Green Grape Jelly

This is a cross-reference entry rather than a standalone recipe: the whole instruction is in the state of the fruit. Everything else is borrowed from the Grape Jelly recipe elsewhere in the chapter.

• Why underripe grapesThe hinge of the recipe is the picking stage. Pectin and acid are both at their highest just before full ripeness and fall off as the fruit sweetens, so grapes "just beginning to turn" set firmly with nothing added. Fully ripe grapes give a softer, less reliable set.
• The parent methodThe referenced Grape Jelly recipe is not reproduced here. Grape jelly of this period generally means: stem and crush the fruit, cook it gently until the skins break down and the juice runs, drain through a jelly bag, then boil the juice and add heated sugar, commonly about three-quarters the measure of the juice. Squeezing the bag gives a cloudy jelly.
• Sugar proportionBecause green grapes are more acidic and less sweet, the three-quarters ratio can taste sharp. Going much below it, though, weakens the set, so adjust cautiously rather than cutting sugar freely.
• Testing the setPeriod jelly recipes rely on the sheeting test off a spoon. A thermometer is faster: 220F at sea level, or 8F above the boiling point of water where you are.
• Tartrate crystalsGrape jelly is prone to gritty crystals of cream of tartar. Letting the strained juice stand overnight in a cool place and pouring it off the sediment before boiling largely prevents them.
• JarsPeriod recipes assume the jelly is poured hot into sterilized glasses and sealed with paraffin. Paraffin seals are no longer considered sound; use standard canning jars and a boiling-water bath, or refrigerate and eat the jelly within a few weeks.
• YieldNo quantity is given. As a working figure, 4 lb of grapes gives roughly a quart of juice, which makes about four or five half-pint jars.