There are many anti oxidants. Vitamin C, Vitamin E, Beta Carotene, Lycopene, Pycnogenol etc. Basically oxygen forms free radicals in our bodies and the anti-oxidants help eradicate the damage caused by them. In other words we are going rusty!
Try this experiment: Your mum probably does it with making a fruit salad. Take an apple, cut it in half and leave one half on the plate, but squeeze lemon jice over the other half.
Email me back about what happens!