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Recommendations for Maintaining Postharvest Quality

Marita Cantwell and Trevor V. Suslow
Department of Plant Sciences, University of California, Davis, CA 95616

Maturity Indices
Corking of the stem and subtle changes in rind color (bright green to dull green in ‘Kabocha’ for example) are the main external indications of maturity. Immature fruit have a fleshy stem, maturing fruit will have some stem corking, and well mature fruit will have a well corked stem. Internal color should be intense and typical of the cultivar. The concentrations of the yellow and orange carotenoids generally increase only slightly during storage. Maturity at harvest is the major determinant of internal color. Immature fruit will be of inferior eating quality because they contain less stored carbohydrates. Immature fruit will have more decay and weight loss during storage than mature fruits.

Pumpkin and winter squash should be full sized and well formed with the stem intact. They should be well matured with good rind development typical of the cultivar. Internal quality attributes are high color due to a high carotenoid content, and high dry weight and sugar and starch contents.

Optimum Temperature
12.5-15°C (55-59°F)
Pumpkins and winter squash are very chilling sensitive when stored below 10°C (50°F). Depending on the cultivar a storage life of 2 to 6 months can be expected at 12.5-15°C (55-59°F). Recent research at Oregon State University showed that for 8 currently produced winter squash cultivars stored at 10-15°C (50-59°C), 90%, 70% and 50% were marketable after 9, 15 and 20 weeks, respectively. For green rind squashes, storing at 15°C (59°F) may cause degreening, undesirable yellowing, and texture loss. The green rind squashes can be stored at 10-12°C (50-55°F) to prevent degreening, although some chilling injury may occur at the lower temperature. High storage temperature (>15°C) will result in excessive weight loss, color loss and poor eating quality.

Optimum Relative Humidity
50-70% with 60% usually considered optimum Moderate relative humidity with good ventilation is essential for optimum storage. High humidity will promote decay. Although 50-70% RH will reduce decay during storage, significant weight loss will occur. For example, mature Kabocha squash lose 1.0 and 1.5% of their fresh weight per week of storage at 12.5°C (59°F) and 20°C (68°F), respectively.

Rates of Respiration
30-60 ml CO2 / kg·hr at 25°C (77°F)
§ To calculate heat production, multiply ml CO2 / kg·hr by 440 to get BTU/ton/day or by 122 to get kcal/metric ton /day.

Rates of Ethylene Production
<0.5µL C2H4 /kg·hr at 20°C. If the fruit are chilled, ethylene production rates can be 3-5 times higher.

Responses to Ethylene
Exposure to ethylene will degreen squash with green rinds. Ethylene will also cause abscission of the stem, especially in less mature fruit.

Responses to Controlled Atmosphere (CA)
Atmospheres containing 7% CO2 can be beneficial by reducing loss of green color.
Yellow squash, however, appear not to be benefited by 5 or 10% CO2 atmospheres.
Lowering the O2 concentration does not appear to provide any benefit.

Physiological Disorders
Chilling injury. Caused if pumpkins and squashes are stored below 10-12.5°C (50-55°F). Symptoms of chilling injury are sunken pits on the surface and high levels of decay once fruit are removed from storage. Storing fruit 1 month at 5°C (41°F) is sufficient to cause chilling injury symptoms. Depending on the cultivar, storage for several months at 10°C (50°F) may cause some chilling injury.
Freezing injury. Can occur at temperatures below -0.8°C (30.5°F).

Pathological Disorders
Several fungi are associated with decay during storage of pumpkins and winter squashes. Fusarium, Pythium and anthracnose (Colletotrichum) and gummy stem blight or black rot (Mycosphaerella) are common fungi. Alternaria rot will develop on chill-damaged winter squashes. Fruit that are overmature at harvest (>2 weeks beyond optimal harvest date) will tend to have more storage decay.

Special Considerations
Curing. The fruits may have tender rinds when freshly harvested. Curing in the field (with protection from the sun by placing under the leaves) before handling and stacking into bins or wagons will help to harden or cure the rind. The recommended storage conditions also favor curing or hardening of the rind.

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