Prune is a sweet plum that has been dried. Prunes are also commonly called dried plums. The drying gives prunes their wrinkled appearance. Plums that are especially well suited to drying have a high sugar content and are called prune plums. Prunes are high in iron and vitamins.
Prune plums were first grown in western Asia, near the Caucasus Mountains and the Caspian Sea. Today they are a leading crop in California, where about 160,000 tons (145,000 metric tons) of prunes are produced yearly. The warm, dry climate of the fertile valleys provides ideal growing conditions. Prunes are also grown in the northwestern part of the United States, central Europe, and South America.
The French prune plum (Petite d’Agen) makes up 90 percent of the prune production in California. It was brought to the United States from France in 1856 by Louis Pellier, a fruit grower. Other prune plums include the Imperial, the Sugar, and the Robe de Sergeant.
Prune plum trees usually produce a crop five to seven years after planting. The trees bloom in spring. The fruit develops in summer. In August or September, the fully developed fruit falls to the ground or is machine harvested. It is then taken to a dehydrator in lug boxes or portable bins, where it is placed for a few seconds in a hot solution of sodium hydroxide (also called lye). The dehydrator dries the plums with a forced draft of hot air. The drying lasts from 14 to 24 hours and reduces 3 pounds (1.4 kilograms) of fresh plums to about 1 pound (0.5 kilogram) of prunes.
The prunes are placed in bins, where they are cured for at least two weeks. Curing gives the prunes a uniform moisture content of between 18 and 20 percent. The prunes are then graded according to size, and may be pitted. Before the prunes are packed to be sold, they are given a hot water or steam bath to pasteurize them and to bring their moisture content to between 26 and 32 percent.
