. This hypothesis was validated by continuously
monitoring weight loss and surface temperature of a single
mushroom at 100% RH (Fig 4). The test revealed that the
mushroom continuously lose weight as against the evaporation
sphere which showed no decrease in weight over time. Mushroom
showed higher surface temperature (13.8 C) than the surrounding
air temperature (13 C) due to respiratory heat generation. This
result was consistent with the report for other fresh commodities,
in which the produce surface temperature rises slightly above
surrounding temperature (Sastry and Buffington, 1982). The
driving force of water loss in harvested fruit and vegetables is
based on the proportional difference between the water vapour
pressure/concentration gradient in the intercellular air spaces of
the produce and the ambient atmosphere surrounding the product
(Ben-Yehoshua and Rodov, 2003). Additionally, the rate of mass
transport of water vapour (TR) is influenced by the resistance of
produce surface to water vapour loss (Ben-Yehoshua et al., 1985).
According to Fick’s law of diffusion, water vapour will move from
the higher concentration to the lower concentration. The
transpiration process under saturated atmospheric conditions is
a complex process which involves different heat components such
as the internal heat generated by produce; the evaporative cooling
effect on the surface of produce; and the convective heat transfer
between the product and the surrounding environment. Thus, at a
constant ambient temperature and saturated atmospheric level
there is no potential for transpiration nor for evaporative cooling to
occur. However, due to respiratory heat generation the produce
surface heats up, leading to an increase in water vapour gradient
for the mass transfer between the product and the surrounding
conditions. Under such scenario, the produce would still loose
moisture as shown in Fig. 4. The fact that the surface temperature
is higher than the surrounding air only implies that the heat of
respiration is heating the product. This is in agreement with Song
et al. (2002) who proposed a mathematical model based on heat
and mass transfer balances accounting for the respiratory and
transpiratory behaviour of fresh produce, and the transport
phenomenon across the package. When the ambient air is below
saturation level, the difference in water vapour pressure between
the product surface and the ambient air will cause moisture
evaporation from the product surface resulting evaporative
cooling. In this case, the produce surface temperature is lower
than the air temperature. This was evident from the measured
surface temperature of mushroom (10.8 C) which was below the
surrounding air temperature and the mushroom showed a rapid
decrease in mass due to transpiration (Fig 4).