If you are wondering on which temperature you should cook your food without losing important nutrients such as vitamin C, the answer is not simple since it depends on lot of factors among others and the source of that vitamin
One of the best examples for that is orange. Orange will keep most of the vitamin C in it while the skin is intact. But once the skin starts to break down, due to fruits ripening, vitamin C also gets destroyed. So, it is good to keep oranges in the fridge, because it that way orange will ripen slower. One of the important factors is the pH of the food because vitamin C is a weak acid, so it will last longer in that kind of environment.
So, there is not just one factor that will destroy (or keep alive) vitamin C. One of them is oxidation. If you cut an orange in half, vitamin C will be destroyed faster. If you squeeze the juice from the orange, that is also a good way to destroy this vitamin. And even worse, if you put that juice in the fridge, it will also make a big damage.
Besides cooling food down, cooking too can destroy important nutrients. But, if you cook in a small amount of water, vitamins will be preserved, as the opposite of cooking in a large amount of water where vitamins get destroyed. Also, cooking in the microwave is less destructible than cooking food in hot water.
One interesting fact is that fries have more vitamin C than mashed potato. Potato is cooked in water (when you want to make mashed potato) and that kills vitamin C, but when it comes to french fries, they are cooked in fat, and that doesn’t destroy vitamin C rapidly.
Vitamin C gets destroyed after some period of time, so if you have a bottle of pills with vitamin C, and it stands on your counter for a while, well, it may not have vitamin C at all in it. Because it gets easily destroyed.