i do this a lot, and hophead is correct. if you see potassium sorbate, it's the main preservative and will prevent fermentation (which is what it's supposed to do). ascorbic acid is vitamin c, and is fine. i tend to use Great Value brand (wal mart's juice) as they don't use preservatives, and to me, it's just as good as the more expensive welch's, mott's, etc
There is no "i" in denial.