Most grapes are dried on paper trays in the field to make raisins, but researchers at UC Davis have found that there are distinct differences between raisins dried in the traditional way and those dried directly on the vine. Their testing looked at factors that included "fruitiness, chewiness and color" and the tasters were split on which raisins they liked more, though it was clear that there was a big difference between the two types. The vine-dried raisins were said to be softer, plumper and more complex, almost like wine, while the tray-dried raisins were stickier and more caramely. The noticeably different flavor profiles may open the door for gourmet raisins if some growers switch over to the vine-dried variety.
Vine-dried raisins are less labor intensive than tray-dried raisins, but they usually need to be harvested with a machine and take much longer to dry, needing as much as a month on the vine, as opposed to a week on the trays. As a result, they are more expensive to produce and not all of California's growers, who produce almost all of the US's raisins and about 40% of the world's supply, would be willing or able to make the switch.