In a nonaluminum saucepan, bring the juice to a boil; pour over mint. Steep for 20 to 30 minutes. Use a strainer or paper coffee filter to strain juice, pressing mint to strain thoroughly. Return liquid to saucepan. Add sugar and vinegar or lemon juice. Bring to a boil; boil until sugar dissolves. Stir in pectin and return to a rolling boil. Stir for 1 minute. Remove from heat; skim off any foam. Pour into sterile half-pint jars. Place jars in a kettle covered with 1 inch of hot water; bring to a boil and boil for 5 minutes. Use canning tongs to remove jars from the water bath; place jars on a rack. Cool completely without disturbing jars. Makes about four 8-ounce jars.