Major losses of vitamin C occur before cooking occur when vegetables are washed in large amounts of water and left to stand in water. Potatoes that are skinned prior to cooking lose more vitamin C while cooking (see Table 10). The slicing and dicing of vegetables will increase the rate of loss before and during cooking.
Ions of copper and iron play a significant role in vitamin C oxidation and thus the selection of process/preparation equipment can have a marked effect on the stability of the vitamin in food and drink products. Contact with copper, bronze, brass, steel or black iron surfaces should be avoided and only stainless steel, aluminum or plastic should be used. (Berry Ottaway, 1993). A steel knife quickly becomes dull if used for slicing lemons. The iron ends up in the lemon juice and vitamin C activity is reduced.