mangoes of Anastrepha fruit flies prior to impor- tation into the US US Department of Agricul-
ture, 1998. The treatment schedules for hot water and high temperature forced air provide the same
level of quarantine security against a similar pest, yet the high temperature forced air schedule re-
quires a more severe heat dose. For example, when hot water is used to heat a 700 g mango,
insect control is achieved after 90 min of immer- sion in 46°C water. After 90 min of immersion,
the center of a 700 g mango heats to 45°C. However, if the same sized mango is heated by
high temperature forced air at 50°C, insect control is not achieved until the center of the fruit reaches
48°C 110 min. The more severe heat dose longer exposure and or higher temperature re-
quired to disinfest mangoes using air suggests that insect tolerance to heat can be lowered by heating
a commodity in water.
The medium used to heat a commodity may also influence its tolerance to heat. Many com-
modities do not tolerate immersion in hot water, yet the same commodity may tolerate an exposure
to heated air at a higher temperature for a longer duration. For example, grapefruit were damaged
after immersion for 4.5 h in water at 43.5°C Miller et al., 1988, even though no damage was
observed in grapefruit heated for 5 – 7 h in air at 46°C McGuire and Reeder, 1992; Shellie and
Mangan, 1996. The vapor pressure of the heated air has been shown to alter a commodity’s toler-
ance to heat. Jones 1939 was the first to demon- strate the importance of water vapor-pressure
during heating in air by lowering the relative humidity of the heated air from 100 to 60,
which resulted in the elimination of visual internal damage symptoms in heated papaya fruit. Hall-
man et al. 1990, McGuire and Reeder 1992 and Shellie and Mangan 1996 have also demon-
strated that grapefruit tolerate heating with forced air at 46°C for 195-min longer when the heated
air is not saturated with water vapor.
The greater heat transfer efficiency of water does not entirely explain the disparity in insect
and commodity tolerance to different heating me- dia. For example, McGuire 1991 and Hayes
1994 simulated identical heat transfer efficiencies to deliver identical heat doses via water or air,
and observed damage to grapefruit only when they were heated in water. Inhibition of fruit
respiration has been hypothesized as predisposing a fruit to injury during heating Jones, 1939;
Hayes, 1994. However, the mechanism of action by which the heating media alters the commodity
and insect tolerance to heat remains in question.
The objective of this research was to identify some physical and physiological explanations as
to why a more severe heat dose is required for insect control when commodities are heated in air.
The specific objectives were to: 1 compare heat transfer efficiencies of water, forced vapor-satu-
rated air, and forced vapor-pressure-deficit air; and 2 explore the hypothesis that immersion in
hot water alters the concentration of O
2
and CO
2
inside a heated commodity, and that this modified atmosphere lowers the tolerance of fruit fly larvae
to heat.
2. Materials and methods
2
.
1
. Heat transfer
:
commodity and heating medium
Mango Mangifera indica, grapefruit Citrus paradisi , orange Citrus sinensis and papaya
Carica papaya fruit of similar weight were pur- chased from a wholesale produce retailer. Three
fruit of each type total of 12 fruit were simulta- neously heated until the average center tempera-
ture of the slowest heating fruit type reached 46°C. The fruit were heated in one of three heat-
ing media: water at 48°C hereafter referred to as HW for hot water, water vapor-saturated forced
air at 48°C hereafter referred to as VFA for vapor forced air, or water vapor-pressure-deficit
forced air at 48°C hereafter referred to as MFA for moist forced air. Fruit heated in HW were
placed into a stainless steel water bath containing water at 48°C. A perforated, plexiglass plate was
floated on the surface of the water bath to prevent fruit from floating. The temperature of the water
in each bath was monitored every 60 s with 36-gauge 0.13 mm Type T copper constantan
thermocouples.
Fruit heated by VFA, or MFA were placed into a treatment chamber similar to that described by
Gaffney 1990. The bin perforated bottom, solid sides containing the fruit was placed over the air
vent inside the chamber so that heated air was forced over the surface of the fruit in a vertical
direction. Air speed inside the treatment chamber was maintained at 2 m s
− 1
. The chamber was equipped with a chilled mirror type hygrometer
and a steam generator. Dewpoint temperature inside the chamber was automatically maintained
relative to fruit surface temperature using com- puter software that continuously regulated the
injection of steam into the treatment chamber. During VFA treatments, a vapor-saturated condi-
tion was obtained inside the treatment chamber by maintaining the dewpoint temperature of the
chamber air equal to the temperature of the coolest fruit surface. A water vapor-pressure-
deficit condition was obtained inside the treat- ment
chamber for
MFA treatments
by continuously maintaining the dewpoint tempera-
ture of the chamber air 2°C cooler than the coolest fruit surface. The fruit surface remained
dry during MFA treatments.
Fruit were weighed and numbered prior to each heat treatment. Fruit density was calculated by
dividing the weight of the fruit by the amount of water it displaced ml when immersed in a 1000
ml graduated cylinder. Three, 36-gauge 0.13 mm Type T copper constantan thermocouples were
inserted into each fruit. One thermocouple was attached to the fruit surface, a second thermocou-
ple was inserted 2 mm deep into the fruit. A third thermocouple was placed 16 mm deep into the
fruit by inserting it through a 1.9-mm outside diameter plastic tube with a 20-gauge stainless
steel pipetting needle. The temperature reading from the third thermocouple was used to deter-
mine when to terminate the heat treatment. Each thermocouple was anchored in place with hot glue
at the point of insertion on the fruit exterior. The temperature recording region of all thermocouples
was free of hot glue. Temperatures were recorded every 60 s throughout the duration of each heat
treatment. Exposure to each heating medium was replicated four times.
2
.
2
. Heating medium
:
fruit internal atmosphere and lar6al mortality
Physiologically mature ‘Rio Red’ grapefruit were harvested from a grove in Weslaco, TX 1
day prior to treatment, and stored overnight at 23°C. The fruit were not washed or waxed prior
to treatment. Grapefruit were artificially infested by placing 25, late third instar Anastrepha ludens
Loew larvae into the fruit center, as described by Mangan and Ingle 1994 and Shellie et al. 1997.
All larvae used in these experiments were ob- tained from an experimental colony, originally
founded in 1953 with material from Mexico, and maintained at the USDA – ARS laboratory in
Weslaco, TX. Larvae were reared on a standard fruit fly diet of rehydrated carrot powder and
torula yeast in a room maintained at 26.7 9 2°C. The third instars used in this research were re-
moved from the rearing medium at age 8 or 9 days.
An identical heat dose was delivered to infested grapefruit via HW, MFA, or forced, water vapor-
pressure-deficit 1 kPa 1 O
2
with 20 kPa 20 CO
2
balance nitrogen hereafter referred to as MFCA for moist, forced, controlled atmosphere.
Graduated temperature-controlled water baths were used to deliver the HW treatment. Computer
control was used to simulate the surface tempera- ture of fruit during exposure to MFA or MFCA
in the water baths. Simulation of the surface temperature of the fruit during MFA or MFCA
treatments in the water baths provided an isother- mal heat dose via all three heating media. The
temperature of the circulating water bath was controlled to 9 0.02°C by a computer-driven tem-
perature
control system
Water Troll,
J.J. Gaffney, USDA – ARS Gainesville, FL. The wa-
ter baths consisted of a series of four stainless steel baths 11 l capacity, each equipped with an
electric stirrer motor with shaft and propeller, a 1000 Watt electrical resistance heater and a Hart
1006 precision thermometer.
The MFCA treatments were conducted inside a gas-tight, horizontal flow chamber as described by
Shellie et al. 1997. Gas velocity inside the cham- ber was maintained at 2 m s
− 1
. A chilled mirror dewpoint hygrometer was used to maintain the
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
