In a small sauce pan, heat 4 cups of either the apple or white grape juice over medium-high heat until just beginning to boil. Place the sachet into the juice, reduce the heat to medium-low and simmer until reduced by about half (20-30 minutes). Remove from heat and allow to cool for 15 minutes. Remove the sachet, squeezing all the juice from the bundle. Pour the syrup through a fine mesh sieve into a clean glass container. Allow to completely cool. Cover and refrigerate until ready to use. The syrup will become thicker as it cools.