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Plum, Prunus domestica / Rosaceae
Postharvest Atmosphere Management
Plums must be cooled soon after the harvesting, afterwards they are conserved under low temperatures and finally the temperature is risen up to 18ºC for complete maturation.
It is also necessary to control the levels of relative humidity, oxygen and carbon dioxide in the atmosphere.

The plum is a climacteric fruit, so it continues the maturation process after its harvesting. However, it is necessary to make a good choice about the dates of harvesting so that the fruit reaches a desirable sugar content. The greater the content is, the better the fruit is preserved.

In cultivars of slow maturation the ethylene implements (100 ppm for 1-3 days at 20ºC) are necessary for a uniform maturation. Among these types of cultivar we find Angelo, Black Beaut, Casselman, Late Santa Rosa, Kelsey, Nubiana, Queen Ann, Red Rosa and Roysum.

Plums are suitable for short fridge preservation, depending on the use of the product (domestic market, export or industrial processing), on the tendency to mature regularly after its taking out of the cold storage rooms, on the resistance of the fruit to internal breakdown, on the duration of the maturation period after the harvesting and, finally, on the edaphoclimatic conditions.

The pre-cooling technique by quickly placing the plums in thermal conservation conditions, either with water (hydro cooling) or with forced air, in order to maintain the quality of the fruit.

The period of cool conservation is, usually, 15-30 days, and afterwards the fruit is put under constant high temperatures (18ºC) until they reach total maturation and a partial recovery of the organoleptic qualities.

It is also very important to maintain a high relative humidity during the fruit conservation, in order to avoid dehydration. Plums for drying are put under constant heat flow (85ºC), with forced air, until they reduc humidity levels to 22%; before their commercialization, they are rehydrated up to 35%.

Plums are better adapted to the conservation systems in Normal Atmosphere (NA), although there are exceptions; thus, the Nuviana varieties and El Dorado are well adapted to Controled Atmospheres (CA), with 11% of O2 levels, 7% of CO2 and temperatures at 0-1ºC well, remaining in these conditions approximately 80 days.

The positive effects of controlled atmospheres during the storage and packing are the maintenance of firmness and fruit colour, that is to say, CAs maintain the quality of fruit, reduce the postharvest alterations and the losses of soluble solids (the greater acceptance of the consumer is obtained with high content fruit).

Self-life varies among cultivars, being significantly altered by temperature handling. The maximum self-life is obtained when the fruit is stored at approximately 0ºC and it may vary between 1 and 8 weeks.

The Web site of the University of Davis postharvest section, ( http://postharvest.ucdavis.edu/Produce/ProduceFacts/Espanol/Ciruela.html ) includes the indexes of harvest and quality, maximum maturity, optimal temperature and relative humidity, the rate of transpiration, ethylene production and its effects, as well as the effects of CA and the genotype and cultural practices in postharvest life. We can also find the physiological disorders and diseases that affect plums and how to fight them.
Postharvest Problems
Physiological alterations and diseases that frequently affect plums after the harvesting: Internal degradation or damage by cold, brown rot, gray mould, Rhizopus rot and green-blue mould.

Plums may have different physiological alterations (softening and breakdown of the pulp, gelation, darkening or blackening, etc.) depending on the culture techniques, the variety, harvesting period and the thermal regime of conservation; or diseases (rot caused by pathogenic microorganisms).

1- Internal breakdown or Damage by cold: this physiological problem affects the pulp and it is characterized by translucent skin, internal browning, mealiness, red inks, maturation incapacity and loss of flavour. These symptoms developed during the maturation of fresh plums or plums for drying, after a period of cool storage. Therefore, they are usually the consumers who find these symptoms. The fruit that is stored within temperatures of 2 to 6°C is more susceptible to this problem.

2- Brown rot: caused by Monilia fructicola, it is the most important postharvest disease. The infection begins during the flowering, and the fruit rot can take place before the harvest but it often does in postharvest. It is characterized by circular brown spots. Among the control strategies are the cleaning of the orchard in order to diminish infection sources, the implement of fungicides in preharvest, and the immediate cooling of the fruit after the harvest. Moreover, a fungicide treatment can be used in postharvest.

3- Gray mould: caused by Botrytis cinerea. It produces fruit rot and afterwards it develops into silver-gray mould in areas of blight, with round shape. It takes place during the storage if the fruit has been contaminated during the harvest or by injuries during the handling. Some effective control measures consist of avoiding mechanical damages and good temperature control.

4- Rhizopus rot: caused by Rhizopus stolonifer, it appears in mature fruits or almost mature maintained at 20-25°C. It characterizes by brown round spots covered with mycellium. In order to fight against this fungus, cool the fruit and maintain it under 5°C.

5- Green-blue mould: caused by Penicilium sp. The contaminated fruit has, at the beginning, soft, round depressed areas; afterwards, a whitish mould develops evolving into green-bluish colour. With dry environment, the conidium spreads easily and contaminates the fruit in the field, warehouses, and even cold storage rooms. Contamination spreads by way of wounds and lenticels. As defense methods, it is necessary to avoid injuring the fruit, get rid of bad fruit and pre-cool immediately after harvesting.
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