Vegetables are the primary source of vitamins and minerals in our daily lives. Vitamins and Vitamin C are both water-soluble vitamins, and they are easily destroyed by heat and light. Vitamin C, in particular, is very "delicate" in the presence of oxygen, heat, and light. Therefore, vegetables should be washed before being cut, and it is even better not to soak chopped vegetables for extended periods to reduce the loss of water-soluble vitamins.
Some people like to prepare vegetables early and then cook them all at once just before the meal. This method leaves the vegetables exposed for too long, increasing the surface area in contact with air and causing the vitamins to oxidize and break down. It is still better to cut and cook them fresh.
When cooking vegetables, it is best to use a high-heat, quick-frying method. This can shorten the cooking time, thereby reducing the loss of nutrients. Experiments have shown that cooking vegetables for 5 minutes can lead to a 5% loss of Vitamin C, while cooking for 10 minutes can result in a 30% loss.
To reduce nutrient loss, you can add a thickener (starch slurry) when stir-frying. This can effectively protect Vitamin C and also blend the sauce with the dish, allowing some of the leached nutrients to be consumed along with the vegetables.