Vitamin C is an antioxidant, which means that it prevents vegetables and fruits from browning, like a peeled pear for example.
Vitamine C is in several fruits and vegetables like blackcurrant and lemon, but it can be bought in powder form for use in cooking. If you intend to buy: You can find it in drugstores or chemist's as "ascorbic acid". Ask for vitamin C for cooking use, and it will be sold to you as a white powder. If you want to use it: Vitamin C is ideal for keeping the attractive appearance of fruits and vegetables which need to be peeled, cut and/or blended. You need only use a pinch each time.