Ripe-Grape Jelly

A cross-reference recipe: the method is the standard currant-jelly one, and the only new instruction is the choice of fruit. That choice is the whole point, and worth understanding before you start.

• Why acid grapesJelly sets on the interaction of pectin, acid and sugar. Fully ripe sweet grapes are low in acid and their pectin has begun to break down, so the batch runs thin no matter how long it boils. Slightly underripe or naturally tart varieties (Concord is the classic American choice) carry both the acid and the pectin.
• The currant-jelly method, in briefCrush the fruit, heat gently until the juice runs freely, then drain through a jelly bag without squeezing. Squeezing forces pulp through and clouds the jelly. Modern guidance for grapes is roughly 3/4 cup sugar per cup of juice; check the proportions given in the book's own currant-jelly entry, which this recipe defers to.
• The set pointBoil hard and test at 220F, or 8F above the boiling point of water where you live. The old "sheeting" test off a spoon is the same endpoint by eye: the last drops run together into a curtain rather than falling separately.
• Tartrate crystalsGrape juice throws gritty crystals of potassium bitartrate that are harmless but unpleasant in the finished jelly. Let the strained juice stand overnight in the coldest place you have, then pour it off the sediment before boiling with sugar.
• No added pectinThis is a long-boil, no-commercial-pectin recipe. Powdered and liquid pectins call for entirely different sugar ratios and boiling times, so do not try to combine the two approaches.
• SealingJelly of this period was commonly poured into glasses and sealed with melted paraffin, though this entry says nothing about sealing. Paraffin seals can fail invisibly and mold grow underneath; current practice is a two-piece lid and a short boiling-water bath. Follow a present-day tested processing time rather than the wax.
• YieldThe recipe specifies no quantities. As a rough modern guide, about 4 lb of grapes gives roughly 4 cups of juice, which makes about four to five half-pint jars.