The first condition is for your vegetables to be fresh and not wilted.
Washing and cleaning them needs to happen immediately before heat treatment, with a stainless steel knife. If you can shred them without cutting them, it's always better.
Another important rule, which many homemakers don't follow, is for the cleaned vegetables NOT to soak in water, since the water-soluble vitamins and mineral salts are transferred into it.
Vegetables need to be placed in boiling water. This is so because at high temperatures, the substance which ruins vitamin C is neutralized, thus making it possible for the vitamin to be preserved.
Only use sturdy aluminum cookware that isn't flaking. Copper or iron cookware is not recommended, because when in contact with the vegetables, the vitamin C is destroyed.
When boiling vegetables, it's good for them to be submerged in water and for the container they are in to be closed with a lid.
Boil the products until softening, then remove from the heat. The longer they boil the more vitamins they lose.
It is best to consume vegetable dishes immediately after their preparation because after sitting for more than 1 - 2 hours, they begin to lose their vitamins. After the 3rd hour, 20-30% of the vitamin C is lost, and after 6 hours, it is completely ruined. If a dish is heated repeatedly, the vitamin C is further spoiled.
Further, you might wish to know that when steaming vegetables, the vitamin C is ruined less than if boiling in water.