Fresh berries are among the most perishable of fruit, so how to preserve them while they're in season is always an urgent question. For example, fresh blackberries -- with their bold color and deep flavor -- will usually only last a few days once picked. You can freeze them for baking purposes, or turn them into jam or preserves with the addition of lots of sugar, but drying them is the simplest and purest way to extend their lifespan.
Blackberries aren't a quick-drying fruit, because each of the juicy little globes that make up a berry has a thin, waxy coating. Blanching the berries briefly in boiling water -- 20 to 30 seconds is adequate -- can strip this coating and cause minuscule ruptures in the berries' skins, which helps them dehydrate more efficiently. Dip the berries into ice water immediately to prevent them from cooking further, and then drain them thoroughly.
Start by picking out any stems and leaves, and discard any berries that are overripe or show signs of insect damage. Arrange the berries evenly on the dehydrator trays, leaving plenty of room between them for air to circulate. Set your dehydrator for 130 to 140 degrees Fahrenheit, if it's adjustable. Load the filled trays into your dehydrator, and turn it on. Dehydrate the berries until they're uniformly tough and leathery, almost hard. Depending on your dehydrator, this might take as little as 10 to 12 hours or as long as 16 to 18.
If your dehydrator circulates air from bottom to top, you might need to periodically shuffle the trays of berries so they dry evenly.
If you don't own a dehydrator, you can dry blackberries using your oven. Arrange the berries in a single layer on a parchment-lined baking sheet. Set your oven for 140 F, or, if it can't maintain a temperature that low, use the lowest possible temperature setting. Stir the berries every 3 to 4 hours to help them dry evenly on all sides. At the same temperature, they'll usually take a few hours longer to dry in the oven than in a dehydrator because the air isn't actively circulating. If your oven is at a higher temperature, the berries will dry more quickly but lose a bit of their fresh flavor.
If you have a convection oven, turn on the fan to improve air circulation and speed drying.
Berries aren't uniform in size, so the best of dehydrators can't guarantee that they'll dry evenly. To counter this, pack the berries loosely in jars or plastic containers. Seal them, and keep the berries at room temperature for at least four days and up to 10. This step, called conditioning, helps the berries' moisture content even out and prolongs their storage life. If you see condensation in the jars, the berries aren't dry enough for safety and should go back into the dehydrator.
Separate the berries into portion sizes that make sense for you. Pack them into airtight bags or containers, and keep them in a cool, dark, well-ventilated place for up to a year.