Grape Jam
Heat grapes over boiling water till soft, then simmer gently thirty minutes. Let cool a little and press through colander. Weigh pulp, allowing one and one-eighth pounds sugar for each pound of pulp. Cook together, simmering and stirring carefully till it thickens into jam consistency, when place in small jars. Either ripe or green grapes or half and half may be used for this (The wild grapes will not harden as well). Lemon rind may be added.
A straightforward weighed-pulp jam that expects you to work by ratio rather than by volume. Two things carry it: the sugar is measured against the strained pulp, not the whole fruit, and the mix of ripe and green grapes is doing the pectin work.
- Heat over boiling waterThis points to a double boiler or a bowl set over a pan of simmering water, so the fruit softens and releases juice without scorching before there is any liquid in the pot. The recipe does not say whether the thirty-minute simmer continues over water or on direct heat; stir often either way.
- Which grapesThe recipe names no variety. It most likely has American slip-skin grapes such as Concord, Catawba or Delaware in mind, where the pulp pops free of the skin and the seeds are large. Seedless table grapes will work but give a thinner, blander jam with less pectin and acid.
- Press through colanderA colander removes skins and seeds; a food mill with a medium plate does the job faster and more thoroughly. Some cooks cook the skins separately and chop them back in for colour and body, which is worth doing with Concords.
- The sugar ratioOne and one-eighth pounds sugar per pound of pulp is 18 oz to 16 oz, close to 1.1:1 by weight. Weigh the pulp after straining, not the fruit you started with, or the jam will be cloyingly sweet and slow to set.
- Green grapes are the pectinUnderripe grapes carry more pectin and acid than fully ripe fruit. The recipe treats ripe, green and half-and-half as equally acceptable; all-ripe fruit may need lemon juice as well as the rind, or a longer boil.
- Judging "jam consistency"Cook to 220F on a thermometer, or test a spoonful on a chilled saucer: it should wrinkle when pushed after a minute in the freezer. Stir constantly toward the end, as the high sugar catches easily.
- Lemon rindAdd it in wide pared strips, no white pith, and fish them out before jarring, or grate it fine and leave it in. Either way it goes in for the last ten minutes so the oils are not driven off.
- Filling the jars'Place in small jars' says nothing about sealing; jams of this period were commonly hot-filled and sealed with paraffin. Current practice is to fill sterilised jars to a quarter-inch headspace and process in a boiling water bath for ten minutes, or simply refrigerate and use within a month.
- Gritty crystalsGrapes are high in tartaric acid, and jam made from them can throw hard crystals in the jar. Chilling the strained juice or pulp overnight and decanting off the sediment before adding sugar reduces this.
Grape jam belongs to the American home-preserving tradition that grew up around native vines rather than European wine grapes. The Concord, selected in Massachusetts in the mid-nineteenth century, spread quickly because it survived cold winters and fungal disease that killed imported stock, and its slip-skin flesh and pronounced acidity made it far better suited to jelly and jam than to the table.