I have run some tests with apple juice with pulp in it. If I use apple juice without pulp, I get a reading of the free ClO2 in the solution, but if I use the apple juice with pulp in it, the free ClO2 drops to 0 PPM.
The interesting thing is that I can duplicate that same results be putting some vitamin C into the solution.
I wonder what it is in the apple pulp that kills off the free ClO2... I don't think that apples are a high source of vitamin C.