Plum trees are set up when they are one year old and bear fruit the next year. There are many varieties of plums grown in many areas of the United States, The prune plum, used for production of dried prunes, is grown mainly in California. Purple plum types, used for canning, are grown mainly in Oregon and Washington.
At the processing plant, plums are washed in diluted acid or alkaline solutions to remove spray residues, and then rinse with water. They are next inspected to remove defective specimens and graded for size. Those to be sold as fresh are packed in boxes and air cooled to 31 to 32 degree F. In this condition, they have storage life of 2 – 4 weeks. Plums to be canned are cleaned, inspected and graded. Then they are placed in cans by hand and covered with 25 – 30% sugar syrup. The filled, open cans are then heat exhausted in water at 180 – 190 degree F for 12 – 15 minutes, after which the cans are sealed and heated in boiling water for 20 – 25 minutes, depending on the size of the container.For production of dried prunes, plums are washed, dipped in boiling lye solution (0.25 – 1.0% sodium hydroxide) for 5 – 30 sec, and washed. The lye treatment enhances drying by its action on the skin of the fruit. The plums are then dried to a moisture content of 22 – 25%.
Dried prunes may be canned in syrup of 20 Brix with 0.4% citric acid, or they may be canned without added fluid (dry packed). Prunes may also be packaged and distributed as such.
A significant quantity of prune juice is produced. Prunes are steamed 8 – 10 min to soften them and to inactivate enzymes. They are then reduced to a puree during which time they are pitted, the puree is then cooled to about 120 degree F. The puree is filtered and the juice obtained is adjusted to about 22.5 Brix, pasteurized, filled into bottles or cans that are then placed on their sides to sterilize the tops, and cooled. Plums are not commercially frozen.
Plums Production and Processing
