These sun-dried Thompson Seedless grapes constitute 95% of the Californian raisin crop. The green grapes naturally develop a dark brown color as they dry in the sun, a process that takes from two to three weeks. Storage: Unopened packages of raisins will keep almost indefinitely in the refrigerator. Once opened, reseal the package, excluding as much air as possible, or transfer the raisins to an airtight jar or bag. Proper storage will deter the fruit from drying out and will prevent the sugar from crystallizing on the surface. If refrigerated, the raisins will keep for up to a year; they will stay even longer in the freezer and they will thaw quickly at room temperature.