Some but not all vitamins are lost in cooking, and the amount varies widely.
Minerals are fairly stable in cooking, but different vitamins are susceptible in different degrees to various factors, including heat, light and air.
Generally, water-soluble vitamins like riboflavin, thiamine and especially vitamin C are more susceptible to losses in heat and water than are fat-soluble vitamins like A, D and E, which are more susceptible to oxidation.
Another factor is food acidity, which preserves vitamin C but degrades vitamin A.
Vitamin C and thiamine are the most delicate vitamins. In conventional canning using high heat, 33 percent to 90 percent of vitamin C might be lost, and 16 percent to 83 percent of thiamine.
A food that loses some vitamins even to high-heat processing is not an empty food, it just has lower levels of nutrients, and it may still be nutritionally valuable.
Storage as well as processing causes losses, and because of the preservative powers of canning and freezing, properly prepared preserved foods, even those processed with heat, may offer amounts of vitamins and minerals comparable to fresh foods, unless they are taken directly from the garden and stir-fried.
In one study, a bowl of peas on the table retained a similar share of vitamin C whether prepared from fresh peas (45 percent), frozen peas (40 percent) or canned or freeze-dried peas (both 35 percent).
In the frozen peas, some vitamin C was lost in processing, but it was not heated as much in the final preparation, while the fresh peas were exposed to more heat in cooking.