If you watch the latest videos, Jim Humble says when asked about the orange juice that he doesn't know why it affects the ClO2, he says it can't be vitamin C in that case.
Did you test clear apple juice and get the same results, If so then it isn't the pulp is it.
I used some clear apple juice from concentrate once and it tasted strong enough
also the pinapple juice I used had 1g of protein per 250ml and I could taste the ClO2 well enough and it certainly worked.
Orange juice could contain a bit more pulp or protein maybe but so does some grape juice, so perhaps a lot more testing is in order to figure out what it is that the ClO2 is reacting with
I'm happy using pinapple juice myself, though perhaps diluted a bit