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1
Place a small saucer in the freezer to chill.
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2
Grate the unpeeled apple on the large holes of a box shredder to the core; save the core.
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3
In a large, heavy saucepan or pot, stir together the grapes, grated apple and core, and 1/3 cup water.
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4
Bring to a boil over medium-high heat, stirring occasionally, until the grapes are broken down and the mixture is very juicy, about 20 to 30 minutes depending upon the grapes.
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5
Transfer to a fine-mesh sieve set over a bowl and, using a rubber spatula, press the juice out of the grapes.
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6
Discard the solids.
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7
Clean the sieve and strain the juice again through the lined sieve into a clean saucepan.
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8
Add the sugar to the grape juice and place over medium-high heat.
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9
Bring to a boil and cook, stirring occasionally, until thickened and syrupy, about 25 minutes.
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10
To test the jelly and see if it is ready, spoon a little onto the frozen saucer.
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11
Let it stand for about 15 seconds.
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12
If the liquid thickens to a jelly-like consistency, its ready.
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13
If not, continue to cook for another minute or two.
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14
Transfer the warm jelly to clean canning jars, filling them to 1/4 to 1/2 inch below the rim.
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15
Process the jelly according to our , or store in the refrigerator for up to 1 month.
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16
The jelly will be quite loose at first; let it set, undisturbed, for 2 days.
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17
It will thicken and gel.
