How does temperature affect Vitamin C content in citrus fruit juices?

Im doing a school science project about how temperature affects Vitamin C content in Orange juice. I have results and they show that from 30 to 70 degrees celsius, the amount of vitamin c increases steadily and on the internet i found some explanation that could support this which is that fruits contain oxidative enzymes which oxidizes vitamin c and therefore destroys it and increased temp could destroy those enzymes and protects the vit c before the vit c can be oxidized. however at 80 degrees celsius, the amount of vit c decreased by 1/3 and then at 90 went back up again and then at 100 the amount of vit c is more than when the juice was at 70 degrees celsius. why is this so? and what would happen if the temp goes higher? i only know that once it reaches 190 degrees celsius, the vit c will be decomposed and lost. and what about when the temp goes below 30 and even below freezing point? Tks for your help i would have been able to experiment with the different temps but the school lab wouldnt allow me to continue using it for this module in sch that im doing. tks:D