Blackberry Jam

A two-day jam that relies on an overnight rest to draw juice and dissolve the sugar before the final boil. The instructions assume a cook who knows what set jam looks like, so the gaps are worth filling.

• The ratio is by weightWeigh the fruit after hulling and picking over; volume measures of berries vary too much to substitute reliably. As a rough guide only, three-quarters of a pound of sugar is about 1 2/3 cups and a pound of blackberries about 3 cups.
• The first cooking is brief"Cook the fruit a little" means heating the berries just until they slump and release juice, five to ten minutes over medium heat, mashing as they go. Long cooking at this stage drives off the pectin advantage of slightly underripe fruit.
• The overnight rest does real workStanding with the sugar dissolves it fully and lets osmosis pull juice from the fruit, so the second boil is shorter and the flavour stays brighter. Cover and refrigerate rather than leaving it on the counter.
• "Until thick" means 220FJam sets at about 220F at sea level, or 8 degrees above the boiling point of water where you are. A cold-plate test works as well: a spoonful on a chilled saucer should wrinkle when pushed after a minute in the freezer.
• SeedsThe recipe as written leaves the seeds in. For a seedless jam, press half or all of the cooked fruit through a sieve before adding the sugar, and expect some loss of yield.
• No pectin, no lemonThe recipe relies entirely on the fruit's own pectin and acid, which is why a proportion of slightly underripe berries helps. Fully ripe fruit alone can take a long boil to set and will taste cooked; a tablespoon of lemon juice per pound is the usual insurance.
• SealingJam of this period was commonly poured hot and sealed with paraffin. Use current water-bath processing instructions for the jars and headspace instead, or simply refrigerate the jam and use it within a few weeks.