4. Discussion
The rate at which heat is conducted into a fruit can be predicted by knowing the surface heat
transfer coefficient of the heating medium and the thermal diffusivity of the fruit Hayes, 1994. The
surface heat transfer coefficient couples the rate of heat flow through the fruit surface to the differ-
ence between the surface temperature and that of the ambient temperature. Once heat has been
transferred into a fruit, it travels by conduction at a rate influenced by the thermal diffusivity of the
fruit. Therefore, for any given thermal diffusivity, the efficiency by which heat is transferred from a
heating medium to the fruit surface drives the rate at which the center of a fruit heats. We have
demonstrated, in this research, that the water vapor pressure of the air during forced air heating
influences the surface heat transfer coefficient, and suggest that the thermal diffusivity of the fruit is
associated with fruit density. Presumably, the greater number of air spaces in less dense fruit
slows the diffusion of heat. Our research also suggests that differences in the surface heat trans-
fer coefficient had a greater relative influence on the rate of heating at the fruit center than differ-
ences in fruit thermal diffusivity. Many researchers have demonstrated that heat-
ing in MFA is less damaging to the fruit than HW or VFA. Hayes 1994 assumed a similar surface
heat transfer coefficient for heating in VFA and MFA, and concluded that thermal stress by either
method would be similar. He also demonstrated a linear, positive relationship between air speed and
the surface heat transfer coefficient. By maintain- ing similar air speed and ambient temperature and
only altering vapor pressure, we have demon- strated a difference in surface heat transfer effi-
ciency between VFA and MFA, and we have shown that thermal stress is greater in VFA. The
faster heating rate for VFA compared to MFA suggests that water vapor pressure influences the
efficiency by which heat is transferred to the fruit surface. Perhaps the latent heat of condensation,
alluded to by Hayes 1994, is responsible for the hotter temperature of the fruit surface relative to
the heating medium when fruit is exposed to VFA. Evaporative or transpirative cooling during
Fig. 4. Average temperature of the center and range among fruit center temperatures of grapefruit heated in vapor-pressure-deficit air MFA circle, vapor-pressure-deficit 1 kPa O
2
with 20 kPa CO
2
MFCA triangle, or water HW square. The temperature of the water was graduated to simulate the surface temperature of fruit heated in MFA or MFCA. Values represent the average of
11, eight, and five treatment replications with a total of 22, 16, or 24 grapefruit for MFA, MFCA, and HW treatments, respectively.
Fig. 5. Concentration of CO
2
and O
2
extracted from the interior of grapefruit exposed to an identical heat dose in vapor-pressure- deficit air MFA circle, vapor-pressure-deficit 1 kPa O
2
with 20 kPa CO
2
MFCA triangle, or water HW square. Standard errors for MFA or MFCA are based upon the average of two fruit over 11 treatment replications, and five fruit over four treatment
replications for HW.
exposure to MFA may contribute to the cooler temperature of the fruit surface relative to the
heating medium. The cooler temperature of the fruit surface relative to the fruit interior after 60
min in MFA lends additional support for surface cooling.
Jones 1939 and Hayes 1994 hypothesized that inhibition of respiration during heating predisposes
a fruit to injury. They postulated that HW caused greater damage to fruit because the water inhibited
gas exchange and fruit respiration. Our data demonstrate dramatic differences in the concentra-
tion of O
2
and CO
2
inside grapefruit when they are exposed to an isothermal heat dose delivered via
water or MFA. We also demonstrated that this altered atmosphere inside the fruit enhanced the
efficacy of heat as a disinfestation treatment. En- hanced efficacy of heating in MFCA was also
reported by Neven and Mitcham 1996 for codling moth Cydia pomonella L.. Our research and that
of Shellie et al. 1997 does not support the hypoth- esis that inhibition of respiration during heating
predisposes fruit to injury. Shellie et al. 1997 showed that grapefruit tolerated exposure to
MFCA 1 kPa of O
2
, balance nitrogen at 46°C for up to 210 min. The sustained elevation of CO
2
concentration inside grapefruit that were heated in water after termination of the heat treatment sug-
gests that heating in water is more stressful to grapefruit than heating in a MFCA that simulates
identical alteration of respiratory gases. Our data suggest that inhibition of fruit respiration per se
does not predispose fruit to injury during heating.
5. Conclusions
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