Prune plums are selected for processing into prunes. Not all varieties of plums are suitable for making into plums, but French La Petite D’Agen and California prune plums are commonly used. These varieties can be dried without any fermentation, with the pits remaining in the fruit. Plums are harvested from commercial orchards about mid-August, when the fruits are fully ripe. This is usually done with machines that shake the trees, making the plums fall. Debris like leaves and twigs are blown off with large fans, and the fruit is put into bins to be transported. At the dehydrating facilities, the plums are washed prior to drying.
Plums are dehydrated right away after being picked, as a first processing step. This was originally done in the sun, but is now mostly done by natural gas heat in dehydrating tunnels. Heating elements and fans are used to force hot air through tunnels full of plums. They are dried to a moisture content between 18 and 21 percent, which is preferred for storage. They can then be stored until needed for more processing and packing. The fruits shrink by about two-thirds during drying, which usually takes about half a day. After drying, they are sorted for size using a grader with screens filled with holes of different sizes. The prunes that fall through each successive screen are packed as the same size. After sorting, they are stored as “natural condition” fruits or shipped for further processing.
When dried prunes arrive at processors, they are washed again and steamed briefly to return some moisture content to the fruits. A preservative is sometimes added to keep prunes from molding or spoiling. This is most commonly potassium sorbate. Some are “cured” with the preservative, if they are to be sold to retailers or stored over a long time. Some are pitted for food industry or commercial packaging, while others are left whole. Some are “hot-packed” without any potassium sorbate for retail consumers who do not want preservative in their food. Other prunes are canned in water, juice or syrup, while still others are shredded, chopped or pureed for juice, diced dried fruit, baby food and other products. All are sorted and inspected before leaving the processing company, and are weighed, measured, packed and labeled into their various containers.