In most of the cultivars growing in California, harvest date is determined by skin color changes that are described for each cultivar. A color chip guide has been designed to determine maturity for each cultivar.
A three tier maturity system is used in California
1) US-Mature (Minimum Maturity)
3) Tree Ripe
Measurement of fruit firmness is recommended for cultivars where skin ground color is masked by full red or dark color development before maturation.
Maximum maturity: Flesh firmness, measured with a penetrometer with an 8 mm-tip, can be used to determine a maximum maturity index, which is the stage at which fruit can be harvested without suffering bruising damage during postharvest handling.
Plums are less susceptible to bruising than most peach and nectarine cultivars at comparable firmness
High consumer acceptance is attained on fruit with high soluble solids content (SSC). Fruit acidity, SSC/acidity ratio, and phenolic content are also important factors in consumer acceptance. There is no established minimum quality standard based on these factors. Plums with 2-3 pounds -force flesh firmness are considered “ready to eat”.
-1.0 to 0°C(30.5-32°F)
Freezing point varies depending on SSC.
Optimum Relative Humidity
90-95% R.H; an air velocity of approximately 50 CFM is suggested.
Rates of Respiration
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
* The lower end of this range is for mature but unripe fruit; higher values are for ripe fruit.
Responses to Ethylene
Most of the plums harvested at the California Well-Mature stage (higher than US-Mature) will ripen properly without exogenous ethylene application. Ethylene application to fruit harvested at the US-Mature maturity will only ripen the fruit more uniformly without speeding up the rate of ripening. However, for the slow ripening plum cultivars, expgenours application of ethylene (100 ppm for 1-3 days at 20°C(68°F) is needed for even ripening. These cultivars are Angeleno, Black Beaut, Casselman, Late Santa Rosa, Kelsey, Nubiana, Queen Ann, Red Rosa, and Roysum.
Responses to Controlled Atmospheres(CA)
The major benefits of CA during storage/shipment are retention of fruit firmness and ground color. Decay incidence has not been reduced by CA of 1-2%O2 + 3-5 %CO2. CA conditions of 6% O2+ 17%CO2 are suggested for reduction of internal breakdown during shipment, but its effectiveness depends on cultivar, preharvest factors, market life and shipping time.