The San Joaquin Valley has been the U.S. center of raisin production for 140 years; this past year, production was 340,000 tons of dried raisins, about 30% of world raisin production. The vast majority of California raisins are natural sun-dried. Since the 1920s, the preferred grape variety has been the Thompson Seedless. Acreage of this variety has decreased because of low raisin and concentrate prices and the high cost of labor associated with traditional raisin systems: the fruit is painstakingly laid on the ground to dry on individual or continuous paper trays, exposing it to increasing risk of rain between the end of August harvest and October.
Because Thompson seedless grapes are late-ripening, there has been a rise in earlier ripening varieties, such as Fiesta, DOVine and Selma Pete. A different method of raisin harvest, Dry on Vine (DOV), can be harvested mechanically. Canes are cut, separating the fruit from its moisture source, but allowing it to dry on the vine, away from the risk of rain. This method requires a heavy investment in harvesting equipment up-front and different trellis systems, but does show promise for those growers willing to switch varieties and systems in order to continue to grow raisins.