If it's turning color quick you may very well be getting it too hot. I also think you would be better off to put your mix in warm water bath (put it in it's jar first and no hotter than 120F). I used a thermal coffee cup, put in 120F water up to the collar of the bottle I was using for my juice, them put the top on. Let it sit for 30 minutes, shake and repeat once or twice. I would let the juice come back to room temperature between warm water baths. Like I said, I now just sit on top of my cable box - I measured it and it's never gets above 110F, so it's no muss, no fuss for me, but I've had cable boxes in the past that were hotter. Warm water is probably the safest way to go without a UC and even with that you have to monitor temperature.
Cheers,
Steve
I tend to agree with Barbara about creating hotspots using the microwave on the juice. I know if I heat up water it can be steaming on top but after stirring it, it's only warm.