In a large saucepan, crush the grapes with the lemon juice using a potato masher. Bring to a boil and simmer gently for about 30 minutes, uncovered.
Line a colander with several layers of cheesecloth and place it over a large bowl.
Pour the grapes in the colander and let drain for about 2 hours without crushing or squeezing the fruit. You should have about 1.5 litres (6 cups) of juice. If the amount of juice obtained is different, remember that you’ll need about 180 ml (3 /4 cup) of sugar per 250 ml (1 cup) of juice.
In the same clean saucepan, bring the grape juice and sugar to a boil. Attach a candy thermometer in the centre of the pan. Simmer until the thermometer reads 104 °C (219 °F). Skim carefully during cooking.
Pour into sterilized jars.
Unsterilized jars will keep for several weeks in the refrigerator.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.