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Vitamins and minerals in vegetables are easily lost or damaged under heat and acid. Therefore, the rule of thumb is not to overcook them. Blanch them in boiling hot water or stir fry them on high heat, but for a short period of time. Stop cooking once the vegetables turn soft. Green leafy vegetales turn yellow and lose most of their nutrition values when overcooked.
It is also important to eat vegetables fresh. Nutrients are also lost through freezing and oxidation. Preserved vegetales like pickles contains mush less nutrients than fresh ones.