Vitamin C is quite unstable in foods, especially under adverse conditions of storage and preparation. The vitamin generally occurs in foods that are quite acidic or in foods containing antioxidants that help to protect or preserve vitamin C content. Vitamin C in fresh whole fruits and vegetables is quite well preserved for days or weeks at a time. When foods are processed the vitamin may be exposed to oxygen, metallic surfaces, or high temperatures, all of which hasten oxidation. Cooking water is usually discarded and vitamin C, which is water soluble, is also lost in this way. Food Composition Tables list vitamin C values for raw foods minus 10-25% for expected losses. However, such calculations usually overstate the true intakes of the vitamin.