In general plums harvested at a mature stage will ripen properly without an eternal ethylene application. Ethylene application to fruit harvested at a lower, minimum maturity will help to ripen the fruit more uniformly without speeding up the rate of ripening. Also, there are some slow ripening cultivars that require an external application of ethylene for even ripening. For full details, please see the information below on the University of California / Davis.
Apply 100 ppm ethylene at 68°F (20°C) for a minimum of 24 hours. Actual time of exposure to ethylene is determined by the maturity of the fruit; a decrease in fruit firmness indicates that the plums are producing ethylene and the external application of ethylene is no longer needed.
Some of these recommendations from:
The specialists at UC Davis can assist with more detail on the ripening of specific cultivars or any other questions. Visit their web site at http://postharvest.ucdavis.edu
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