All foods (fruits and vegetables included) start to naturally oxidize once they are picked, shelled (nuts) or otherwise removed from their natural state. Eat as close to fresh as you can. Frozen isn't too bad if they are flash frozen shortly after picking. Only eat things that are actually in a season - they taste better because they actually have the nutrients they are supposed to. And for your own health, I would suggest only eating organic label foods. You can read up on all the chemicals and toxins in food at naturalnews.com. Final thought, try and find a farmer's market around you.