Our expert says:
The water-soluble vitamins (the entire B-complex and vit C) will oxidise when exposed to the oxygen in air and high temperatures, and they will dissolve in the cooking water. Thus any cooking will result in losses of water-soluble vitamins. In the case of stir-frying the water-soluble vitamins will be exposed to oxygen when the food is cut into small pieces and the high cooking temperature will promote the process. However, the short time of exposure to high heat and the fact that the vitamins will not dissolve in water, will limit these losses again. The fat-soluble vitamins (vits A, D, E and K) may also be affected by high temperatures, but in the case of vit A and its precursor, beta-carotene, the use of fat or oil in the cooking process will enhance absorption thus cancelling out losses due to heat. So the Golden Rules of preparing vegetables are: don't cook if possible and eat with skin/peel as fresh as possible; if you cook with wet heat (boiling, steaming) do not cut up the vegetables into small pieces if possible and cook as rapidly as possible and use the water the vegs have been cooked in for sauces or soups; for other forms of cooking like stir-frying - work as rapidly as possible and do not allow cut-up vegies to stand around for hours being exposed to oxygen.
I do hope this clarifies the issue
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