated in water or con- trolled atmosphere contained similar internal con-
centrations of O
2
and CO
2
3 and 21 kPa, respectively after 3 h of heating, and remained at
about that level until the treatment was termi- nated. The change in internal concentrations of
O
2
13 kPa and CO
2
8 kPa were of less magni- tude when fruit were heated for 3 h in MFA. The
concentrations of O
2
and CO
2
inside fruit heated by MFA or MFCA returned close to pre-heat
treatment levels within 90 min after heating, whereas fruit heated by HW remained altered at 5
and 20 kPa, respectively.
Larval mortality was highest when artificially infested grapefruit were heated in MFCA or in
HW Table 2. No larvae survived 4.5 h of heat- ing in MFCA or HW, yet 10 of larvae survived
4.5 h of heating in MFA. After 2 h of heating in MFCA, 21 vs 8 of larvae were killed, respec-
tively. Larval mortality was higher after a 2 h isothermal exposure to MFCA than a similar
exposure to MFA 88 vs 12, respectively.
Table 2 Average adjusted percent mortality of late third instar
Anastrepha ludens larvae heated in grapefruit or diet to an identical temperature in air, controlled atmosphere
a
, or water
b
Artificially infested Diet
grapefruit 6 g per cup
Exposure: 2 h
4.5 h 2 h 44°C
8.3
a
89.8
a
11.7
a
Hot air 20.7
b
Hot CA
a
100.0
b
88.1
b
100.0
b
NE
c
16.1
a,b
Hot water
a
1 kPa O
2
with 20 kPa CO
2
balance nitrogen.
b
Mean separation in columns by Duncan’s multiple range test, P50.05 superscript, bold letters.
c
NE = not evaluated.
4. Discussion
