Humidity
Moisture loss from harvested vegetables causes changes in structure,
texture and appearance (Woods, 1990). Reduction of water loss during
the postharvest period is therefore critical for maintaining freshness and
quality. During growth, water taken up by the roots serves as a trans-
port medium for minerals in the xylem and organic compounds (carbo-
hydrates, amino acids) in the phloem. Water surplus escapes as water
vapor from the entire plant surface but mainly through the stomata on
the leaves, and regulation of the rate of transpiration by stomata is well
known. After harvest, water supply from the roots is terminated and the
ability to withstand water loss is now mainly dependent on the water
vapor pressure deficit between produce and surrounding air as well as
the surface transfer resistance to water vapor movement since stomata
are closed. The former could be considered as a pure physical phenom-
enon while the latter is determined by shape as well as the resistance to
water movement inside the tissue and the surface transfer resistance nor-
mally defined as the transpiration coefficient (Sastry et al., 1978).
Plants grown under arid conditions build up a strong inherent resis-
tance to water loss compared with plants grown under more humid con-
ditions. Preharvest conditions thus exert a strong influence on the ability
to retain turgor and freshness during the postharvest period. Two routes
are available for moisture through the surface, evaporation from the pro-
duce surface and transpiration via small openings between the epider-
mis cells. The plant surface could consist of a thin layer of lightly
suberized living cells, a suberized wall of dead cells or a wall of living
cells covered by a waxy cuticle. Surface composition is not constant for
a particular vegetable but may change both during growth and develop-
ment and due to postharvest senescence. Being the barrier protecting the
tissue against environmental forces, the evaporating surface exerts a
strong influence on the resistance to moisture loss from a produce. Apart
from leafy vegetables, only a very small proportion of the plant surface
may consist of apertures connecting intercellular space with the sur-
rounding air for transfer of oxygen and carbon dioxide. Water vapor
movement is reported to be low through these apertures (Burton, 1982).
This may also explain why micro-perforated packaging materials pro-
tect against water loss without establishing a modified atmosphere in-
side the cover.
The transpiration coefficient as a measure of surface transfer resis-
tance not only vary considerably between different produce (Sastry et
Influences of the Environment 103