To facilitate the growth of a commercial pawpaw (Asimina triloba) industry, several problems with harvest and postharvest handling of fruit need to be resolved. Pawpaw fruit ripening is characterized by an increase in soluble solids content, fl esh softening, increased volatile production, and a loss of green color intensity. Within 3 days after harvest, ethylene and respiratory climacteric peaks are clearly evident. Softening of fruit is due to the action of at least four enzymes, with the softening proceeding from the surface to the interior tissue. Fruit on a single tree can ripen over a 2-week period, creating labor problems. When immature fruit is harvested it does not ripen, even if treated with ethephon at 1000 mg·L-1 (ppm), but the use of commercially available growth regulators both pre- and postharvest warrants further study. Fruit soften very rapidly at room temperature after harvest and have a 2-to 4-day shelf life. However, we have stored pawpaw fruit for 1 month at 4 °C (39.2 °F) with little change in fruit firmness and fruit apparently continue normal ripening upon removal to ambient temperature. The optimum temperature and duration for holding fruit will need to be determined. Further extension in pawpaw storage life may be feasible with controlled or modified atmosphere storage. Although there are a number of practical problems with pawpaw harvest and postharvest storage that need to be addressed, we hope to develop recommendations for harvest and handling of fruit in the near future.

Pawpaw fruit were harvested at the advent of the ripening process and were ripened at room temperature. Based on fruit firmness and respiration and ethylene production rates at harvest and during ripening, fruit were classified into one of four categories: preripening (no to very slight loss of firmness; preclimacteric), early ripening (some softening; increasing rates of ethylene and CO2 production), mid-ripening (soft; at or just past climacteric), and late ripening (very soft; postclimacteric). The activities of the cell-wall degrading enzymes polygalacturonase (PG), endo-(1→4)ß-D-glucanase (EGase), and endo-ß-1,4-mannanase (MAN) were low in the preripening and early ripening stages, increased dramatically by mid-ripening coincident with the respiratory and ethylene climacterics, and decreased at late ripening. However, pectin methylesterase (PME) activity per milligram protein was highest at the green stage when the fruit firmness was high and decreased as ripening progressed. Tissue prints indicated both EGase and MAN increased as ripening proceeded. The EGase activity was evident near the seeds and the surface of the fruit at preripening and eventually spread throughout, while MAN activity was evident near the fruit surface at preripening and was progressively expressed throughout the flesh as fruit ripened. The greatest decline in fruit firmness occurred between pre- and early ripening, before the peak activities of PG, EGase, and MAN, although MAN exhibited the greatest relative increase of the three enzymes in this period. The data suggest that PME may act first to demethylate polygalacturonate and may be followed by the action of the other enzymes resulting in cell wall disassembly and fruit softening in pawpaw.

Pawpaw fruit ethylene production, 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS) and ACC oxidase (ACO) activities, and tissue content of the ethylene precursor ACC and conjugate malonyl-ACC (MACC) were measured during postharvest ripening. Fruit were harvested near the advent of the ripening process and were ripened at room temperature. The fruit displayed increases in ethylene production and respiration rate during ripening with maxima for both 3 days after harvest. Mean ethylene maxima on a fresh weight basis were 4.7 and 7.6 μg·kg-1·h-1 and mean respiratory (CO2 production) maxima on a fresh weight basis were 220 and 239 mg·kg-1·h-1 in 1999 and 2001, respectively. The increase in ethylene evolution coincided with an increase in respiration and a rapid decline in fruit firmness. Internal and external fruit firmness declined in a parallel manner. The ethylene climacteric peak occurred after the greatest decline in fruit firmness, indicating that low levels of ethylene may be sufficient to initiate the ripening process. The ethylene climacteric peak also coincided with the highest activities of both ACS and ACO as well as the maximum tissue ACC content. As ACC content increased, MACC content declined, suggesting a regulation of ethylene production via free ACC levels by malonylation of ACC. Thus, the climacteric development of ethylene production may be regulated by an increase of ACS activity and a decrease in ACC malonyltransferase activity, making more free ACC available for the production of ethylene by increased activity of ACO.

No abstract

The effects of postharvest treatment with hexanal vapour on longan fruit decay, quality, hexanal residue, phenolic compound content, and polyphenoloxidase (PPO) and peroxidase (POD) activities were studied during storage at 5°C for 30 days. Hexanal exposure for 2 h at 900 lL L )1 before cold storage reduced the percentage of fruit with decay and was deemed the optimum treatment. Hexanal exposure resulted in a pericarp that was more reddish brown and less intense in colour. Hexanal residue in the pericarp and aril of fumigated fruit was several fold higher than that of nonfumigated fruit, although levels were low at the end of cold storage. Electrolyte leakage of pericarp increased during 5°C storage and was further increased by hexanal exposure. Hexanal reduced pericarp phenolic content, and increased PPO and POD activities. Overall, use of hexanal vapour reduced postharvest disease of longan fruit but increased the likelihood of pericarp browning.