Wash the ripe ume thoroughly to remove any dirt.
ripe ume plums
Drain well and pat completely dry.
Remove the stem ends with a toothpick or bamboo skewer, then trim away any bruised or damaged spots.
Weigh the prepared ume in an enameled pot. Add sugar equal to 70% of the fruit's weight and toss well. Let stand for 15–30 minutes until the sugar begins to dissolve.
sugar
Cook over very low heat, stirring constantly. As the fruit softens, gently mash the ume with a spatula.
Once the flesh has separated from the pits, remove from the heat. When cool enough to handle, remove the pits.
Transfer to a clean storage container, cover, and refrigerate.
To serve, mix with cold water, hot water, or sparkling water.