Left to its own devices, fruit quickly spoils or ferments as bacteria and mold gleefully seize upon its natural sugars. Drying the fruit extends its shelf life dramatically, by reducing its moisture content to a point that leaves microorganisms too desiccated to survive. Still, dried fruit such as raisins should retain enough moisture to be pleasantly chewy rather than tough. If they pass that point, drying to tooth-sticking leatheriness, they’ll need softening before you eat or bake with them.
To restore your raisins to their original tenderness, simply drizzle them with a few tablespoons of warm water and microwave them on High for 12 to 15 seconds. Pour off any excess water, cover the bowl, and let the raisins absorb the moisture until they return to room temperature. Alternatively, preheat your oven to 200 degrees Fahrenheit and spread the raisins between layers of moistened paper towels. After 20 to 25 minutes, the raisins should be nicely softened. In either case, they should be eaten or used on the same day or refrigerated, because the added moisture makes them perishable.
Plumping the Raisins
Rehydrating the raisins before baking with them prevents them from drawing too much moisture from the batter, but some recipes take this further and call for the raisins to be plumped. This simply means soaking them in boiling water — or for holiday baking, water mixed with rum or bourbon — for 10 to 15 minutes until they’re partially reconstituted. You can add spices or citrus zest to the liquid for added flavor, or soak them in heated fruit juice instead. The plump, soft raisins slice more readily in the finished goods, and add both flavor and moisture to their texture. The same techniques can be used for other dried fruits and berries, as needed.
References and ResourcesBulk Barn: Jumbo Flame Raisins
Los Angeles Times: Test Kitchen Tips -- Rehydrating Raisins and Other Dried Fruit
The Kitchn: Tip -- How to Plump Up Your Raisins
On Food and Cooking: The Science and Lore of the Kitchen; Harold McGee