When I recently shared my recipe for a Kale Salad With Oven-Dried Grapes and Blue Cheese, I got a couple humorous comments that gently questioned the logic of making raisins at home when they're readily available by the box. But I also heard from a lot of people who wanted to know more. Chief among them was our own Stella Parks, who was excited by the potential baking applications.
The idea is exciting because oven-dried grapes aren't at all the same as store-bought raisins. They're larger, plumper, and juicier, and, if you use a variety of grapes, they have the potential to deliver a range of grape flavors, too. That grape flavor, meanwhile, is closer to that of fresh grapes than a fully dehydrated raisin is—it's just more intensely concentrated, with a light caramelization from the sugars on the exteriors of the grapes (more on that below). This is because ovens, even at their lowest settings, are hotter than a dedicated dehydrator. Oven-drying them yourself also gives you control over the degree of dryness: Do you want them more plump and fresh, or more concentrated and shriveled?
In practice, what this recipe means, aside from a really fun addition to salads, is that you can stud baked goods, like Homemade Bagels and Bran Muffins, with these fat, juicy beauties and get results very different from what you'd see with regular raisins (even rehydrated ones). Other ideas: work them into a bread pudding, use them as a more interesting accompaniment to cheeses on a cheese platter, and much, much more.
In my original round of testing, I tried out a few different methods of oven-drying the grapes, based on tips I'd read online. Some recipes suggest blanching them first, for instance, which they claimed would crack the skins, giving moisture more escape routes and leading to faster dehydration. Others said to cut the grapes in half, another road to exposing the interior of the grape and speeding the drying process.
In reality, though, my tests revealed that the time savings provided by those extra steps were small and, at least in my eyes, not worth the effort of either pre-blanching the batch or hand-slicing each individual grape. That's good news, because it means less work for you. All you have to do is put the grapes on a rimmed baking sheet, pop them in a low oven, and wait.
This time around, I decided to try out a larger variety of grapes, using black, red, and green seedless, just to see how much of a flavor difference there was from one to the next. Because of the time of year, I couldn't find less common varieties of grapes, like Concords, which would have been fun. In any event, even among basic commercial varieties, there were noticeable differences in flavor as the grapes dried, so it stands to reason that other grapes, with even more unique flavors, would lead to even bigger flavor gains. My green ones were the most tart, with a delicate floral taste; black grapes came out the sweetest; and reds somewhere in the middle.
About that caramelization I mentioned above. Fructose, the primary sugar in grapes, doesn't begin to caramelize until it hits 230°F (110°C). Given how water-laden grapes are and the fact that the water's boiling point will prevent the interiors of the grapes from surpassing 212°F (100°C), at least until a whole lot of it has cooked off, there's not much caramelization happening inside the grapes. But as they cook on a baking sheet, their juices leak out and pool below them, quickly drying on the sheet into a thick syrup. That syrup does reach caramelization temperatures, and some of it clings to the exteriors of the grapes. It's extremely delicious (I even licked it off my baking sheets afterward), but if you don't want a caramelized flavor at all, you could suspend the grapes on a wire rack set over the baking sheet, which would keep them from sitting in those juices as they dry in the oven.
In all of my tests, I used the lowest temperature allowed by my oven, but Stella did her own tests and reported that you can get away with bumping the temperature up as high as 300°F (150°C) if you're willing to trade the fruit's vibrant color for darker, faster results. Exactly how much time is shaved off depends on your oven, your grapes, and how much you allow them to dry out. I had batches that were ready in anywhere from three to four and a half hours at 225°F (107°C), and Stella's were done in four hours at 300°F (though she made a triple batch, and, when we compared notes, it looked like she dried hers more than I did mine).
The last big question, of course, is storage. Fully dehydrated raisins keep for many months, if not longer, but my testing revealed that these juicier oven-dried grapes have a shorter lifespan. After about three weeks in the refrigerator, mine started to taste kind of strange...not spoiled, really, but just flat and no longer fresh. Stella's batch, meanwhile, is still going strong in her fridge after almost a month. The most likely explanation is that since hers were more dried than mine, their more concentrated sugars delivered a longer shelf life.
You could try to control for this by weighing the grapes before and after, to be more precise in how much you allow them to dry (and therefore get a more accurate sense of how long they'll last—if you're willing to do the testing to figure that out), but that seems a little too fussy to me. Better, I think, is to keep in mind that there's a relationship between degree of dryness and shelf life, and not count on having your batch last more than two or three weeks.
Ultimately, the moral of the story is that you have control over how your oven-dried grapes turn out, which is more than store-bought raisins can say for themselves.