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Papaya (Carica papaya L.)
S. P. Singh, Curtin University of Technology, Australia and D. V. Sudhakar
Rao, Indian Institute of Horticultural Research, India
Abstract: Papaya is a commercial fruit crop in many tropical regions of the world. The
fruit is a rich source of vitamins, minerals and dietary antioxidants. Short postharvest life
and susceptibilities to physical damage, water loss, chilling injury, diseases, and insectpests are the major postharvest constraints for papaya fruit. This chapter reviews the
economic importance, postharvest physiology, maturity indices and effects of pre and
postharvest factors on papaya fruit quality. Postharvest quality and marketing losses
attributed to non-pathological, pathological and insect-pests problems are discussed.
Finally, the processing of fruit into fresh-cut and other products is reviewed.
Key words: Carica papaya, maturity, postharvest physiology, quality, storage, fresh-cut.
6.1
Introduction
6.1.1 Origin, botany, morphology and structure
The papaya (Carica papaya L.) belongs to the family Caricaceae. It is often
called ‘pawpaw’ in Australia and ‘tree melon’ in some other countries, but it is
different from the North American ‘pawpaw’ (Asimina triloba Dunal), which is a
member of the family Annonaceae. Carica papaya is believed to be native to
tropical America, its region of origin perhaps being southern Mexico and
neighbouring Central America (Morton, 1987). Spanish explorers took it to the
Caribbean and South East Asia in the sixteenth century. The large number of seeds
in the fruit and their long viability are among the factors that are thought to have
contributed to the wide geographical distribution of the fruit (Chan and Paull,
2008).
The papaya is a herbaceous perennial 2–10 m in height. It usually has a single,
semi-woody, hollow, erect stem, which terminates with a cluster of large,
palmately-lobed leaves with 25–100 cm long petioles and latex vessels in all
tissues. Five-petalled, fleshy and slightly fragrant flowers borne in modified
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Papaya (Carica papaya L.)
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cymose inflorescences in the axils of leaves are primarily of three types: staminate
(male), pistillate (female) and hermaphrodite or perfect (having both female and
male organs). The papaya plants may be classified into three categories – male,
female or hermaphrodite depending on their flower type. Some plants may bear
both male and female flowers, and are called monoecious. Sex expression in
papaya is strongly influenced by genotype and climatic conditions. Male and
hermaphrodite flowers may undergo sex reversal and morphological changes
under the influence of environmental conditions.
In the tropics, the interval between seed sowing to the fruit harvesting is
generally 8–9 months. The plant flowers and sets fruit throughout the year, so
harvesting takes place all year around (Chan and Paull, 2008). The fruit shape
varies from oval to somewhat round, pyriform, or elongated club-shaped,
depending on the flower-type. The fruit that develops from a female flower is
round or ovoid in shape, while a hermaphrodite flower develops into a fruit that is
elongated and cylindrical or pyriform in shape. The fruit size may vary from
15–50 cm long, 10–20 cm thick, and 1–3 kg or more in weight (Morton, 1987).
Cultivars with small size (~1 kg) fruit are preferred as they are more convenient to
pack and market. The immature or mature unripe fruit has green skin and greenish
to white flesh and is rich in white milky latex. The skin colour changes to deep
yellow or reddish-orange as the fruit ripens. The flesh of the ripe fruit is aromatic,
juicy, and yellow, orange, pink or salmon-red in colour with numerous small, dark
grey to black, ovoid, peppery seeds clustered in the central cavity, which may be
round or star-shaped. These seeds are usually attached to the flesh by soft, white
fibrous tissue, and have transparent and gelatinous arils (Morton, 1987).
Consignments of black peppercorns have been on occasion fraudulently
adulterated with papaya seeds.
6.1.2 Worldwide distribution and economic importance
In the past decade, papaya has attained great popularity among growers due to the
fact it can be intensively cultivated, its rapid returns and the increased demand for
the fresh fruit and its processed products. It is commercially cultivated between
23 ° North and 32 ° South latitude (Chan and Paull, 2008), an area which includes
many tropical and sub-tropical countries of the world. World papaya production
in 2008 was about 9.1 million tonnes (FAOSTAT, 2010), registering a growth of
40% between 1998 and 2008. The share of different geographical regions in
global papaya production is depicted in Fig. 6.1. India was the world’s largest
producer of papaya and contributed about 33% of global production in 2008.
Brazil, Nigeria, Indonesia, and Mexico were the other leading papaya producing
countries (Table 6.1). Other than the top-ten producing countries, papaya is
cultivated on a commercial scale in countries such as Peru, Venezuela, China,
Thailand, Bangladesh, Cuba, Kenya, Malaysia, El Salvador, Costa Rica, Ecuador,
Mozambique, Mali, South Africa, and the United States of America (countries
arranged in the decreasing order of their production volumes in 2008, FAOSTAT,
2010). The international trade in papaya fruit has exhibited buoyant growth during
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Fig. 6.1 The share of different geographical regions to the global production of papaya
(FAOSTAT, 2010).
Table 6.1 Total area and production of papaya in the top-ten producing
countries of the world in 2008
Country
Area (thousand hectares)
Production (thousand tonnes)
India
Brazil
Nigeria
Indonesia
Mexico
Ethiopia
D.R. Congo
Colombia
Guatemala
Philippines
80.3
36.8
92.5
9.0
16.1
12.5
13.5
5.5
3.5
9.2
2686
1900
765
653
638
260
224
208
185
183
Source: FAOSTAT, 2010
the past ten years. The export value of papaya increased by 2.7-fold from US$70
million in 1997 to 186 million in 2007. Out of the total papaya export volume of
276 thousand tonnes in 2007, 145 thousand tonnes (53%) was from Central
America. In monetary terms, the international export market was dominated by
Mexico (US$55 million, 36%), followed by Brazil (US$34 million, 22%), the
USA (US$18 million, 10%), the Netherlands (17 million tonnes, 9%), Belize
(US$13 million, 8%), and Malaysia (US$8.4 million, 5%). The other major
producing countries such as India, Indonesia, and Nigeria contributed very little
to the global trade. In 2007, the USA was the largest importer of papaya fruit in
the world, and imported about 138 thousand tonnes, which is about 54% of total
world imports. Singapore, Canada, the Netherlands, China, the United Kingdom,
Germany, Spain, and the United Arab Emirates are the other major papaya
importers in the world.
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Papaya (Carica papaya L.)
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Table 6.2 Papaya cultivars/hybrids grown in different parts of the world
Country
Cultivar(s)
Australia
‘Petersen’, ‘Improved Petersen’, ‘Sunnybank’, ‘Richter Gold’,
‘Arline/57’, ‘Bettina’, ‘Guinea Gold’ ‘NT Red’, and ‘Yarwun
Yellow’
Brazil
‘Golden’, ‘Sunrise Solo’, and ‘Tainung-01’
India
‘Honey Dew’, ‘Coorg Honey Dew’, ‘Ranchi’, ‘Washington’,
‘CO–2’, ‘CO–7’, ‘Pusa Delicious’, ‘Pusa Dwarf’, ‘Pusa Majesty’,
‘Surya’, and ‘Red Lady’
Indonesia
‘Dampit’, ‘Jingga’, and ‘Paris’
Malaysia
‘Eksotika’, ‘Eksotika II’, ‘Sekaki’, ‘Batu Arang’, ‘Subang 6’, and
‘Sitiawan’
Mexico
‘Maradol’, ‘Cera’, ‘Chincona’, ‘Gialla’ and ‘Verde’
Philippines
‘Cavite Special’ and ‘Sinta’
South Africa
‘Hortus Gold’ and ‘Honey Gold’
Thailand
‘Khaek Dam’, ‘Kaegnuan’, ‘Koko’, ‘Sainampeung’, ‘Tainung’
series, and ‘Red Lady’
USA (Hawaii)
‘Kapoho Solo’, ‘Sunrise’, ‘Waimanalo’, and ‘Rainbow’
USA (mainland)
‘Betty’, ‘Cariflora’, and ‘Homestead’
Source: Chan and Paull, 2008; Chan, 2009.
A number of papaya cultivars are available in different parts of the world
(Table 6.2). The breeding programmes run by many countries have evolved
cultivars which are diverse in size, shape, skin and flesh colours, suitability for
eating fresh and processing, and resistance to diseases such as papaya ring spot
virus (PRSV). ‘Solo’ types are much in demand in the international market due to
their small size (Firmin, 1997). Consumers in western countries prefer a fruit
without the musky, sweet odour found in some cultivars of South East Asia. The
odour is due to methyl butanoate, levels of which are low in ‘Solo’ types, but high
in other cultivars (Flath and Forrey, 1977; MacLeod and Pieris, 1983).
6.1.3 Uses, nutritional value and health benefits
The ripe papaya fruit is eaten as dessert, in fruit salads, and is processed into a
variety of products such as jam, jelly, nectars, ice cream, canned and dried fruit.
Mature unripe fruit has culinary value and is used in green salads, candy-making
and fermented products. Papaya leaves and flowers are also used in Asian cooking.
Papain, a proteolytic enzyme, is extracted from the immature fruit by lancing their
surfaces and collecting the white exuding latex, then drying it into a powder. The
lanced fruit may be allowed to ripen partially, and can be used to make fruit
leathers, candy, papaya powder, and pectin extract. Papain has diverse commercial
uses. In the food processing industry, it is used in meat tenderizing and for
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chill-proofing of beer among other applications. Papain has widespread uses in
the pharmaceutical and medical industries in manufacturing drugs and other
formulations to cure various digestive ailments, in the preparation of vaccines, for
deworming cattle, in the treatment of wounds, reducing swelling and fever. Other
industrial uses include degumming of silk and softening of wool in the textile
industry and tanning in the leather industry. Several cosmetic products such as
shampoo, soaps and skin-care products also have papain as an important
ingredient. The major producers of crude papain are Democratic Republic of
Congo, Tanzania, Uganda and Sri Lanka (Anonymous, 2010).
Papaya fruit is low in calories and rich in vitamins and minerals. The
average concentration of ascorbic acid (vitamin C) in Hawaiian papaya cultivars
ranges from 45.3 to 65.4 mg 100−1 (Wall, 2006). Therefore, one cup of papaya
cubes (140 g) can provide about 80–96% of the dietary reference intakes (DRI,
established by the US Food and Nutrition Board, National Academy of Sciences)
for vitamin C and 8–11% of the DRI for Mg for adult males and females (Wall,
2006). The ascorbic acid (48.4 mg 100 g−1) in the fruit flesh contributed about
97% of the total hydrophilic antioxidant capacity of the fruit (Isabelle et al.,
2010). The carotenoids present in the fruit flesh contribute to its vitamin A
level and lipophilic antioxidant capacity. The red-fleshed cultivars have high
concentrations of lycopene (about 63% of the total carotenoid content) and have
relatively lower retinol activity equivalents compared to the yellow counterparts,
in which β-cryptoxanthin and β-carotene are the major carotenoid pigments.
Lycopene is vitamin A inactive but is a more efficient antioxidant than β-carotene
(Di-Mascio et al., 1989) and has been linked with a reduction in the risk of cancer
especially lung, stomach and prostate cancers (Giovannucci, 1999). Therefore,
the antioxidant activity of red-fleshed cultivars may make a greater contribution
to human health than their vitamin A activity (Wall, 2006).
Ripe papaya fruit has many medicinal uses. It is a digestive aid and is a
stomachic, carminative, diuretic and expectorant. Its applications to combat
dysentery and chronic diarrhoea, wounds of the urinary tract, ringworm and skin
diseases have also been reviewed elsewhere (Krishna et al., 2008). The extracts
from unripe fruit, leaves, seeds, and roots have antimicrobial properties and have
been traditionally used to cure several ailments related to digestive and urinary
complaints. The abortifacient properties of unripe fruit and seeds make them
unsafe for consumption by pregnant women (Krishna et al., 2008).
6.2
Fruit development and postharvest physiology
6.2.1 Fruit growth, development and maturation
Botanically, papaya fruit is a fleshy berry. Its growth and development rate
depends on the cultivar, age of bearing trees, climatic conditions and the selected
maturity index (Selveraj et al., 1982b; Nakasone, 1986). In general, it takes about
140–180 days after anthesis (DAA) to reach full maturity. The pattern of fruit
growth corresponds to a single sigmoid growth curve with two major phases. The
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Papaya (Carica papaya L.)
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first phase lasts for about 80 days. A large increase in dry weight occurs during the
second phase just before maturity (Chan and Paull, 2008). In fact, the fruit
accumulates about 50% of its dry matter during this second phase of development
(120–168 DAA), the duration of which is less than one-third of the complete fruit
developmental period (Calegario et al., 1997).
As mentioned previously, the climatic conditions during anthesis and fruit
growth have a profound effect on the number of DAA to reach harvest maturity.
For instance, in Hawaii (USA), fruit development is normally complete after
about 150–164 days, but it is extended by another 14–21 days in colder months.
‘Sunset’ papaya fruit matured in 140 and 180 DAA from June and December
flowers, respectively (Zhou and Paull, 2001). The difference in fruit maturation
time was due to fruit growth and development being retarded mainly by
temperature, and to a lesser extent by tree age and fruit competition. Fruit
growth and development is even slower under sub-tropical conditions and may
take 190–270 days (Chan and Paull, 2008).
During the final phase of maturation (>120 DAA), the skin colour changes
from dark green to light green. The appearance of yellow string on the fruit surface
marks the completion of fruit maturation and when the yellow colour predominates,
the fruit is ripe. Higher yellow colouration at the harvest has been linked to better
fruit pulp eating quality. However, the number of days postharvest to reach eatingripe stage decreases as surface colouration at harvest increases. For example, a
Venezuelan papaya cultivar ripened normally in 5–7 days when the initial yellow
colour intensity on the major part of fruit exceeded the Hunter b* value of 20,
whereas those with b* values of 18–20 took about 8–10 days. The majority of
fruit with lower b* values at harvest did not ripen normally and those which did
took about 11–14 days to reach acceptable ripeness (Peleg and Brito, 1974).
Soluble solids concentration (SSC), which remains almost constant during
the initial phases of growth (up to 120 DAA), undergoes a dramatic increase
during the final growth phase (Ghanta, 1994; Calegario et al., 1997). In a study by
Calegario et al. (1997), the increase in SSC observed was from 6% at 120 DAA to
15% at 168 DAA. A minimum of 11.5% SSC is required to meet the Hawaiian
grade standards, which corresponds with the 6% skin coloration (Akamine and
Goo, 1971a). Studies on sugar metabolism during growth and development of
papaya fruit showed that sucrose synthase (SS) activity was very high in young
fruit (14 DAA), decreased to about one-fourth of initial activity within 56 DAA
and then remained relatively low during the subsequent period of fruit development
(Zhou and Paull, 2001). The high levels of SS activity in the initial stages of fruit
development seem to play an important role in papaya fruit sink establishment.
On the contrary, the activity of acid invertase (AI) was very low in the young fruit
and increased four weeks before fruit maturity and commencement of ripening
during the last phase of fruit development (90–125 DAA). The increase in AI
activity paralleled the increase in sugar accumulation in the fruit. The reduced AI
activity two weeks prior to ripening together with an increased sucrose phosphate
synthase (SPS) activity may contribute to sucrose accumulation in the vacuole.
Papaya mesocarp does not contain measurable quantities of starch and other
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carbohydrate storage compounds. Sugar accumulation in the fruit is mainly
reliant on continued sucrose import rather than starch degradation (Zhou
and Paull, 2001).
6.2.2 Respiration and ethylene production
Papaya is a climacteric fruit (Jones and Kubota, 1940; Nazeeb and Broughton,
1978; Fabi et al., 2007) and exhibits increases in rates of respiration and ethylene
production during ripening. The rates of respiration and ethylene production are
determined by many factors including cultivar, fruit maturity, and storage
conditions. The peaks in rates of respiration and ethylene production coincide
with full skin colour development (Jones and Kubota, 1940; Manenoi et al.,
2007). The ethylene peak generally precedes the respiratory peak during papaya
fruit ripening (Nazeeb and Broughton, 1978; Wills and Widjanarko, 1995; Fabi
et al., 2007; Manenoi and Paull, 2007). Rates of respiration and ethylene
production began to rise after two days at 22 °C in ‘Rainbow’ papayas harvested
at colour-break stage (<10% yellow) and reached their maximum after 10 and
eight days, respectively (Manenoi et al., 2007). However, the rate of respiration in
relation to skin colour changes and fruit ripening varies greatly among papaya
cultivars. The time between the start of skin yellowing and the rise in respiration
at 22 °C varied from about two days in ‘Kapoho’ and ‘Sunrise’ to about four days
in RL1–3 and eight days in RL1–12 (Zhang and Paull, 1990). Papaya cv. ‘Taiping’
exhibited higher rates of respiration and ethylene production compared to
‘Bentong’. Consequently, ‘Taiping’ ripened in four and six days at 20 and 26 °C,
respectively, while ‘Bentong’ took seven and 11 days at the same temperatures
(Nazeeb and Broughton, 1978). The number of days to reach respiratory and
ethylene climacteric peaks and fruit ripening during postharvest also decreases
with the advancement of fruit maturity at harvest. For example, ‘Golden’ papayas
harvested at different stages, viz. mature green, up to 15% yellow, up to 25%
yellow, and up to 50% yellow, reached the edible-ripe stage after 7, 6, 4, and 3
days at 23 °C, respectively (Bron and Jacomino, 2006).
Storage temperature remarkably modulates the respiratory and ethylene
production behaviour of papaya fruit (Nazeeb and Broughton, 1978; Lam, 1990),
and thus the postharvest life of fruit. The respiratory and ethylene peaks in papaya
cultivars, ‘Taiping’ and ‘Bentong’, appeared earlier at higher temperatures and the
magnitude of the peaks also increased with temperature. Papaya cv. ‘Taiping’
showed the ethylene peaks after 7, 9, 17, and 28 days when the fruit were stored
at 25, 20, 15 and 10 °C, respectively, whereas the respiratory peaks were observed
on days 8, 12, 22 and 24 (Nazeeb and Broughton, 1978). Therefore, fruit ripening
proceeded at a faster rate at higher temperatures and climacteric peaks were of
greater magnitude. The respiration rates of papaya at 10 °C and 15 °C were almost
double those at 5 °C (Lam, 1990). Humidity in the storage atmosphere has also
been reported to influence the ethylene and respiratory rates of papaya fruit. The
number of days to reach CO2 and ethylene peaks were significantly larger under
high humidity conditions compared to low humidity conditions (Nazeeb and
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Broughton, 1978). The exogenous application of ethylene has been known to
enhance the rates of respiration and ethylene production in papaya fruit, resulting
in accelerated and uniform fruit ripening (Nazeeb and Broughton, 1978; Fabi
et al., 2007). The removal of ethylene from the storage atmosphere and blocking
of its action by the postharvest application of 1-methylcyclopropene (1-MCP)
suppress the respiratory and ethylene production in papaya, resulting in delayed
fruit ripening. Knowledge of the factors affecting the rates of ethylene production
and respiration is therefore pivotal for the development of postharvest strategies
aimed to retard them for quality maintenance and enhanced postharvest shelf life.
6.2.3 Biochemical changes during fruit ripening
Fruit ripening in papaya involves several biochemical changes that convert
mature–hard and inedible fruit into sweet, soft, juicy and aromatic edible
fruit. During fruit ripening, chlorophyll in skin tissue is degraded, underlying
carotenoids are unmasked and de novo biosynthesis occurs in chromoplasts
leading to development of yellow or orange skin colour, depending on the cultivar
(Selveraj et al., 1982a; Paull, 1993; Paull et al., 2008). Carotenoids are the major
pigments in the skin and flesh of the fruit and variation in carotenoid composition
profiles is often present among cultivars (Chandrika et al., 2003; Wall, 2006). The
relative amounts of different types of carotenoids determine the skin or flesh
colour and its intensity. The flesh colour intensifies parallel to the changes in skin
colour during ripening. The major carotenoids present in yellow-fleshed cultivars
are β-cryptoxanthin, β-carotene, lutein, and ζ-carotene; while red-fleshed cultivars
contain lycopene as the major carotenoid pigment in addition to the presence
of others (Chandrika et al., 2003; Wall, 2006). Lycopene is absent in yellowfleshed cultivars as lycopene β-cyclase catalyses conversion of lycopene to
β-carotene in these cultivars. Recently, two different genes encoding lycopene
β-cyclases (lcy-β1 and lcy-β2) have been characterized from red (‘Tainung’) and
yellow (‘Hybrid 1B’) papaya cultivars (Devitt et al., 2010). The lcy-β2 transcript
levels in ‘Hybrid 1B’ increased about four-fold from colour break to full ripe
stage, while such increase was not noticed in ‘Tainung’ cultivar. The nonfunctional lcy-β2 might be responsible for the accumulation of lycopene in the
red-fleshed ‘Tainung’ cultivar (Devitt et al., 2010). In ‘Golden’ papaya, the
concentrations of the three main carotenoids, all-trans-lycopene, all-trans-βcryptoxanthin, and all-trans-β-carotene, increased by about 2.5-fold during fruit
ripening at 25 °C for 7 days (Fabi et al., 2007). The concentration of total
carotenoids, in general, increases during fruit ripening in papaya (Selveraj et al.,
1982a; Wills and Widjanarko, 1995; Singh and Rao, 2005a; Fabi et al., 2007).
The enzymatic and non-enzymatic modifications in the cell wall carbohydrates
matrix are responsible for the tissue softening during fruit ripening. The
activities of various hydrolytic enzymes such as pectin methylesterase (PME)
polygalacturonase (PG), β-1,4-glucanase, galactosidase, endoxylanase, cellulase,
and proteinase and changes in cell wall composition have been associated with
the fruit softening in papayas (Paull and Chen, 1983; Lazan et al., 1995, 2004; Ali
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et al., 1998; Chen and Paull, 2003; Shiga et al., 2009). Paull and Chen (1983)
reported that PME and cellulase activities gradually increased from the start of the
climacteric rise attaining a peak, two days after the respiratory peak. PG and
xylanase exhibited an increase during the climacteric and showed a close
relationship with the rate of respiration. During the post climacteric phase, the PG
declined to a level of one-quarter of peak activity with xylanase activity returning
to zero, while proteinase activity declined throughout the climacteric and
postclimacteric phases. A recent study has shown that the transcription of a
gene (cpPG) encoding PG was ethylene-dependent, and coincided with the
enhanced PG activity and flesh softening during fruit ripening (Fabi et al., 2009).
Endoxylanase also played a very important role in xylan degradation of the cell
wall matrix during the middle and late phases of fruit softening in papaya, but not
in the early stages (Manenoi and Paull, 2007). The transcripts levels of CpaEXY1
and activity of endoxylanase increased during the climacteric rise in the rates of
respiration and ethylene production during fruit ripening and also coincided with
the firmness decline. The activity of β-galactosidase also increased about four- to
eight-fold during the early stages of fruit ripening and coincided with the ripening
phase when the fruit firmness begin to decline more rapidly (Ali et al., 1998). The
inner mesocarp showed faster fruit softening compared to the outer part and was
attributed to a lack of synchronization of cell-wall degradation processes in these
regions. Furthermore, the activities of PG and β-galactosidase increased with the
increase in tissue depth of mesocarp, resulting in the differential tissue softening
in fruit (Lazan et al., 1995, 2004).
The increased solubility and depolymerization of pectins and hemicelluloses
has been linked with the fruit softening in papayas (Paull et al., 1999; Ali et al.,
2004). During fruit ripening, pectin molecular size decreased with a 6-fold increase
in water-soluble pectin. The hemicelluloses also showed a significant change in the
molecular size with an increase in solubility of hemicellulose in KOH fractions
(Paull et al., 1999). The typical changes occurring in the cell wall of ripening fruit
are increases in the levels of polyuronides (UA, uronic acid) or neutral sugars (NS)
(Lazan et al., 2004). Most of the polyuronides (~75%) were attributable to polymers
that are water-soluble. There was a loss of about 25% of UA from the cell wall
materials during fruit ripening and in the fully ripe fruit, soluble polyuronides
accounted for about 85% of the total polyuronides compared to about 46% and
62% in unripe (5% yellow-skin) and half-ripe (50% yellow-skin) fruit, respectively.
The ratio of UA/NS also decreased from 3.7 to 2.4 during ripening, indicating that
chelator-soluble pectin polymers that were solubilized from alcohol-insoluble
solid contained increasingly greater proportion of neutral sugars (Lazan et al.,
2004). The concurrent solubilization and depolymerization of pectin and
hemicellulose polymers are therefore responsible for the destabilization of the
complex carbohydrates matrix in the fruit cell wall, causing tissue softening.
The soluble sugars levels increase significantly during the final phase (~110
DAA) of fruit development in papaya. Sucrose contributed to about <18% of the
total sugar content at 110 DAA, but increased rapidly to make up 80% of the sugars
at about 135 DAA (Chan et al., 1979). Postharvest fruit ripening involves a small
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change (1–2%) in the soluble sugars levels in papaya fruit (Paull, 1993; Zhou and
Paull, 2001). Papaya cultivars have also been reported to differ in the relative
amounts of sucrose to total sugars. For instance, average sucrose concentration in
‘Kapoho’ cultivar was significantly higher than ‘Sunset’ ‘Waimanalo’, ‘UH 801’
and ‘Line 8’ cultivars. The contribution of sucrose to total sugars either decreased
slightly in some cultivars or remained unchanged in others during the seven days
of fruit ripening period (Zhou and Paull, 2001). Fabi et al. (2007) reported that
sugars composition profile also changed during seven days of fruit ripening in
‘Golden’ papaya; the concentration of sucrose increased during the first three days
of ripening and then showed a continuous decline during the next four days. On the
other hand, the concentrations of fructose and glucose increased throughout the
fruit ripening for seven days in the same cultivar. A large increase in acid invertase
(AI) activity and decreases in activities of sucrose synthase and sucrose phosphate
synthase have been reported during postharvest fruit ripening in papaya (Zhou and
Paull, 2001). The higher AI activity may be associated with the enhanced cleavage
of sucrose into glucose and fructose during papaya fruit ripening (Fabi et al., 2007).
The low titratable acidity (TA) in papaya fruit limits its role in influencing fruit
flavour. A general trend of either slight decrease or no change in TA has been
noticed during fruit ripening in papaya (Selveraj et al., 1982a; Singh and Rao,
2005a; Nunes et al., 2006; Azevedo et al., 2008). Citric and malic acids are the
predominant organic acids that contribute almost equally to the total acidity. The
other organic acids present in minor concentrations include ascorbic, quinic,
succinic, tartaric, oxalic, galacturonic, α-ketoglutaric and fumaric acids (Chan
et al., 1979; Hernandez et al., 2009). Contrary to TA, the concentration of ascorbic
acid increases during fruit ripening in papayas (Selveraj et al., 1982a; Wills and
Widjanarko, 1995; Singh and Rao, 2005a).
The increases in concentrations of ascorbic acid and carotenoids contribute to the
increased hydrophilic and lipophilic antioxidant capacity of fruit a respectively.
Papaya fruit also contains high amounts of benzylglucosinolates (BG) and benzyl
isothiocyanates (BITC) and these compounds are also important from a nutritional
viewpoint. The BG levels increased in the fruit pulp during the late stages of fruit
ripening, while no significant change occurred in BITC (Rossetto et al., 2008). The
ripening process, therefore, improves the nutritional quality of papaya fruit, in
addition to other favourable changes such as skin colour and flesh softening. The
production of aroma volatile compounds during fruit ripening also determines the
flavour of fruit. Many volatile compounds (~300) which contribute to the aroma of
fruit have been identified and quantified from different cultivars (Flath and Forrey,
1977; MacLeod and Pieris, 1983; Heidlas et al., 1984; Flath et al., 1990; Pino et al.,
2003). Linalools, BITC, methyl butanoate and ethyl butanoate were the most
abundant volatile compounds, depending upon the cultivar and geographical region.
For example, in Hawaiian papaya, terpenoids were the most abundant group of
volatile components (81% of the total volatiles) (Flath and Forrey, 1977). The most
abundant aroma volatile compounds, linalool and BITC, in the ripe ‘Solo’ papaya
contributed about 68% and 13% to the total volatiles, respectively. Both compounds
were present in glycosidically bound form in the intact fruit and were released by
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the activity of β-glucosidase enzyme during tissue disruption (Heidlas et al., 1984).
The relative amounts and total concentrations of volatile compounds have been
reported to vary with the stage of fruit ripeness (Katague and Kirch, 1965; Flath
et al., 1990). The concentration of the linalool increased about 400-times from
mature green stage to full ripe stage, while the benzyl isothiocyanate increased
by about seven-fold during the same time (Flath et al., 1990). In a Sri Lankan
cultivar, the principal volatiles (52% of the total volatiles) were represented by
esters; the methyl butanoate being the major aroma compound imparting sweaty
odour to the fruit (MacLeod and Pieris, 1983). The presence of BITC was also
relatively lower (1.5%) compared to 13% in ‘Solo’ papayas (Flath and Forrey,
1977). The Sri Lankan and Hawaiian papaya cultivars thus showed a contrast in the
composition of their aroma volatile profiles. The volatile components of ‘Maradol’
cultivar were also dominated by esters (62, about 40.8% of the total volatiles), with
the major representatives being methyl butanoate and ethyl butanoate (Pino et al.,
2003). The effects of various preharvest and postharvest factors on the production
of aroma volatile compounds in papaya require more investigations.
6.3
Maturity indices
Harvesting at appropriate maturity is important for development of excellent eating
quality of papaya fruit and for better consumer outcomes. Fruit harvested before
optimum maturity fail to ripen properly, with unacceptable skin and flesh colour,
lower SSC and rubbery texture (Paull et al., 1997). The skin colour break is
considered the best harvest maturity index for papaya fruit, depending on the
destination of fruit (Akamine and Goo, 1971a; Selveraj et al., 1982b). The
appearance of yellow string on the blossom-end of the fruit surface indicates
the optimum maturity for long-distance transport. In Hawaii, fruit must reach
11.5% SSC to meet the quality standard and the fruit harvested at 6% surface
coloration have been shown to meet this standard (Akamine and Goo, 1971a).
Abrasion injury is a major problem in fruit harvested at <25% surface colour
(Quintana and Paull, 1993). For local markets, fruit harvesting may be delayed till
the fruit surface is ≥50% yellow. However, the postharvest life of fruit declines
with the on-tree advancement of fruit maturity (Bron and Jacomino, 2006), while
fruit quality in terms of skin and pulp colour and soluble sugars improves with the
delayed harvesting. The susceptibility to fruit fly attack increases when the surface
yellowing exceeds >25% (Seo et al., 1982). The fruit with 40 to 60% skin yellowing
are also more prone to impact and compression injuries (Quintana and Paull, 1993)
and diseases (Alvarez and Nishijima, 1987) during postharvest handling.
6.4
Preharvest factors affecting fruit quality
Orchard practices such as defoliation, fruit thinning and fertilization have
been shown to influence fruit quality in papayas. Leaf pruning of ‘Solo’ papaya to
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15 functional leaves did not adversely affect the production and SSC of the fruit
(Ido, 1976). The removal of 50% of the leaves has been reported not to influence
fruit set and SSC of fruit, but excessive pruning involving removal of 75% of the
leaves reduced the fruit set, flesh dry matter and SSC of ripe fruit. However,
continual defoliation appeared to reduce the supply of photosynthates below the
compensation point resulting in smaller fruit with lower SSC (Zhou et al., 2000).
Fruit thinning is another practice that can ensure an optimum leaf to fruit ratio,
thereby maintaining a favourable source–sink relationship and achieving desirable
fruit size, uniform fruit production and fruit sweetness (Zhou et al., 2000). Fruit
thinning to one fruit per node level has been reported to increase fruit size without
any effect on sugar content of fruit (Martinez, 1988). The beneficial effects of fruit
thinning in ‘Sunset’ papaya included increased new fruit set and higher SSC in
ripe fruit (Zhou et al., 2000). The removal of old fruit stimulated fruit development
and accumulation of sugars in young fruit and also resulted in increased fruit size.
These authors concluded that each mature leaf can provide sufficient photosynthate
for growth and development of about three fruit in the cultivar ‘Sunset’ under
Hawaiian conditions.
A comprehensive study on the effects of fertilization on papaya fruit quality
and the mineral composition of fruit was conducted by Qiu et al. (1995) in Hawaii.
Papaya trees that were six months old and had just begun to flower, received
additional fertilization with CaCO3 (48.5% CaO) at the rate of 192 g tree−1 per
month, KCl (61% K2O) at the rate of 197.5 g tree−1 per month, and urea (46% N)
at the rate of 158.7 g tree−1 per month. The extra application of nitrogen
significantly delayed skin and flesh colour development during fruit maturation
and postharvest fruit ripening. Calcium fertilizer treatment alone or in combination
with potassium significantly increased Ca concentration in the fruit mesocarp,
while potassium fertilization alone reduced the Ca concentration in the fruit
mesocarp. The presence of a higher concentration of Ca in the mesocarp tissue
was positively correlated with higher firmness retention in the ripe fruit. For
instance, Ca and Ca plus K fertilization significantly increased fruit firmness with
an average deformation force of 76 and 78 N, respectively, against 68 N in the
control treatment. Fruit softening was delayed during the ripening process when
the mesocarp Ca concentration was ≥130 μg g−1 FW. The fruit with the mesocarp
Ca concentrations lower than this threshold were more susceptible to soft fruit
disorder. The strategy to increase the Ca concentration in the fruit mesocarp by
spraying papaya fruit six times over 12 weeks with CaCl2 (2% w/v) during fruit
growth and development was not successful (Qiu et al., 1995).
Boron (B) fertilization is also important as its deficiency causes a serious
‘bumpy’ fruit disorder (Wang and Ko, 1975). The localised areas of fruit that are
deficient in boron cease to increase in size, resulting in an uneven growth of fruit.
Proper fertilization of papaya trees during fruit growth and development with
boron alleviates this disorder. Foliar applications of borax (0.25%) and soil
application of 1–3 kg ha−1 elemental boron have been effective in preventing the
development of this disorder (Nishijima, 1993). A boron concentration in the leaf
petiole of more than 25 ppm is essential to reduce the incidence of this disorder. A
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recent study in India has also shown that the fertilization of ‘Surya’ and ‘Red
Lady’ cultivars that were grown in a boron-deficient soil, with 2 kg boron per
hectare (as Colemanite) significantly increased the Ca concentration in fruit
tissue, improved skin smoothness, fruit size and seed health resulting in better
fruit yield and quality (Raja, 2010). The postharvest shelf life in ‘Surya’ and ‘Red
Lady’ cultivars was also enhanced to 10 and 12 days in the B-fertilized fruit
against four and six days in control, respectively.
Preharvest factors also affect the development of superficial skin freckles or
spots on fruit surface. The skin freckles begin to develop in half-mature fruit and
their size and incidence increases during the final phase of fruit maturation. The
exposed fruit surface is more prone to skin freckles. The development of freckles
has been physiologically associated with a rapid growth rate in the final phase of
fruit development, a thicker cuticle, and greater latex soluble solids leading to a
higher osmotic, and hence turgor pressure in the laticifer (Eloisa et al., 1994).
Low and high temperatures, two months before harvest, increased stress leading
to occasional rupture of laticifers and thus development of skin freckles. Bagging
of fruit before the final growth phase significantly reduced freckle incidence and
can be an important control measure to prevent the losses caused by the skin
freckles (Eloisa et al., 1994).
6.5
Postharvest factors affecting fruit quality
6.5.1 Temperature management
The management of storage temperature can regulate the respiratory and ethylene
production behaviour of papaya fruit, thereby offering an opportunity to increase
the postharvest life of fruit through retardation of these processes (refer to section
on respiration and ethylene production). Therefore, low temperature storage is
recommended to retard the physiological activity of fruit and extend the storage/
shipping/shelf life potential. The optimum storage conditions for papaya fruit
(breaker stage) are 7 °C temperature and 85–90% relative humidity (Paull, 1999).
The tolerance of papayas to temperatures below 10 °C varies with the maturity
of the fruit and the duration and temperature of exposure (Chen and Paull, 1986).
For example, mature green fruit can be stored at 10 °C for < 14 days and the fruit
ripen at 22.5 to 27.5 °C in 10–16 days. Fruit at colour break stage can be stored
at 7 °C for 14 days, while quarter-ripe (25% skin yellowing), half-ripe (50%
yellow) and ¾ ripe-fruit (75% yellow) can be stored at 7 °C for less than 21 days
(Paull et al., 1997).
The storage of fruit below optimum temperature or even at optimum
temperature for a longer duration leads to the development of chilling injury
symptoms (Thompson and Lee, 1971; Chen and Paull, 1986; Chan, 1988; Paull
et al., 1997; Wills and Widjanarko, 1997). Symptoms of chilling injury are skin
scald, water soaked areas and hard lumps in the mesocarp and failure to ripen
properly. The fruit also has increased susceptibility to diseases. The development
of water-soaked lesions on the skin appears during cold storage, and the above
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described symptoms manifest and intensify when the fruit are transferred to
ambient conditions for ripening. As the storage temperature decreased from 20 °C
to 5 °C during 7 to 21 days of storage, the number of days required for fruit
ripening and severity of chilling injury also increased when the fruit were
transferred to 20 °C (Wills and Widjanarko, 1997). Chilling injury symptoms have
even been observed on ripe fruit stored for seven days at 15 °C (Nazeeb and
Broughton, 1978). ‘Bentong’ and ‘Taiping’ papaya cultivars stored below 15 °C
for more than seven days developed chilling injury and failed to ripen when
returned to 20 and 25 °C (Nazeeb and Broughton, 1978). However, Thompson
and Lee (1971) reported that Trinidad ‘Solo’ papaya stored at 13 °C for 21 days
ripened normally when transferred to ambient conditions (25–28 °C). These
contradictions in the literature may arise due to the differences in the maturities of
the fruit used in the studies.The susceptibility to chilling injury might have been
enhanced due to the use of mature green fruit in the experiments by Wills and
Widjanarko (1997) and Nazeeb and Broughton (1978). The storage potential of
papaya fruit therefore depends on the fruit maturity, storage temperature and
duration of storage.
A study on the effects of simulated commercial temperature regimes during
air-transport on the quality of papaya fruit throughout the handling chain showed
that the fruit handled in the fluctuating cold or warm temperature regimes normally
experienced during the actual conditions lost more weight, developed objectionable
colour, were softer and more shrivelled, had more decay, and had lower soluble
solids, acidity and ascorbic acid contents than papayas handled in the semiconstant temperature regime (12 °C for 52 h, 8 °C for 24 h and seven days at 20 °C)
(Nunes et al., 2006). A brief exposure of fruit to 1 °C for two hours developed
chilling injury symptoms during seven days of ripening period at 20 °C in ‘Redy
Lady’ papayas handled in a fluctuating cold temperature regime (at 8–15 °C for
76 h). Therefore, proper temperature management without any significant
deviations from the recommended conditions is crucial to provide consumers with
high quality fruit and limit the possibility of rejection of papaya consignments.
Ways to reduce the incidence and severity of chilling injury are discussed in more
detail in section 6.6.1.
6.5.2 Physical damage
The fruit skin in papaya is delicate in nature which predisposes it to physical
damage that leads to excessive weight loss and pathogen infections (Alvarez and
Nishijima, 1987; Quintana and Paull, 1993; Paull et al., 1997). The fruit at ripe
stage show some sunken areas that fail to degreen, called ‘green islands’ that are
primarily caused by mechanical injury. Mechanical injury has been reported in
14.8% of the papaya shipments evaluated in New York markets (Quintana and
Paull, 1993) and is one of the major causes of postharvest losses in papaya (Paull
et al., 1997). The skin injury caused by mechanical damage increased in fruit as it
moved through postharvest handling system. A five-fold increase in the skin
injury was observed from at harvest to after packing stage in a commercial packing
house (Quintana and Paull, 1993). The fruit samples from the sides of field bins
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Postharvest biology and technology of tropical and subtropical fruits
showed skin injury, but it was absent in fruit sampled from the centre of bins. This
study clearly indicated that abrasion and puncture injury were more important
than impact injury for papaya fruit. Fruit with 60% or more colour are susceptible
to impact injury causing internal bruising (Paull et al., 1997). However, the impact
and abrasion injury in papaya fruit did not stimulate the respiration rate or ethylene
production during ripening.
The severity of skin injury could be reduced by waxing of fruit with a carnaubabased wax (FMC-819) or a polyethylene-paraffin wax (FMC-820) applied before
or after heat treatment (Quintana and Paull, 1993). Papaya fruit should be carefully
handled postharvest during all stages of supply chain to prevent bruising on the
fruit surface. Bin liners may be used to reduce the damage. The adoption of bins
made of materials less prone to have rough surfaces can reduce mechanical injury
(Paull et al., 1997). The use of cushioning material in packing can also minimise
bruising damage. The identification of critical points in handling system associated
with mechanical injuries and taking corrective or precautionary measures can
help reduce the qualitative and quantitative losses in papaya fruit.
6.5.3 Weight loss
Weight loss in papayas occurs mainly due to water lost through the skin and the
stem–scar (Paull and Chen, 1989). The amount of weight loss in papayas is
dependent on several factors such as cuticle thickness, fruit maturity, storage
conditions (temperature and relative humidity) and postharvest treatments (Paull
and Chen, 1989; Singh and Rao, 2005a; Nunes et al., 2006). Weight loss symptoms
in papaya fruit are shrivelling, low gloss, and rubbery texture. The loss of about 8%
of initial weight from ‘Sunset’ and ‘Sunrise’ papayas harvested at mature green
stage produced these symptoms and rendered the fruit unacceptable (Paull and
Chen, 1989). The cuticle thickness has been found to decrease when the fruit skin
colour changes from half yellow to yellow. Therefore, fruit harvested at advanced
maturity exhibit more weight loss than the less mature fruit during the postharvest.
The maintenance of high relative humidity (>90%) has been reported to be
beneficial to reduce postharvest weight loss, thus preventing the development of
associated symptoms and resulting marketing losses (Nunes et al., 2006). Modified
atmosphere packaging (MAP) is another effective way to reduce the weight loss of
papaya fruit (Paull and Chen, 1989; González-Aguilar et al., 2003; Singh and Rao,
2005a, 2005b). The weight loss in papaya fruit during two weeks at 10 °C plus two
days at ambient could be reduced by 14–40% in waxed fruit and by about 90% in
plastic shrink wrapped fruit (Paull and Chen, 1989). The packaging of individual
papaya fruit harvested at colour-break stage in a 25 μm thick low density
polyethylene (LDPE) film or Pebax-C® film significantly reduced the weight loss
during 14 days storage at ambient conditions (27–32 °C and 50–55% RH) and 20
days storage at 13 °C plus seven days at 20 °C (Singh and Rao, 2005a). The
individual shrink-wrapping of ‘Solo’ papayas in Cryovac® D-955 or LDPE (25 μm
thick) significantly reduced the weight loss to <2% during 14 days storage at
ambient conditions (27–32 °C and 50–55 % RH) against 20% weight loss in
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control fruit (Singh and Rao, 2005b). The development of off-flavour caused
by accumulation of CO2 concentration higher than 7–8% has also been reported in
papaya fruit wrapped in plastic films (Paull and Chen, 1989; Singh and Rao,
2005a). The adoption of MAP technology should therefore be considered carefully
to prevent the build-up of CO2 concentration and/or exhaustion of O2 below a
certain level capable of inducing anaerobiosis.
6.5.4 Storage atmosphere
Modified/controlled atmospheres (MA/CA) have been used for long distance
shipping of tropical fruits. The modification of storage atmospheres with low O2 and
high CO2 extends the storage/shipping potential of tropical fruits including papaya
(Akamine, 1959; Hatton and Reeder, 1969; Maharaj and Sankat, 1990; Cenci et al.,
1997). The ideal storage atmospheres for papaya fruit should range between 2–5 kPa
O2 and 5–8 kPa CO2 (Yahia, 1998; Yahia and Singh, 2009). However, the response
of fruit to CA/MA depends on several other factors including cultivar, fruit maturity
and storage temperature (Yahia, 1998). Delayed fruit ripening and reduced decay in
papaya were the major benefits associated with CA/MA. ‘Solo’ papaya fruit kept in
10 kPa CO2 at 18 °C for six days showed less decay compared to those held in
normal air or atmospheres with higher levels of CO2 (Akamine, 1959). Papaya fruit
held in 1 kPa O2 and 3 kPa CO2 at 13 °C for three weeks and then ripened at 21 °C
showed 90% acceptability, whereas 10% of the fruit held in normal air for the same
duration were acceptable (Hatton and Reeder, 1969). Delayed fruit ripening in
‘Bentong’ and ‘Taiping’ cultivars in Malaysia was achieved by removal of ethylene
from the storage atmosphere and enrichment of the storage atmospheres with 5%
CO2 at 15 °C for about 25 days (Nazeeb and Broughton, 1978). The removal of CO2
from the storage atmosphere accelerated the onset of ethylene rise in both cultivars
and fruit ripened at a faster rate. Maharaj and Sankat (1990) reported that ‘Known
You No. 1’ and ‘Tainung No. 1’ papayas at the colour break stage could be stored for
29 days in atmospheres containing 1.5–2.0% O2 and 5% CO2 at 16 °C, compared to
17 days in air. Similarly, the storage life of ‘Sunrise’ papaya could be extended to
31 days at 10 °C in atmospheres containing 8% CO2 and 3% O2 and fruit ripened
normally in five days at 25 °C (Cenci et al., 1997). In addition to delayed fruit
ripening, CA/MA have been found very effective to alleviate chilling injury in some
tropical fruits (Yahia, 1998). However, CA containing low O2 (1.5 to 5%) with or
without high CO2 (2 or 10%) did not reduce chilling injury symptom development
in ‘Kapoho’ and ‘Sunrise’ papayas (Chen and Paull, 1986). The exposure of ‘Sunrise’
papayas to an insecticidal atmosphere (0.17 to 0.35 kPa O2, balance is N2) up to
five days at 20 °C delayed fruit softening without any external or internal injury. The
development of a very weak fermentative odour occurred after three days and its
intensity increased with the increase in exposure period to low O2 (Yahia, 1991,
1993; Yahia et al., 1992). Therefore, the tolerance to these insecticidal atmospheres
was less than three days for papaya fruit.
Hypobaric storage has also been suggested to extend storage life and reduce
the decay incidence in papaya. The exposure of papaya fruit to subatmospheric
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pressure (20 mm Hg, 10 °C and 90–98% RH) for 18–21 days during shipment in
hypobaric containers from Hawaii to Los Angeles and New York inhibited both
fruit ripening and disease development (Alvarez, 1980). The storage conditions
after removal from the hypobaric containers did not affect the ripening process.
The disease incidence was significantly reduced in the hypobaric-stored fruit;
these fruit had 63% less peduncle infection, 55% less stem-end rot and 45% fewer
fruit surface lesions than those stored in a refrigerated container at normal
atmospheric pressure (Alvarez, 1980). The inhibitory effect of hypobaric storage
on the disease development in papaya was further confirmed by Chau and Alvarez
(1983). The fruit artificially inoculated with Colletotrichum gloesporioides held
at 15 mm Hg at 10 °C for three weeks, and then ripened at ambient conditions for
five days showed less anthracnose than the control fruit. A significant delay in the
infection progress on fruit has been reported under hypobaric conditions (Chau
and Alvarez, 1983).
The gaseous atmosphere surrounding a fruit can also be modified passively or
actively by packaging of an individual or a group of fruit in polymeric films. In
passive MAP, the O2 levels are depleted and the CO2 levels increase inside the
package due to fruit respiration. Active MAP involves the flushing of a mixture of
gases with desired concentrations so that an equilibrium modified atmosphere is
established rapidly. MAP also enriches the surrounding atmosphere with high
humidity that results in reduced water loss, increased shelf life and better textural
properties (Singh, 2010). The compositions of atmosphere inside the MA packs is
dependent upon several factors such as film permeability to O2, CO2, and water
vapour, produce respiration and the influence of temperature on these processes.
Therefore, choice of an appropriate packaging film is a key factor in order to
maintain optimum MA.
The use of polymeric films to achieve atmospheric modification has been
demonstrated to extend the storage life of papaya fruit (Paull and Chen, 1989;
González-Aguilar et al., 2003; Singh and Rao, 2005a; Singh and Rao, 2005b).
MAP of papaya with Cryovac® D-955 film increased the shelf life of fruit to
two weeks at room temperature (26–32 °C, 32–45% RH) and up to four weeks at
18 °C, 72–80% RH (see Plate XI in the colour section between pages 238 and
239). MA (3–5 kPa O2 and 6–9 kPa CO2) were created inside the LDPE (43 μm
thick) package during storage of ‘Sunrise’ papayas at 10 °C for 32 days and did
not result in any off-flavour development (González-Aguilar et al., 2003).
Similarly, the individual packing of ‘Solo’ papaya in a LDPE (25 μm thick) film
created the MA containing CO2 and O2 levels in the range of 3–7% and 8–15%,
respectively at 7 or 13 °C during 20–30 days storage, while Pebax-C® film
maintained 2–3% CO2 and 10–12% O2 (Singh and Rao, 2005a). The internal
concentration of CO2 more than 7–8% resulted in development of off-flavour
when the fruit were wrapped in plastic films (Paull and Chen, 1989). The range of
CO2 concentration (8–10%) was slightly higher in the atmospheres generated
inside the polypropylene (PP) film compared to LDPE and Pebax-C® films (Singh
and Rao, 2005a). Thus, the development of off-flavour has been reported in the
fruit packed in the PP film. The concentration of gases in the MA packs and their
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effects on fruit flavour should therefore be closely monitored by considering the
multiple variables such as fruit maturity, storage temperature and potential
temperature fluctuations in the supply chain.
6.6
Physiological disorders
6.6.1 Chilling injury
Chilling injury is the major postharvest physiological disorder limiting the storage
and transport of papaya fruit at low temperature. Symptoms of chilling injury and
a discussion of the effects of low temperature storage on development of the
disorder can be found in section 6.3. The recommended conditions for papaya
fruit storage are: temperature 7–13 °C relative humidity 90–95% (Paull, 1999;
Chen et al., 2007). However, chilling injury can occur under these conditions, as
the duration of exposure to low temperature and level fruit maturity determine its
incidence and severity in papaya fruit (Wills and Widjanarko, 1997; GonzálezAguilar et al., 2003; Singh and Rao, 2005a). This section will mainly focus on the
strategies that have been tested to alleviate chilling injury in papaya fruit.
The best method to alleviate chilling injury is to avoid the exposure of fruit to
the storage temperatures and durations that cause it. Optimum storage conditions
are primarily governed by the fruit maturity. The chilling tolerance in papaya fruit
increases as the fruit ripens (Chen and Paull, 1986). For instance, fruit harvested
at mature green stage should not be stored at temperatures below 10 °C and should
not be stored for more than 14 days at 10 °C. The fruit showing one-quarter, half
and three-quarters skin yellowing should not be stored at 7 °C for more than
21 days (Paull et al., 1997). Fully-ripe fruit can be stored at 1–3 °C for more than
a week (Chen et al., 2007). Short-term exposure of less ripe papaya fruit to very
low temperatures during commercial handling can also induce chilling injury
symptoms in the fruit that were otherwise kept at optimum storage conditions
(Nunes et al., 2006). Temperature fluctuation during postharvest handling should
therefore be minimised to prevent the fruit quality losses.
Postharvest heat treatments and temperature preconditioning have been known
to enhance the chilling tolerance of papaya fruit. Pérez-Carrillo and Yahia (2004)
showed that postharvest exposure of papaya fruit to dry air (50% relative humidity)
at 48.5 °C for 4 h was safer than the moist air (100% relative humidity) at the same
temperature. The dry air treatment alone or in combination with a fungicide,
thiabendazole, decreased the severity of the chilling injury and inhibited fungal
growth in ‘Maradol’ papaya during six weeks storage at 5 °C (Pérez-Carrillo and
Yahia, 2004). The temperature preconditioning involves the exposure of the fruit
to temperatures slightly above the critical chilling temperature for a short duration
and that increases the resistance in fruit during subsequent storage at lower
temperature. The temperature preconditioning increased the fruit ripeness in
papaya which imparted tolerance to chilling stress as the sensitivity of fruit to
chilling injury depends on the fruit maturity (Chen and Paull, 1986; Chan, 1988).
The modification of storage atmosphere with low O2 alone or in combination with
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high CO2 has not been beneficial to alleviate the chilling injury in papaya fruit
(Chen and Paull, 1986). MAP, seal packaging, waxing and treatment with methyl
jasmonate have been found to be effective in alleviating chilling injury in papaya
fruit. The postharvest treatment of ‘Sunrise’ papaya fruit with methyl jasmonate
vapours (10−5 or 10−4 M) for 16 h at 20 °C has been reported to reduce chilling injury
during 14–32 days storage at 10 °C and four days shelf life at 20 °C (GonzálezAguilar et al., 2003). The combination of MAP using LDPE film with methyl
jasmonate treatment significantly improved the quality retention of fruit with lower
severity of chilling injury and better visual quality of fruit. Similarly, the individual
packing of ‘Solo’ papaya in LDPE or Pebax-C® film significantly reduced the
incidence of chilling injury during 30 days of storage at 13 °C and the fruit ripened
normally in seven days at 20 °C, while non-packed fruit showed these symptoms
after 14 days of storage at 13 °C (Singh and Rao, 2005a). Though, MAP also
prevented the appearance of chilling injury in these fruit during cold storage at 7 °C
for 30 days, but the fruit failed to ripen properly showing uneven surface colour
development and skin bronzing when these were transferred to ambient conditions.
Individual shrink wrapping of fruit with LDPE or Cryovac® D-955 film significantly
alleviated chilling injury symptoms in ‘Solo’ and ‘Red Lady’ papayas during four
weeks storage at 13 °C (Singh and Rao, 2005b; Sudhakar Rao unpublished). These
studies suggest that maintenance of high humidity by MAP could be the major
factor contributing to the alleviation of chilling injury in papaya fruit.
6.6.2 Soft fruit
Soft fruit is another problem encountered during the payapa postharvest supply
chain. The problem of soft fruit is sporadic in nature and appears most commonly in
the autumn in Hawaii, USA. Some batches of fruit ripen very rapidly, limiting the
long–distance transport and marketing of fruit (Paull et al., 1997). This problem may
be caused by bruising or crushing injury during handling, or, as mentioned in
section 6.4, by low Ca levels in the fruit (Qui et al., 1995; Paull et al., 1997). The
concentration of Ca in the fruit mesocarp has been positively correlated with the
retention of firmness during ripening. Fruit with high Ca concentration (>130 μg g−1)
are expected to be less susceptible to soft fruit problem (Qui et al., 1995). The uptake
of Ca in papaya fruit is influenced by the preharvest environmental conditions and
the fertilisation (Qui et al., 1995). The Ca concentration generally varies with the
harvest date and it can result in the problem of soft fruit only in a particular season.
Soil application of Ca is effective to raise its levels in the mesocarp and can possibly
reduce the problem of soft fruit. Careful postharvest handling to prevent bruising and
compression can also be helpful to mitigate the soft fruit.
6.7
Postharvest pathological disorders
Postharvest losses due to diseases can reach about 93% depending upon the
postharvest handling and packing procedures (Alvarez and Nishijima, 1987). The
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inspection of shipments that had arrived in New York terminal markets revealed
that anthracnose rot caused by Colletotrichum gloeosporiodes affected 62% of
shipments, while other diseases such as Rhizopus rot, stem-end rot and grey
mould affected about 35% of the shipments (Cappellini et al., 1988). Californian
inspections also showed that 73% of the inspected cartons have decay and mould
growth and 52% had sunken defects (Paull et al., 1997). These statistical figures
on postharvest losses caused by decay in papaya clearly demonstrate the scale and
importance of this problem.
Postharvest diseases of papaya fruit can be categorised into three groups: fruit
surface rots, stem-end rots and internal fruit infections (Alvarez and Nishijima,
1987). The fruit surface rots are further of two types depending upon the nature
of pathogen. The first group includes the fungal pathogens which invade the
immature fruit still attached to the plant in the orchard. Anthracnose, chocolate
spot, Phytophthora fruit rot and Cercospora black spot are the examples of this
group. The symptoms of Phytophthora rot and Cercospora black spot are not a
major problem during postharvest handling of fruit because the symptoms appear
on fruit before harvest and the fruit can be culled in the packinghouse. The second
group of surface rots includes the weak fungi that infect the fruit through various
wounds or injuries occurring during and after harvest, for example, Mycosphaerella,
Phomopsis, Alternaria, Fusarium and Guignardia (Alvarez and Nishijima, 1987).
Anthracnose rot, caused by a fungus Colletotrichum gloeosporiodes (Penz.)
Sacc., is the major postharvest disease in papaya fruit. The infection occurs on the
developing fruit in the field, but it remains latent until the fruit begin to ripen. The
symptoms appear on fruit surface in the form of round, water-soaked and sunken
spots. Several small spots may enlarge and coalesce to form a bigger lesion that can
be as large as 5 cm in diameter. These lesions are light brown to salmon colour
initially and eventually turn dark brown or black. Although they are sunken, they
rarely extend deep into the flesh tissue. Chocolate spot is also caused by C.
gloeosporiodes, but the initial symptoms are superficial reddish brown lesions. These
lesions become sunken with water-soaked margins as the fruit ripening progresses.
Among the other surface rots, dry rot and wet rots are caused by Mycosphaerella
sp. and Phomopsis sp., respectively. The dry rot symptoms include surface lesions
with brown and translucent margins. The wet rot causing Phomopsis sp. is
frequently associated with the stem-end rots. The other fungi causing stem-end rot
are Botryodiplodia theobromae, Alternaria alternata, and Mycosphaerella sp.
The spores of these fungi enter through the crevices between the fruit peduncle
and the flesh, producing stem-end rot symptoms. The affected area is soft and
translucent, and the rot progresses rapidly from the surface into the fruit cavity.
Another common fruit rot is caused by a fungus, Rhizopus stolonifer, which
penetrates through wounds and causes fruit rotting without affecting the fruit
cuticle. It is considered one of the most destructive postharvest fungal pathogens
of papaya fruit due to its ability to rapidly develop and spread. Internal fruit
infections are mainly caused by fungi such as Penicillium sp., Cladosporium sp.,
and Fusarium spp. These fungal pathogens make their entry into seed cavity
through the small narrow passage at the blossom end. The seed cavity of the
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infected fruit is filled with fungal spores, seeds and the internal flesh is damaged.
These fruit also show uneven ripening behaviour (Alvarez and Nishijima, 1987).
Internal yellowing is a bacterial disease of ripe papaya fruit that is caused by the
enteric bacterium Enterobacter cloacae. The fruit infected with E. cloacae can
affect the coliform bacterial counts in the fresh-cut products.
6.7.1 Postharvest disease management
Orchard sanitation is the most important consideration in reducing the pathogen
inoculum from the field. The discarded and infected fruit should be removed
immediately to control spread and survival of the pathogen. The preharvest
application of fungicide sprays is an effective approach to reduce the incidence of
postharvest diseases. The protective fungicides should be sprayed once every
7–14 days in rainy season when high disease pressure is expected, but the spray
intervals can be increased to 14–30 days in dry conditions (Alvarez and Nishijima,
1987). Packinghouse hygiene and decontamination of packing line equipment is
essential to prevent the build-up of inoculum and re-infection of the fruit subjected
to disease control measures. There should be utmost care during harvest and
postharvest handling to minimise the physical damage to fruit. The mechanical
injuries caused during different packinghouse operations provide excellent
opportunities for the entry of wound pathogens that causes serious fruit rot
problems. The use of plastic liners for field bins, proper fruit arrangement and
cushioning material in the box, and avoiding impact and compression damages
during handling could be some of the ways to reduce the fruit rots problems
caused by secondary pathogens (Paull et al., 1997).
Postharvest temperature management is an important key to prevent
the development of latent infections and also to suppress the growth and
development of wound pathogens. The conditions favourable for slow ripening of
fruit (15–18 °C) allow the latent fungus to grow appreciably. Therefore, rapid
postharvest cooling of fruit to 13 °C before cold storage or transport and then
faster ripening of fruit at 22.5 to 27.5 °C can be helpful to reduce the losses caused
by diseases. Postharvest heat treatments have been used to meet quarantine
requirements either to eliminate fruit fly eggs and larvae or to control diseases
(Nishijima, 1995). Single hot water dip treatment (49 °C for 20 min) has been very
promising to control postharvest diseases in papaya fruit without detrimental
effect on fruit quality. The quarantine treatments aimed to control fruit fly such as
forced air dry heat (FADH) and vapour heat (VH) also provide some control of
postharvest diseases. Moreover, the single hot water dip before or after FADH and
VH treatments can provide additional disease control to the same level as provided
by the combination of fungicide thiabendazole with FADH and VH treatments
(Nishijima et al., 1992). The double hot water dip treatment (42 °C for 30 min and
then 49 °C for 20 min) that was used as a quarantine treatment in Hawaii from
1984 to 1992, also provided a good control of postharvest diseases, but the fruit
quality was affected due to high levels of heat stress, which were close to the
maximum that papaya fruit can tolerate.
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Though fungicide treatments are the most effective and least expensive method
to control postharvest diseases in fruit (Nishijima, 1995), there are increasing
concerns about the harmful effects of the fungicides on human and environmental
health. As a result, the consumers demand residue-free fruit and the regulatory
authorities have discontinued the use of several fungicides on fruits. Fungal
pathogens have also developed resistance to synthetic fungicides due to their
continuous use for several years. All these reasons have spurred investigations into
alternative methods of disease control in papaya fruit. There is increasing interest
in the exploitation of antibacterial and antifungal properties of essential oils,
chitosan and other generally recognised as safe (GRAS) compounds to control
postharvest diseases in fruits. A recent study has shown that incorporation of
essential oils of thyme and Mexican lime with fruit coating material reduced the
incidence of anthracnose and Rhizopus rot in ‘Maradol’ papayas (Bosquez-Molina
et al., 2010). The fruit immersed in mesquite gum emulsion and formulated with
both the essential oils, it was possible to reduce the disease incidence caused by C.
gloeosporioides by 100% and Rhizopus rot by 60% with the thyme (0.1%) and
Mexican lime (0.05%) essential oils. The postharvest treatment of papaya fruit
with 1.5% chitosan before inoculation with C. gloeosporioides also provided
adequate control of anthracnose during five days of shelf life at ambient conditions
(Bautista-Baños et al., 2003). Gamagaea et al. (2003) reported that the postharvest
application of sodium bicarbonate (2%) reduced the incidence and severity of
anthracnose disease during 14 days storage at 13.5 °C and subsequent two days
shelf life at 25 °C. The efficacy of sodium bicarbonate to control anthracnose
increased when combined with a biocontrol agent, Candida oleophila (yeast; strain
1–182). The incorporation of sodium bicarbonate with a paraffin-wax and C.
oleophila has been suggested to be the best combination to control anthracnose
(Gamagaea et al., 2003, 2004). The survival of the biocontrol agent in 2% sodium
bicarbonate-incorporated wax coating was not adversely affected (90%) during 14
days storage at 13.5 °C. Another study on testing the efficacy of antagonistic yeasts
against the Colletotrichum gloeosporiodes showed that fruit treated with the
Cryptococcus magnus at concentrations of 107 to 108 cells ml−1, as early as 24 h,
preferably 48 h, before inoculation with the pathogen reduced the development of
disease (de Capdeville et al., 2007). The research on the application of biocontrol
agents and alternative disease control methods has gained momentum in the recent
past and appears to be promising for commercial situations in the near future.
6.8
Postharvest insect pests and phytosanitary treatments
The international trade in papaya is constrained by the problem of regulated
insect-pests. Fruit flies are the predominant insect-pests species of global concern.
The major fruit flies species that infest papaya include Mediterranean fruit fly
(Ceratitis capitata, Wiedemann), oriental fruit fly (Bactrocera dorsalis, Hendel),
and melon fly (Bactrocera cucurbitae, Coquillett) (Seo et al., 1974, 1982;
Armstrong et al., 1989). Fruit harvested after colour break stage are more prone
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to fruit fly infestation (Seo et al., 1982). The presence of the regulated insect-pests
hampers the marketing of fruit not only between countries, but also between
geographical areas within countries (e.g. Florida to California; Hawaii to the U.S.
mainland; Queensland to Victoria, Australia; Okinawa to the Japan mainland)
(Follett and Neven, 2006). Therefore, the strict phytosanitary procedures are
followed to eliminate, sterilise, or kill the regulatory pests in exported papayas to
prevent their introduction and establishment to new areas in countries such as the
U.S.A. (mainland), Japan, Australia and New Zealand. Heat treatments and
irradiation are legally approved and commercially used for papaya exports
depending upon the destination country.
6.8.1 Heat treatments
Heat treatments have been widely used and accepted measures of providing
phytosanitary security against a number of insect-pests in tropical fruits (Paull
and McDonald, 1994). The other benefits of postharvest heat treatments include
disease control and modification of the ripening and storage behaviour of fruits
(Paull and Chen, 2000 and references therein). All quarantine treatments including
heat must provide probit 9 quarantine security (99.9968% mortality); this implies
that no more than 32 individuals will survive from a treated population of
1 000 000 at the 95% confidence limit (Sharp, 1993).
The application of heat can be achieved by different methods that include hot
water immersion, dry hot air and vapour heat (VH) treatments. Akamine and
Arisumi (1953) showed that single hot water dip (49 °C for 20 min) can control
postharvest diseases in papaya fruit and it has been used by the papaya industry
since then. This treatment was modified into double hot water immersion (Couey
and Hayes, 1986) and was also accepted by the USDA–APHIS as a quarantine
treatment in 1990. The first step involved hot water dip of one-quarter ripe fruit
for 30 min at 42 °C followed by the second step of 20 min at 49 °C, and then
immediate cooling of fruit to less than 30 °C with ambient water dips or sprays.
There were several issues related to the effectiveness of this treatment against the
larvae present in deep pulp (Hallman, 2000) and the quality of fruit (Paull and
Chen, 1990). The one-quarter or less ripe fruit are expected not to be containing
the third instar larvae of fruit flies. However, the blossom end defects allowed the
fruit flies to oviposit in the papaya at an earlier stage than usual and for larger
larvae than first instar to be found deeper in the pulp than usual (Zee et al., 1989),
rendering the double dip hot water treatment ineffective to meet the quarantine
requirements. The treated fruit also failed to ripen properly due to uneven or overheating with hard lumps in the mesocarp and poor skin colour (Paull and Chen,
1990). The double hot water dip treatment significantly altered the physiology of
fruit ripening in papaya and the major processes affected by this treatment
included skin colour development, respiration, ethylene production and its
biosynthetic pathway, and flesh carotenoid biosynthesis (Chan, 1986; Paull and
Chen, 1990). The sensitivity of fruit to heat is also determined by several other
factors. For example, fruit harvested in winters showed more sensitivity to heat
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treatments. The fruit with lower calcium concentration in the mesocarp and
exposed to a three day preharvest mean low temperature higher than 22.4 °C did
not show heat sensitivity (Paull, 1995). The sensitivity of papaya fruit to heat
decreased due to accumulation of heat shock proteins caused by a preconditioning
treatment of 4 h at 42 °C or 1 h at temperatures >35 °C followed by 3 h at 22 °C
(Paull and Chen, 1990). The research interest in hot water treatments for quarantine
purposes has diminished due to their lack of approval by various quarantine
regulatory authorities in the world.
VH treatment involves heating fruit with warm air saturated with water vapours
at temperatures between 40 and 50 °C to kill insect eggs and larvae. Seo et al.
(1974) reported probit 9 security against oriental fruit fly (Bactrocera dorsalis)
eggs and larvae in Hawaiian papayas using an average 11-hour approach time to
raise the fruit temperatures from 23.3 to 44.4 °C, followed by maintaining the fruit
temperatures at 44.4 °C for 8.75 hours. According to the current VH treatment
protocol, the centre of fruit requires to reach 47.2 °C in >4 h, with 90–100%
relative humidity in the last hour of the treatment, followed by the rapid cooling
of fruit to <30 °C within an hour. The longer duration of VH treatment is
disadvantageous from operational efficiency perspective. The VH treatment is an
approved quarantine treatment for imports of Hawaiian papayas into the mainland
U.S.A. and Japan (Armstrong and Mangan, 2007).
Hot air treatment involves exposure of fruit to hot air in a closed system and is
similar to VH treatment except the relative humidity is relatively lower to avoid
water condensation on fruit. High temperature forced air (HTFA) treatment (43–
49 °C) provided phytosanitary security in Hawaiian-grown papayas against the
eggs and larval stages of Mediterranean fruit fly, melon fly, and oriental fruit fly.
The fruit that were exposed to forced hot air at 49 ± 0.5 °C with 40–60% relative
humidity reached the 47.2 °C temperature at their centre in less than one hour
(Armstrong et al., 1989). The HTFA is an approved quarantine treatment for
papaya imports in the USA, Japan and New Zealand (Armstrong and Mangan,
2007). However, the papaya imports into New Zealand from the Pacific region
(Fiji, Samoa, Tonga, and Cook Islands) require the fruit to be held at 47.2 °C for
an additional 20 min. The HTFA is less detrimental to fruit quality compared to
vapour heat and hot water immersion treatments. The exposure of papaya fruit to
moist air (100% relative humidity) at 48.5 °C for 4 h produced more fruit damage
compared to dry air (50% relative humidity) treatment for the same duration at the
same temperature. Unlike double dip hot water treatment, there is no restriction
on the ripeness of fruit to be exposed to the HTFA, but best results are obtained
when the fruit are treated at colour break to 1/8-colour stage (Cavaletto, 1989,
cited in Moy, 1993).
6.8.2 Irradiation
Irradiation is an ideal technology for developing ‘generic’ quarantine treatments
because it is effective against most insect and mite pests at dose levels that do not
affect the quality of most commodities (Follett, 2009). According to Follett and
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Neven (2006), ‘a generic treatment is a single treatment that controls a broad
group of pests without adversely affecting the quality of a wide range of
commodities’. In 2006, low-dose generic radiation of 150 Gy for tephritid fruit
flies and 400 Gy for all insects, except pupa and adult stages of Lepidoptera, was
approved for the first time by the U.S. Department of Agriculture–Animal and
Plant Health Inspection Service (USDA–APHIS, 2006). In 2009, the International
Plant Protection Convention (IPPC) approved and annexed the generic dose of
150 Gy for all tephritid fruit flies to International Standards for Phytosanitary
Measures No. 28, Phytosanitary treatments for regulated pests (IPPC 2007, cited
in Follett, 2009). A practical advantage of generic treatments is that if a new fruit
fly species or other quarantine pest should invade a new area, the fruit export trade
using radiation as a disinfestation treatment would not be interrupted because the
generic doses also would apply to the new invasive species (Follett, 2009).
The ban on the postharvest use of chemical fumigants such as methyl bromide
and the quality concerns from heat treatments increased the scope and prospects
of irradiation as a phytosanitary treatment (Moy and Wong, 2002). Papayas
irradiated with 400 Gy have been exported from Hawaii to the U.S.A. mainland
since 2000 and this dose provided phytosanitary security against all tephritid fruit
flies, white peach scale (Pseudaulacaspis pentagona, Targioni Tozzetti), and
mealybug (Paracoccus marginatus, Williams & Granara de Willink) (Follett,
2009). Australia also exports papaya to New Zealand after radiation treatment at
250 Gy, a generic dose developed for their specific quarantine pests (Follett,
2009). A variation in the absorbed doses of radiation is often encountered under
commercial situations. The doses are generally higher on the outside of the stack
and lower in the centre of the stack. A recent study on dose mapping showed that
dose variation (ratio of maximum to minimum values) was about 1.3 for papaya
fruit packed alone and 1.37 for a mixture of papaya, longan and banana (Follett
and Weinert, 2009). The treatment of papaya alone or in a mixed fruit box resulted
in the absorption of the required minimum dose of 400 Gy. This study shows that
there is a great scope of using generic irradiation treatments for single and mixed
loads of fruits.
Irradiation has shown to be superior to several thermal treatments as a
phytosanitary measure for papayas and other tropical fruits in efficacy, product
quality and economics (Moy, 1993). Many researchers have reported the beneficial
response of papaya fruit to irradiation doses in the range of 250 to 1000 Gy (Balock
et al., 1966; Akamine and Goo, 1971b; Akamine and Moy, 1983; Paull, 1996; Zhao
et al., 1996; Camargo et al., 2007). These doses are sufficient to meet quarantine
requirements without any adverse effect on fruit quality. The response of papaya
fruit to irradiation depends upon several other factors such as fruit maturity, amount
of dose absorbed, preharvest and postharvest conditions. Papaya can tolerate
radiation dose up to 1000 Gy without any surface scald (Zhao et al., 1996). Sensory
and nutritional quality of most of the tropical fruits including papayas is not
affected by irradiation treatment (Moy, 1993; Moy and Wong, 2002).
Harvest maturity is an important factor that governs the potential effects of
irradiation treatment on the rate of fruit ripening, especially the fruit softening
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(Paull, 1996). Paull (1996) reported that irradiation (250 Gy) of 30% yellow fruit
was effective in delaying skin yellowing, fruit softening and flesh colour
development. However, the fruit irradiated at colour break stage (<10% yellow)
with 250 Gy dose exhibited increased rates of respiration and ethylene production
and faster skin yellowing compared to non-irradiated fruit. The radiation doses
higher than 250 Gy (up to 1.5 kGy) have been tested on Hawaiian papayas.
‘Sunset’ papayas at 25 to 30% yellow stage, when irradiated with 0.5 to 1.0 kGy,
had less pectic depolymerisation, and firmer texture at ripe stage than nonirradiated ones (Zhao et al., 1996). The firmness of these irradiated fruit was
retained for two days longer than the non-irradiated control. However, the higher
doses of irradiation (1.5 kGy) induced solubilisation of pectic substances and
resulted in premature softening of fruit. In conclusion, the irradiation treatment
with a dose of up to 1.0 kGy does not adversely affect the fruit quality in papayas
treated at 25–30% skin colour stage.
It is common to adopt a single postharvest phytosanitary treatment to prevent
the introduction of exotic pests into the new regions. However, alternative
approaches such as combination treatments, non-host status, identification of pest
free areas, pest eradication, system approaches, and special inspection procedures
can also provide the basis for establishing phytosanitary security (Follett and
Neven, 2006).
6.9
Postharvest handling practices
6.9.1 Harvest operations
Fruit maturity in papaya is mainly judged by the skin colour. The fruit at colour
break stage are harvested by hand. The fruit pickers keep them in shoulder bags
and then they are placed into plastic crates or field bins. There is a great need to
educate fruit pickers to handle the fruit as gently as possible. Scars from pickers’
nails may become apparent injury when the fruit is ripe. The field bins and crates
should preferably be lined with plastic liners and efforts should be made to
minimise the bruising injury to the fruit. After harvesting, the fruit are transferred
to the packinghouse for further operations.
6.9.2 Packinghouse practices
The typical packinghouse operations followed in Hawaii are summarised in
Fig. 6.2 (Paull et al., 1997). The fruit received in the packing shed are washed with
a sanitising agent and sorted for colour, size, and defects. Culling involves the
removal of fruit with defects such as mechanical injuries, deformities, scars, overripeness and size. After culling, the fruit are subjected to disinfestation treatment
involving vapour heat. Vapour heat treatment involves increasing the fruit centre
temperature to 47.5 °C over a period of 6–8 h, followed by cooling of fruit to
< 30 °C with water (Paull et al., 1997). A very rapid cooling after heat treatment
should be avoided to prevent the development of skin scald. After disinfestations
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Fig. 6.2 A flow diagram of the postharvest handling system of fresh papaya in Hawaii.
The time scale on the left hand side is for the movement of fruit if there is no holding of
fruit before the next step. [Adapted from Paull et al., 1997, with permission from Elsevier
Ltd., UK].
treatment, the fruit are waxed and treated with fungicide before packing and sealing
into 4.54 kg cartons. The bottom of cartons should be cushioned with foam mesh
sleeves and foam padding; additionally, paper wrapping is important to prevent
abrasion injury and development of ‘green islands’ on fruit. These cartons are then
ready for air or surface shipment at 8–10 °C.
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According to CODEX standard for fresh papaya (Codex, 2001; amended
2005), fruit can be graded into three classes: Extra class, Class I, and Class II.
Extra class fruit must be of superior quality and free from defects, with the
exception of very slight superficial defects, provided these do not affect fruit
appearance, quality, keeping quality and presentation in the package. Class I fruit
must be of good quality, but with slight defects on the skin due to mechanical
bruising and other superficial defects not exceeding 10% of the total surface area.
Class II includes the fruit which do not qualify for the higher classes, but should
meet minimum quality requirements including the total surface area with defects
not to exceed 15%. There are ten size codes depending upon the fruit weight (in
grams): A 200–300; B 301–400; C 401–500; D 501–600; E 601–700; F 701–800;
G 801–1100; H 1101–1500; I 1501–2000; J >2001.
6.9.3 Control of ripening and senescence
Papaya fruit ripening can be regulated by exposure to ethylene and 1-MCP, in
addition to temperature management practices. A temperature of ~25 °C is
considered ideal fruit ripening in papaya. The fruit harvested at colour break stage
take about 10–16 days to reach full yellow stage at 22.5 to 27.5 °C (An and Paull,
1990). The ripening temperature of more than 27.5 °C increased weight loss and
external abnormalities in fruit. Flesh softening and ripening progresses from
inward to outward in papaya. The exogenous application of ethylene promoted
skin yellowing, softening and flesh colour development in the outer mesocarp
(Fabi et al., 2007), whereas the inner mesocarp near to seed cavity was not
responsive to ethylene (An and Paull, 1990). ‘Red Lady’ fruit at colour break
stage when treated with 100 ppm ethylene reached full yellow stage in three days
at room temperature (26–32 °C, 46–65 % RH) while untreated fruit reached the
same stage in seven days (see Plate XII in the colour section). Ethylene treatment
is not recommended commercially because it limits the marketing period (Paull
et al., 1997). However, it may be used to ensure uniform and rapid fruit ripening
to regulate marketing at the retail level.
1-MCP has emerged as a wonderful postharvest tool to control ripening and
senescence, and thus to enhance the storage/shipping potential of fruits. Many
researchers have reported the beneficial effects of 1-MCP on the ripening and
storage behviour of papaya fruit (Hofman et al., 2001; Karakurt and Huber, 2003;
Ergun et al., 2006; Karakurt and Huber, 2007; Manenoi et al., 2007; Manenoi and
Paull, 2007). 1-MCP treatment has been shown to suppress the rates of respiration
and ethylene production in different cultivars of papaya such as ‘Sunrise Solo’,
‘Golden’, and ‘Rainbow’ (Ergun and Huber, 2004; Fabi et al., 2007; Manenoi
et al., 2007). Manenoi et al. (2007) showed that treatment time of 4–24 hours had
similar effect on the response of fruit to 1-MCP doses in the range of 50–1000 nL
L−1. The development of skin and flesh colour and fruit softening was also delayed
significantly by the 1-MCP treatment (100 nL L−1 for 12 h). However, the response
of fruit to 1-MCP in papaya varied greatly due to fruit maturity. The fruit treated
with 1-MCP at < 25% yellow stage failed to ripen properly, and developed a more
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elastic flesh texture, termed ‘rubbery’. Contrarily, the fruit at more ripe stage (40–
60% yellow) showed limited response to 1-MCP, in terms of number of days to
reach edible ripe stage. Therefore, the beneficial effects of 1-MCP in papaya fruit
can be obtained in a very narrow window of fruit maturity (25–30% yellow).
Manenoi et al. (2007) reported that fruit treated with 1-MCP (100 nL L−1 for
12 h) at the colour break storage showed a delay in skin colour development of
about seven days and at 25 and 50–70% yellow approximately five and one or two
days, respectively. These treated fruit also showed a dramatic delay in softening
and developed the ‘rubbery’ texture when fully ripe. The development of rubbery
texture has been attributed to the lower enzyme activity of endoxylanase in the
flesh tissue (Manenoi and Paull, 2007). The fruit with more than 25% yellow skin
when treated with 1-MCP softened normally without any rubbery texture.
Ethephon treatment before or after 1-MCP application was unable to overcome
the effect of 1-MCP on fruit treated at colour break stage (Manenoi et al. 2007).
The concentration of soluble solids and titratable acidity are not affected much
by the 1-MCP treatment, but this treatment has also been shown to delay the
disease development in papaya fruit. ‘Surya’ papayas treated with 1-MCP (100 nL
L−1 for 18 h) at colour break stage could be stored for three weeks at 18 °C without
any decay and flesh softening (Sudhakar Rao, unpublished data). Though the skin
colour of the 1-MCP-treated fruit was completely yellow after storage, but failed
to soften to reach edible-ripe stage. These studies clearly indicate that papaya
should be treated with 1-MCP (100 nL L−1 for 12 h) when it attains > 25% skin
colour.
6.9.4 Recommended storage and shipping
Storage and shipping at low temperature (7–13 °C) and high relative humidity
(90–95%) is recommended to maintain fruit quality in papaya (Paull et al., 2007).
Long-term cold storage is limited by susceptibility of fruit to chilling injury. As
mentioned previously, less mature (< 25% yellow) fruit are more susceptible to
chilling injury at ≤ 10 °C, while more mature (25–75% yellow) fruit can tolerate
the chilling conditions (7–10 °C) for about 2–3 weeks. The short-term (~24 h)
exposure of fruit to severe chilling conditions such as 1–2 °C can be injurious to
fruit. Temperature fluctuations during the surface or air-shipment of papayas can
increase marketing losses due to more decay and poor fruit quality (Nunes et al.,
2006). The application of MAP technology, in supplementation with optimum
storage temperature, can also be very beneficial to reduce weight losses and retard
fruit ripening during storage and long-distance shipping of papaya fruit.
6.10
Processing
6.10.1 Fresh-cut
Demand for fresh-cut fruits (i.e. products have been subjected to various
degrees of peeling, trimming, coring, slicing, shredding or dicing (Karakurt and
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Huber, 2003)) has increased several fold in the past decade. Production of freshcut products involves wounding the fruit tissue to a degree, which triggers several
physiological and biochemical changes in the fruit that are generally associated
with abiotic stresses. The most common symptoms are browning of cut surfaces,
increased rates of respiration and ethylene production, loss of flavour, tissue
softening, weight loss, and decline in phytonutrients (Hodges and Toivonen,
2008). The problems associated with fresh-cut products vary according to the fruit
species, degree of maturity and handling and storage conditions. Fresh-cut papaya
is a popular product. Two reasons for the rapid growth of the fresh-cut papaya
industry may be that consumers consider the fresh-cut fruit more convenient than
the whole fruit (which has to be peeled, deseeded and sliced before consumption)
and find the large size of some papaya cultivars, such as ‘Maradol’ off-putting
(Rivera-López et al., 2005).
Fresh-cut papaya is not chilling sensitive and can be stored for about 8–10 days
at 4–5 °C. A storage temperature of 5 °C has been suggested to be optimal
(O’Connor-Shaw et al., 1994; Karakurt and Huber, 2003; Rivera-López et al.,
2005; Ergun et al., 2006). An early report suggested that the potential shelf life of
fresh-cut papaya was limited to two days at 4 °C, with tissue softening the limiting
factor (O’Connor-Shaw et al., 1994). The susceptibility to tissue softening is
mainly influenced by the degree of fruit maturity, post-cut treatments, packaging
and storage conditions. Selecting fruit at optimum maturity is paramount for
consumer acceptability of fresh-cut product. Paull and Chen (1997) reported that
fruit with < 25% yellow skin had no soft edible flesh; an increase in skin yellowing
to > 55% also increased the percentage of edible flesh to more than 60%. Fruit
with < 55% skin yellowing showed more wound-induced respiration and ethylene
production due to slicing and deseeding and fully ripe fruit were easily bruised
and difficult to handle. Therefore, selecting fruit with 55–80% yellow skin, which
ensures > 50% edible flesh recovery, has been recommended for production of
fresh-cut papaya (Paull and Chen, 1997).
Tissue softening in fresh-cut papaya is primarily due to changes in cell wall
composition induced by the activities of various hydrolytic enzymes and stressrelated proteins. Wounding of 60–70% yellow fruit used to make fresh-cut product
enhanced the activities of various enzymes such as polygalacturonase,
α-galactosidase, β-galactosidase, lipoxygenase, phospholipase D, and ACC
synthase and ACC oxidase within 24 h, and levels remained significantly higher
compared with those in intact fruit during 8 days storage at 5 °C (Karakurt and
Huber, 2003). The total amount of pectin in fresh-cut papaya also declined,
increased in solubility and exhibited depolymerization. In addition, this study
confirmed that tissue softening was due to the physiological and biochemical
alterations in the cell wall and membranes rather than to microbial activity. Gene
expression analysis of the fresh-cut papaya has also revealed that wounding
induces the expression of proteins involved in membrane degradation, free radical
generation, and enzymes involved in stress responses (Karakurt and Huber, 2007).
It has been suggested that tissue softening in fresh-cut papaya can be delayed by
treatment of 70–80% ripe fruit with 1-MCP (2.5 μL L−1 for 12 h) before slicing
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(Ergun et al., 2006). Softening in fresh-cut product from 1-MCP-treated fruit was
significantly delayed during 6–10 days storage at 5 °C. The slices from 1-MCP
treated fruit were acceptable to the sensory panel for six days while those from
control fruit could only be stored for 2–3 days.
The potential shelf life of fresh-cut ‘Maradol’ papaya has been suggested to be
2, 6 and 10 days at storage temperatures of 20, 10 and 5 °C, respectively. The
degree of tissue softening and weight loss were also lower at 5 °C compared to 10
or 20 °C (Rivera-López et al., 2005). Storage of fresh-cut papaya at 5 °C also
helped to prevent losses of soluble solids, ascorbic acid, β-carotene, and
antioxidant capacity. This study also showed that slices were a better shape for
fresh-cut products than cubes as the latter presented higher weight loss, lower
SSC and lower overall quality. Another study on the comparison of cut shapes
showed that papaya flesh cut into spheres (1.55 cm radius) showed lower
weight loss, firmer texture, higher SSC and ascorbic acid and lower microbial
count compared to the cubes (1.4 cm side) during 10 days storage at 4 °C (Argañosa
et al., 2008). Furthermore, edible coatings have great potential to reduce problems
of weight and textural losses in fresh-cut products, increasing their shelf life. The
application of alginate- (2 % w/v) or gellan-based (0.5 % w/v) coating formulations
containing 1% ascorbic acid on fresh-cut papaya reduced weight loss through
improved water vapour resistance and delayed tissue softening during eight days
storage at 4 °C (Tapia et al., 2008). González-Aguilar et al. (2009) reported that a
medium molecular weight chitosan coating (0.02 g mL−1) maintained the quality
of fresh-cut papaya in terms of higher colour values (L* and b*) and firmness. It
showed antimicrobial activity and suppressed plate counts of mesophiles, moulds
and yeasts during 14 days of storage at 5 °C. To summarize, the huge research
interest in fresh-cut products has led to the development of techniques that involve
minimum use of synthetic food additives which result in better retention of fruit
quality. Both technological developments and consumer preferences indicate
great scope for expansion of the fresh-cut papaya industry.
6.10.2 Other processed products
Ripe papaya fruit can be processed into a number of other products such as pure
juice, blended beverages, jam, jelly, dehydrated, fruit bars, candy, intermediate
moisture and frozen products. Purée is the major intermediate product of papaya.
It is further processed into products like juice, nectar, jam, jelly and leather. The
slices and chunks of semi-ripe fruit can also be canned.
6.11
Conclusions
Papaya fruit is a rich source of vitamins, minerals and dietary antioxidants. The
mature unripe and ripe fruit including seeds have been used in traditional medicine
since ancient times. There is a great demand for papaya fruit in the fresh market
and processing industry. A consistent supply of high quality fruit to the consumers
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and processors is a great challenge for papaya industry around the world. The
tropical environmental conditions, where the papayas are grown, are congenial
for the development of various diseases and insect-pests, and for promoting
postharvest losses in fruit. The lack of cold chain, proper postharvest handling
facilities, and limited market access due to phytosanitary requirements could be
some of the reasons for the small share of major papaya producing countries in the
world trade.
Harvest maturity in papaya is a critical factor that determines fruit quality,
shelf life, storage/shipping potential at low temperature, and susceptibility to
mechanical injuries and diseases. Harvesting at colour break stage (10–12%
yellow) is a commercial practice to ensure better postharvest life and long-distance
shipping of fruit. The fruit harvested before optimum maturity fail to develop
good eating quality. The delayed harvesting (>25% yellow) improves fruit quality,
but limits shelf life and increases susceptibility to fruit-fly attack. The harvest
maturity should therefore be determined by considering all these factors. Fruit
harvested at colour break stage can be stored for 2–3 weeks at 7–13 °C temperature
and 90–95% relative humidity. Fruit at advanced stages of ripeness are more
tolerant to chilling conditions compared to less mature ones. The delicate nature
of fruit skin predisposes it to mechanical injuries during harvest and postharvest
handling operations, resulting in increased susceptibility of fruit to rots caused by
wound pathogens. The marketability of fruit can be significantly increased by
proper care to avoid the mechanical injuries and weight loss. MAP of papaya fruit
with very low permeability films is beneficial to retard weight loss, fruit ripening
and alleviate chilling injury during cold storage. The benefits associated with
blocking of ethylene action by 1-MCP can be obtained only if the fruit are treated
at >25% skin yellowing stage. The possibility of integration of 1-MCP into the
current papaya handling protocol is limited because fruit treated at colour break
stage fail to soften and produce ‘rubbery’ texture. However, the exposure of
quarter- to half-ripe fruit to 1-MCP can delay the fruit softening and provide some
benefits at the retail end.
The phytosanitary treatments such as vapour heat, forced hot air and irradiation
have been adopted commercially for fruit to be exported to countries such as the
U.S.A. (mainland), Japan, Australia and New Zealand. The response of papaya
fruit to these treatments varies greatly due to a number of factors including fruit
maturity and growing conditions. The thermal treatments have been shown to
cause some damage to fruit quality; while irradiation has been proven to be a safe
technology without adverse effects on fruit quality. The postharvest diseases such
as anthracnose, Rhizopus rot and stem-end rot are responsible for huge economic
losses in fruit. The incidence and severity of these diseases can be reduced by
integrated management practices such as orchard hygiene, preharvest and
postharvest fungicide applications, packinghouse sanitation, heat treatments, and
use of GRAS compounds alone or in combination with biocontrol agents.
There is a great demand for fruit in the fresh-cut and processing industries. The
fruit at 75% ripe stage are the most suitable for fresh-cut products, which can be
safely handled at 5 °C for 5–10 days. The ‘tissue-softening’ is commonly a limiting
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factor in the stability of the fresh-cut papaya. This problem can be minimized by
treatment of fruit with 1-MCP before cutting. The fruit can also be processed into
a number of products such as puree, juice, jam, jelly, fruit bars, etc. The demand
for healthy and nutritious fruits and their products is increasing as the consumers
are adopting a healthy lifestyle. These trends present a positive outlook for the
papaya industry in the near future.
6.12
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Plate X
(Chapter 5) Trunk shaking harvester in high density hedgerow orchard.
(A)
(B)
Plate XI (Chapter 6) Effect of modified atmosphere packaging (MAP) on shelf life of
‘Red Lady’ papaya. Fruit were harvested at colour break stage, treated with fungicide
(prochloraz; 100 ppm) and sealed in Cryovac® D-955 film.
A. Storage for 12 days in MA plus 2 days in ambient air (2 weeks in total) at room
temperature (RT; 26–32 °C, 32–45% RH) (D. V. Sudhakar Rao, unpublished).
B. Storage for 21 days in MA plus 7 days in ambient air (4 weeks in total) at 18 °C,
72–80% RH (D. V. Sudhakar Rao, unpublished).
© Woodhead Publishing Limited, 2011
(a)
(b)
(c)
(d)
Plate XII (Chapter 6) Effect of exogenous application of ethylene on fruit ripening in
‘Red Lady’ papaya harvested at colour break stage. Ethylene (a & b) and control (c & d)
fruit after 3 and 7 days at room temperature (RT; 26–32 °C, 46–65% RH) (D. V. Sudhakar
Rao, unpublished).
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