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The primary impediment to introduction of pawpaw into both fresh and processing markets is its perishability (Popenoe 1916, 1917; Peterson 1991). In order to facilitate the growth of a commercial pawpaw industry, several problems with harvest and postharvest handling of fruit will need to be resolved.
Pawpaw fruit ripening is characterized by an increase in soluble solids concentration (up to 20%), flesh softening, increased volatile production, and, in some genotypes, a decline in green color intensity (McGrath and Karahadian 1994). The volatile profile during ripening consists primarily of ethyl and methyl esters (Shiota 1991; McGrath and Karahadian 1994). Within 3 days after harvest, ethylene and respiratory climacteric peaks are clearly evident as pawpaw fruit rapidly soften (Archbold et al. 2003; Archbold and Pomper 2003; Koslanund 2003). Other members of the Annonaceae such as cherimoya, sweetsop (sugar apple), soursop, and atemoya, are also climacteric (Biale and Barcus 1970; Kosiyachinda and Young 1975; Paull 1982; Wills et al. 1984; Brown et al. 1988). Fruit of these tropical species exhibit similar climacteric maxima, although some have two respiratory peaks. Preliminary analyses conducted by Koslanund (2003) indicate that the decline in firmness of pawpaw fruit is due to the action of at least four enzymes: polygalacturonase, cellulase, pectin methylesterase, and endo-β-mannanase.
Color change is generally not a reliable indicator of pawpaw fruit ripeness. Although a decline in green color intensity during pawpaw ripening has been reported by McGrath and Karahadian (1994), we have found that it is not consistent among genotypes, occurs later in ripening if at all, and is not easy to identify visually. A common practice to determine maturity is to touch each fruit to determine if it is ready to harvest; ripe softening pawpaw fruit yield to slight pressure, as ripe peaches do, and can be picked easily with a gentle tug. Thus, fruit are harvested when they have already begun ripening and have lost some firmness. This is labor intensive and may result in slight bruising injury, perhaps leading to off-flavors (Peterson 1991). Also, a natural abscission zone forms where the fruit attaches to the peduncle when fruit ripen. "Wiggling" the fruit can determine how well this abscission zone has formed. Ripening fruit also give off a strong aroma. Fruit on a single tree do not ripen within close proximity in time to one another. An extended harvest period of two weeks or longer from an individual tree is not uncommon. The protracted harvest may be due in part to the staggered bloom period in the spring, but it is not known if the bloom to harvest period is the same for all fruit on a tree. Although each fruit cluster develops from an individual flower, fruit within a cluster often ripen at different times. Currently, multiple harvests from one tree are conducted to obtain high-quality fruit. Cultivar variation in harvest period also exists. Because of the lack of easily identifiable ripening traits, it is difficult to determine fruit maturity except by individual "feel" or to determine softening. Softer fruit needs to be marketed immediately, while firmer fruit should be held in cold storage at 4circC.
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cluster develops from an individual flower, fruit within a cluster often ripen at different times. Currently, multiple harvests from one tree are conducted to obtain high-quality fruit. Cultivar variation in harvest period also exists. Because of the lack of easily identifiable ripening traits, it is difficult to determine fruit maturity except by individual "feel" or to determine softening. Softer fruit needs to be marketed immediately, while firmer fruit should be held in cold storage at 4circC.
Pawpaw fruit soften rapidly at room temperature after harvest. McGrath and Karahadian (1994) and Layne (1996) indicated a 2- to 3-day shelf life, although fruit that are just beginning to soften have a 5- to 7-day shelf life (Archbold et al. 2003) at room temperature. However, we have observed that pawpaw fruit can be stored for 1 month at 4circC with little change in fruit firmness; fruit then ripen upon removal to ambient temperature. The optimum temperature and duration for holding the fruit needs to be determined. Immature fruit does not ripen, even if treated with ethephon at 1000 mg L-1. Controlled or modified atmosphere storage has not been evaluated. Fruit packaging needs to be developed to minimize bruising. Because fruit is non-uniform in size and shape, packing for shipping presents some unique challenges.
Annonaceous acetogenins have been extracted from pawpaw twigs and have potential as medications and pesticides (Zhao et al. 1994; Johnson et al. 1996; McLaughlin 1997; McCage et al. 2002). About 250 of these compounds have been isolated and characterized (McLaughlin 1997). Three compounds—bullatacin, bulletin, and bullanin—have high potencies against human solid tumor cell lines in vitro (Zhao et al. 1994). Several acetogenins have been patented for pesticidal use (McLaughlin 1997) and as anti-tumor agents (Zhao et al. 1994; McLaughlin 1997). Another patented product made from annonaceous acetogenins is a head-licie-remover shampoo developed by Jerry McLaughlin of Nature's Sunshine Products (Spanish Fork, Utah; patents 4,721,727, 4,855,319, and 09/213,164 (pending); United States Patent and Trademark Office, www.uspto.gov) (McCage et al. 2002). Botanically-derived pesticides that are environmentally compatible and biologically degradable may be obtained from pawpaw (Ratnayake et al. 1993). Pests are less likely to develop resistance to botanically-derived pesticides, as they often have a larger pesticidal spectrum (Arnason et al. 1989). Asimicin, extracted from pawpaw, has been shown to have significant pesticidal activity against mosquito larvae (Aedes aegypti L.), blowfly larvae (Colliphora vicina Meig), two-spotted spider mite (Tetranychus urticae Koch),