> Perhaps the brand I buy has a lot less sorbate than what's available
> to you. But I had no trouble in fermenting juice from tetrapaks.
We don't have Tetra Pak products here, so I had to do a bit of research. Tetra Pak is both the name of the company and its packaging process. They use ultra-high temperature pasteurization combined with an aseptic process to produce shelf stable juices. Here's a link that provides more details:
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It appears that most juice products packaged this way don't require preservatives. Here's a link to one such Tetra Pak apple juice product:
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I'd be willing to bet that the juice you've used has no sorbate. Sulphites are just a bump in the road for a wine yeast, and wouldn't even prove much of a challenge for a wild yeast. Ascorbic acid also presents no problem for wine yeast.
Greg G.