Postharvest Biology and Technology 21 2000 51 – 60
Postharvest disinfestation heat treatments: response of fruit and fruit fly larvae to different heating media
Krista C. Shellie , Robert L. Mangan
United States Department of Agriculture, Agricultural Research Ser6ice, Crop Quality and Fruit Insect Research Unit,
2301
S. International Boule6ard, Weslaco, TX
78596
, USA Received 24 March 2000; accepted 28 July 2000
Abstract
The surface heat transfer efficiency of three heating media water, and forced air with and without a water vapor-pressure deficit was compared using four commodities: mango, papaya, grapefruit, and orange. Forced
vapor-saturated-air VFA, and water HW transferred heat most efficiently to the fruit surface. Thermal stress to the fruit was greatest during heating in VFA. Thermal diffusivity of fruit corresponded to fruit density, and was highest
for papaya and mango. Differences in the oxygen O
2
and carbon dioxide CO
2
concentration inside grapefruit were apparent among those exposed to an identical heat dose in HW, forced water vapor-pressure deficit air MFA, or
a forced, vapor-pressure deficit atmosphere of 1 kPa O
2
with 20 kPa CO
2
MFCA. The altered atmosphere that developed inside fruit during heating in HW or MFCA was shown to enhance efficacy of heat as a disinfestation
treatment. Results from this research suggest that heating in an atmosphere that inhibits fruit respiration does not in itself predispose a fruit to injury. The water vapor pressure of the atmosphere used to heat a commodity influences
the thermal stress delivered to that commodity during heating. Forced, water vapor-pressure-deficit atmospheres with altered levels of O
2
and or CO
2
have commercial potential for providing quicker, less severe heat disinfestation treatments. Published by Elsevier Science B.V.
Keywords
:
Qquarantine; Fruit fly; Vapor pressure; Respiration; Heat tolerance; Controlled atmosphere; Hot water; Forced air; Anastrepha
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1. Introduction
The marketing of fresh horticultural commodi- ties can be disrupted by the presence on the
commodity of insects recognized as pests. Produce harvested from growing regions where insect pests
may be present must be disinfested before it can be marketed in a geographic location that has a
legislated restriction for that particular insect. Heat is one option that can be used commercially
as a postharvest commodity treatment to rid fresh produce of unwanted insects. Hot water immer-
sion or high temperature forced air schedules T102-a, or T103-c-1 can be used to disinfest
Corresponding author: Fax: + 1-956-5656652. E-mail address
:
kshellieweslaco.ars.usda.gov K.C. Shel- lie.
0925-521400 - see front matter. Published by Elsevier Science B.V. PII: S 0 9 2 5 - 5 2 1 4 0 0 0 0 1 6 4 - 2
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
