To make grape jam, peel the skins off Concord grapes, and cook the skins along with water for approximately 10 minutes. Combine sugar and the flesh of the fruits with the cooked skins, and cook for approximately 20 minutes. Pass the cooked grapes through a food mill, and cook the puree for about 30 minutes. Finally, test the jam for readiness over a chilled saucer.
For optimal results, choose succulent, unblemished Concord grapes, preferably with seeds. To peel the fruits, squeeze them and allow the pulp to fall into a bowl. When cooking the grape skins separately in a covered saucepan, add about 1 tablespoon of water for every pound of the fruit. Cook over low heat.
To the grape flesh add 1 cup of sugar per pound of fruit, as well as the cooked grape skins. Cook over medium heat until the grape pulp breaks down.
Passing the cooked grapes through a food mill separates the seeds and skins and purees the flesh. Transfer the pureed mixture into a saucepan, and cook over medium heat, stirring at regular intervals.
To test the readiness of the jam, carry out the gel test. To do this, place one or two saucers in the freezer when beginning the cooking process. After cooking for 30 minutes, drop a spoon of the hot jam mixture on a chilled saucer, and tilt to see if the jam runs. If it does, cook the mixture for five more minutes and repeat the test. The jam is ready when it does not run during the gel test.