156 Papayamarket shelf-life of 8 days, while gamma irradiation (25–50 krads) delayed ripening
up to 7 days. Fruits treated at 100 krads also slightly accelerated ripening in storage.
The carotenoid content was unaffected but ascorbic acid was slightly reduced at all
exposures. Even the lowest level of irradiation inhibited fungal growth. Partly ripe
papaya stored below 10°C will never fully ripen. This is the lowest temperature at
which ripe papayas can be held without chilling injury.
The degree of maturity will indicate whether papaya fruit should be exposed to
ethylene. Papayas that are fully mature at harvest should not be ripened with ethylene
if they are to be stored for an extended period of time. Papayas of minimum commer-
cial maturity will benefit from a treatment of ethylene with an improvement in tex-
ture and colour. A standard banana ripening room may be used for papayas as well.
If not using pressurised ripening rooms, then air stack the boxes (at least 2′′ between
boxes) to ensure proper air circulation. Leave 1 1/2 feet between walls and pallets
and about 6′′ between pallets. Depending upon desired shipping time, bring the pulp
temperature to the range of 20–25°C and apply 100 ppm ethylene for 24–48 h. The
green fruits are ripened successfully by 6–7 days treatment with ethylene gas in
airtight chambers at 25°C and 85%–95% humidity, followed by hot water treatment.
The actual time of exposure to ethylene depends upon maturity of the fruit. The yel-
lowing of fruit indicates that papayas are producing ethylene and the generator is no
longer needed. Once the desired level of ripeness is attained, reduce the temperature
up to 10°C for partially ripe papaya or 7°C for ripe fruits.
Papaya is a climacteric fruit with characteristic respiratory peak and ethylene pro-
duction pattern during fruit ripening. Respiration rates alter during a natural process
of fruit ripening, maturity and senescence (Desai and Wagh 1995; Irtwange 2006).
Papaya experiences a marked and transient increase in respiration during its ripening,
which is associated with increased production of and sensitivity to ethylene (Desai and
Wagh 1995). The sudden upsurge in respiration is called the “climacteric rise”, which
is considered to be the turning point in the life of the fruit. After this senescence and
onset of fruit deterioration, reduced food value for consumers and increased loss of
flavour and salable fruit weight results. To extend the post-harvest life of climacteric
fruits, their respiration rate should be reduced as far as possible (Irtwange 2006).
At the onset of ripening, respiration rises to a maximum (climacteric peak) and
ethylene production increases with a similar pattern (Bron and Jacomino 2006). One
of the effects of storage under MAP is for levels of ethylene produced by the fruit
to diminish, along with changes in colour and texture, while changes in sugars and
acids responsible for some of the flavour proceed normally (Wills et al. 1989). Lazan
et al. (1990) reported that there was a concomitant decrease in internal ethylene
concentration of papaya fruit packaged with polyethylene film, which may be instru-
mental in delaying ripening of the sealed fruit. The rate of ethylene production of
papaya fruit stored at 20°C ranges from 10 to 100 μl kg−^1 h−^1 (Nakasone and Paull
1999). The ethylene forming enzyme activity was found to be maximum in the exo-
carp of three-quarter-ripe fruit (Sankat and Maharaj 2001). The level of 1-amino-
1-cyclopropane carboxylic acid (ACC), the substrate for ethylene forming enzyme, is
initially low in fruit mesocarp tissue during ripening, increasing threefold when the
peak of ethylene synthesis occurs. Ethylene-treated papayas ripened faster and more
uniformly in terms of skin de-greening, softening and flesh colour. The enzymes,