Blackberries have a sweet tartness that comes alive when you use them to make jam. We add lime, which gives the beautiful purple jam a zest of citrusy sunshine and brightens the flavor of the blackberries. This jam begs to be put on your morning toast.
Put a saucer in the freezer, along with three metal spoons.
Rinse the blackberries in a colander under cold running water and drain. Put the berries in a large heavy nonreactive saucepan and toss them with the sugar, lime zest, and lime juice. Cook over low heat, stirring, until the sugar has dissolved and the berries have released a lot of juice, about 10 minutes. Clip a candy thermometer to the pan, gradually raise the heat to high, stirring frequently, and then continue to cook, stirring constantly and scraping the bottom of the pan so the mixture does not stick (if the mixture does begin to stick, turn down the heat slightly), until the jam registers 220°F on the candy thermometer, 15 to 20 minutes after it reaches a rolling boil. The jam will look glossy and dark. To check for doneness, put a teaspoon of the jam on the cold plate and return the plate to the freezer for 3 to 4 minutes. If you can make a line through the jam with your finger that doesn’t disappear, it’s ready. If the jam is still too runny, continue to cook and test until ready.
Immediately transfer the jam to a medium-mesh strainer set over a heatproof mixing bowl and force the jam through the strainer; discard the seeds. Carefully skim any foam from the top with a spoon. Let cool.
Store the jam in Mason jars or in an airtight container in the refrigerator for up to 2 weeks.
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