Fruiting, Harvesting and Post-Harvest 157
polygalacturonase (PG), pectin methylesterase (PME), xylanase and cellulose were
reported. There is a relationship between PG and xylanase and fruit softening (Paull
et al. 1999). The peak in xylanase and PG activity occurs when the fruit has 40%–60%
skin yellowing (Paull 1993). Since papaya ripens from the inside outwards, the effect
of ethylene treatment is to accelerate the rate of ripening of the mesocarp tissue
nearer the skin that has not started to soften. The already well-softened mesocarp
that is near to the seed cavity is not responsive to ethylene. Ethylene is not recom-
mended commercially, as the rapid softening severely limits marketing time (Paull
et al. 1997). Thus, fruits can be kept in pre-climacteric by controlling atmospheric
gas composition in special storage or modifying the atmosphere with in a package.
Packaging that absorbs ethylene, carbon dioxide, or oxygen is being developed to
control or retard the ripening process (Desai and Wagh 1995). The post-harvest life
of fruit can, thus, be extended through low-oxygen concentration and slightly high
carbon dioxide level and decreasing the storage temperature. The optimum gas com-
position is the range of oxygen and carbon dioxide level that would minimise physi-
ological disorder, and reduce respiration rate and ethylene production during storage
(Kays 1997). A decrease in the rate of respiration increases the shelf-life of fruits
(Wills et al. 1989). The removal of oxygen from the storage environment is another
important effect of respiration. As a consequence, the rate of respiration is important
for determining the amount of ventilation required in the storage area. This is also
critical in determining the type and design of packaging material that can be used, as
well as the use of artificial surface coatings on the product (Kays 1997).
Harvested fruits continue to respire and lose water even when they are attached
to the parent plant, only difference being that losses are not replaced in the post-
harvest environment. Water loss through transpiration is the first stage for loss of
marketable weight and textural quality, softening, loss of crispness and juiciness,
followed by reduction in nutritional quality (Wills et al. 1989). Papaya belongs to
fruits with high-moisture loss rate (Nakasone and Paull 1999). Gradient in water
deficit, which is a parallel gradient in tissue softness, occurs in the mesocarp tissue
of papaya fruit (Lazan et al. 1990). At high relative humidity, produce maintains
salable weight, appearance, nutritional quality and flavour, while wilting, softening
and juiciness are reduced (Nakasone and Paull 1999). Packaging of fruit with poly-
ethylene film retards development of water stress and softness in the fruit tissues.
14.4 Processing
Papaya fruits are very sensitive to pressure. The fungus Colletotrichum gloeospo-
rioides can easily spread through cuts or where the fruit is attached to the stem.
The fungicide baths used in conventional papaya plantations are not permitted.
Generally, fruits between 220 and 600 g are in demand on the international markets.
It is recommended to harvest the fruits for export as soon as the tips turn yellow
from green. The fruit pulp can attain a Brix value of 10%–11.5%. After harvesting,
the fruits are washed in hot water, which varies in temperature and duration accord-
ing to the type of fruit. On average, the papayas are bathed at 49°C for 20 min or
at 42°C for 30 min. After this treatment, the fruits are slowly cooled down to room
temperature, dried, sorted, classified, packed and then stored in the cool place until