Fruit and Vegetable Quality

al., 1978) but also for the same produce dependent on internal turgor
and external air humidity as well as stage of development. Wrapping of
desiccation-sensitive produce with either a plastic film, waxing or ap-
plication of different edible coatings (Nussinovitch and Lurie, 1995) is
therefore a common practice. The multilayered structure of onions and
iceberg lettuce is carrying an inherent protection since water transport
between leaves occurs via the dwarf stem. The main water loss is there-
fore from the outer leaves, and as they dry they will form a barrier against
further water loss. This desiccation is acceptable for dry onions but for
iceberg lettuce wilted outer leaves are not acceptable.
Without any air movement, a boundary layer of water vapor-saturated
air is established close to the vegetable that increases the resistance to
transpiration. Air movement should therefore be as low as possible but
the only possibility for removing respiratory heat is via convection,
which implies air movement. Rapid precooling with water vapor-
saturated air reduces water loss. Film-wrapped vegetables usually re-
quire longer cooling time than unwrapped produce, which could result
in water condensation within the package (Kadar et al., 1989). After
cooling to the desired temperature, the respiratory heat of the vegetables
will maintain a water vapor pressure deficit with continued transpiration
even though the air inside the package is saturated.
For those involved in storage, distribution and retailing of vegetables
both the influence and measurement of temperature are easy to under-
stand. However, humidity and its measurement is much more difficult
to grasp. Air humidity is commonly specified as relative humidity (RH),
which is the ratio of water vapor pressure in air to saturation vapor pres-
sure at the same temperature, expressed as a percentage. The use of rel-
ative humidity measurements has the disadvantages that as long as
produce temperature differs from that of the surrounding air, a water
vapor pressure deficit is present between produce and surrounding air
even if air humidity approaches saturation.
Although most vegetables have a high water content, differences in
the osmotic pressure of the cell sap are present between various species
and tissues. The water vapor pressure exerted by the produce in the in-
tercellular space is therefore lower than that of distilled water. If placed
in a sealed enclosure under constant temperature, the vegetable will gain
or lose water until the humidity inside the enclosure reaches a value that
is characteristic of that vegetable at that temperature. This value is called
the equilibrium relative humidity (ERH) (Rooke and van den Berg,
1985). Most vegetables have an ERH between 97–98% compared with

104 POSTHARVEST HANDLING AND STORAGE OF VEGETABLES