Wash the grapes and remove the stems. Put them in a pot and mash them coarsely with a potato masher.
Add 100 ml of water and bring the grapes to a boil. Simmer for about 8 minutes, remove the pot from the heat and add the rosemary sprigs. Let steep for about 15 minutes.
Hang a large strainer in a pot and line it with a straining cloth (muslin cloth). Pour in the grape mixture and let the juice drip into the pot overnight.
Measure 700 ml of juice and mix with the jelling sugar (2:1) and lemon juice. Boil for 4 minutes until bubbling and do the jelly test.
To do this, drip some of the juice onto a cold plate. If it becomes solid, the jelly is ready. If not, let the mixture boil a little longer.
Immediately pour the jelly into prepared twist-off jars. Place on a tea towel with the lid down and allow to cool.
Preparation Tip:
A visual and flavorful topper for the Isabella grape jelly is very finely chopped strawberry pieces, which you add to the jars before you pour in the jelly.