This syrupy liquid is a traditional ingredient in many Roman recipes.
Take desired amount of grape juice (at least twice as much as you need of Caroenum ) and boil it until it is reduced to half the amount with which you started. If desired, add honey to taste and boil some more (until syrup thickens) to form Passum, another traditional Roman ingredient.
I assume it would probably taste like grape syrup. Does this help?