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  • Author or Editor: Federica Galli x
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Pawpaw [Asimina triloba (L.) Dunal] has significant potential as a new fruit crop. During ripening, loss of firmness is extremely rapid, and this trait may be the biggest obstacle to the development of a broader market. Cold storage of pawpaw fruit seems limited to 4 weeks at 4 °C, though fruit softening merely slows during storage. A study of several cultivars with commercial appeal has shown that none have superior cold storage life. Extending the cold storage beyond 4 weeks resulted in increasing loss of fruit firmness, poor poststorage ripening, and development of quality traits, and many fruit exhibiting flesh and peel discoloration. Cold storage duration affected fruit volatile production. By 4 weeks of cold storage, ethyl hexanoate and ethyl octanoate were the major volatiles produced, replacing methyl hexanoate, which was the major volatile produced by ripe fruit after harvest. By 8 weeks of cold storage, volatile ester production was generally low and ethyl hexanoate became the only major volatile. This loss of volatile production was accompanied by a decrease in alcohol acetyl transferase activity. Also, during cold storage, there was an increase in total phenolic content, lipid peroxidation products, and polyphenol oxidase activity. These changes may contribute to the black discoloration that developed in fruit cold-stored for 8 weeks or more. It is apparent that cold storage alone may not be sufficient to extend the storage life of most, if not all, current pawpaw cultivars beyond 4 weeks.

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Ripening pawpaw [Asimina triloba (L.) Dunal] fruit exhibit climacteric peaks of ethylene and CO2 production 48 to 72 hours after harvest, and thus may be considered climacteric. The development of desirable quality traits and the loss of fruit firmness during ripening is extremely rapid, and a variety of strategies to slow these processes via manipulation of ethylene production and/or response and by more direct techniques like postharvest heat treatment have been attempted. Fruit, branches with fruit, and/or whole trees have been sprayed with ethephon or aminoethoxyvinylglycine to hasten or delay ripening, respectively. After harvest, fruit have been treated with commercial and higher rates of 1-methylcyclopropene for various durations at ambient and cold storage temperatures. Fruit have also been heat-treated at various temperatures, using both brief “shock” treatments above 40 °C and longer periods at 35 °C to 40 °C. In addition, in an attempt to alleviate the loss of ripening capacity as well as the development of injury symptoms from cold storage for longer than 4 weeks, cold-stored fruit were warmed to ambient temperature intermittently and then returned to cold storage. While some effects of the treatments were noted, the responses to all of these treatment strategies have failed to appreciably alter fruit ripening, the rapid loss of firmness, or otherwise maintain fruit quality beyond that without treatment.

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Pawpaw[Asiminatriloba (L.) Dunal] is a highly perishable climacteric fruit. Generally, fruit may be stored at 4 °C for 4 weeks with minimal loss in quality or subsequent ripening capacity. However, comparisons among cultivars and advanced selections for ripening behavior and postharvest storage life have not been reported. Ideally, cultivars with superior ripening traits (higher firmness, or a slower rate of firmness loss) and longer storage life may be identified for the commercial market. To determine if differences among genotypes may exist, respiration, C2H4 production, and fruit firmness of six varieties, 8-20, 9-58, `Middletown', `PA Golden', `Taytwo', and `Taylor', were measured during ripening after harvest and after 1, 2, 3, 4, 5, and 6 weeks of 4 °C storage. No differences were observed among the cultivars regarding respiration and C2H4 production. Respiratory and ethylene peaks were detected within 48 hours after harvest or removal from cold storage. Rapid loss of firmness was measured during ripening at ambient temperature after harvest and after removal from cold storage (days 0–3), with some loss during cold storage itself (weeks 1–6). As expected, firmer fruits at harvest had a lower respiration rate. However, no obvious differences in ripening behavior or cold storage response were observed among the six genotypes. As a general guideline, only firmer fruits should be cold stored, since fruit softening did not stop at 4 °C. A broader analysis of all of the named cultivars and advanced selections of pawpaw will be needed to determine if the present results are generally representative of pawpaw.

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Pawpaw (Asimina triloba) fruit stored longer than 4 weeks at 4 °C fail to ripen normally and may develop internal discoloration, indicative of chilling injury (CI). To determine if loss of antioxidant protection in the fruit tissue during cold storage could be the cause of these problems, the levels of total, reduced, and oxidized glutathione and ascorbate and the key enzymes glutathione reductase (GR) and ascorbate peroxidase (APX) of the ascorbate-glutathione cycle were studied in fruit at 4 and 72 h after harvest and after 2, 4, 6, and 8 weeks of 4 °C storage. The total phenolic level was also studied due to its potential antioxidant role, and the activity of polyphenoloxidase (PPO) was assayed, as it may contribute to phenolic oxidation and tissue browning. Fruit ethylene production and respiration rates were in typical climacteric patterns during ripening after harvest and after up to 4 weeks of cold storage, increasing from 4 to 72 h after removal from cold storage, though maximum ethylene production declined after 2 weeks of cold storage. However, fruit showed higher respiration rates at 4 versus 72 h of ripening at 6 or 8 weeks of cold storage, opposite to that at earlier storage dates, possible evidence of CI. Ripening after harvest generally resulted in an increase in total and reduced glutathione, reduced ascorbate, and total phenolics. However, levels of total and reduced glutathione, total ascorbate, and total phenolics declined as storage time progressed. Neither GR nor APX exhibited changes during ripening or trends over the cold storage period. PPO activity increased as the storage period lengthened. Thus, the declining ability of these components of the protective antioxidant systems during the prolonged stress of low temperature storage may be one of the major causes of pawpaw CI limiting it to 4 weeks or less of cold storage. An increase in reactive oxygen species with prolonged storage, coupled with the increase in PPO activity, may have led to greater oxidative damage and been a major cause of the loss of ripening potential and the tissue browning that occurs in fruit stored for more than 4 weeks.

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Pawpaw [Asimina triloba (L.) Dunal] is a highly perishable climacteric fruit, softening rapidly once ripening commences which may limit its marketability. In studies to determine the optimum cold storage temperature and maximum storage life of the fruit, pawpaw fruit were stored at -2, 2, and 6 °C for 1, 2, 4, 8, and 12 weeks, and then ripened upon removal to ambient temperature. Through 4 weeks, fruit exhibited adequate firmness upon removal from cold storage, but at 8 and 12 weeks fruit held at 2 and 6 °C were very soft. Irrespective of storage temperature, at 8 weeks fruit showed a delay in a climacteric-like respiratory increase, and by 12 weeks a respiratory climacteric was not apparent. An ethylene climacteric was evident after all temperature and storage periods except those held at 6 °C for 12 weeks. Significant symptoms of cold injury were found by 8 weeks of 2 °C cold storage. In addition to a delayed respiratory climacteric, pawpaw fruit stored for 8 and 12 weeks exhibited flesh browning within 48 h of moving to ambient temperature. A change in fruit aroma volatile profile suggested injury might have been developing by 4 weeks of cold storage even though other symptoms were not evident. Immediately after harvest, methyl octanoate was the dominant volatile ester followed by methyl hexanoate. By 4 weeks of postharvest cold storage, ethyl hexanoate was the dominant ester followed by ethyl octanoate, but methyl octanoate production was still substantial. At 8 weeks, volatile ester production was generally lower with ethyl hexanoate the major volatile followed by ethyl octanoate. These symptoms indicate that pawpaw fruit can suffer cold injury during extended periods of cold storage.

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