Place seeds in a heavy-duty zip-top plastic bag; seal, pressing as much air as possible out of bag. Gently mash seeds with a rolling pin or bottom of a bottle. Place mashed seeds, juice, and water in a medium nonreactive saucepan, and bring to a boil. Cook 10 minutes. Strain mixture through a sieve into a bowl; discard solids. Store in refrigerator or proceed with recipe as directed.
Be sure to use a nonreactive saucepan, such as one with a nonstick coating. Aluminum will react with the acid in the seeds, giving the juice a slightly metallic or off taste.