If they are pure juices you should be fine Polly. Take care with labels though. Anything with an artificial sweetener will never ferment out to taste 'dry' - you'll always have an underlying sweetness. Sometimes this isn't stated obviously - they may say - 'no added sugar' which can mean they have added something else!
I once had a real disappointment with a pineapple juice which had artificial sweetner in. It was just yukky when fermented.
Lucky you!

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vegheaven.blogspot.com Updated July 7th 2008