While all meats require thorough cooking to prevent food-borne illness, vegetables are generally more nutritious when they're consumed raw. Beneficial vitamins may wash out of veggies when they're cooked in water, or nutrients may be lost in recipes that require high heat or long cooking times. However, you can still benefit from the nutrients in your food by learning which cooking methods best preserve vitamins.
Freezing and Reheating
The Self Nutrition Data website reports that freezing foods generally results in a typical maximum loss of only 5 percent in nutrients such as Vitamin A, Beta-Carotine, Folic Acid, Lutein, Lycopene and Thiamin. Vitamin C is the notable exception: frozen foods can lose as much as 30 percent of Vitamin C. The site notes that reheating foods can also cause a loss of nutrients ranging from 10 to 50 percent, with Vitamin C once again at the end high of the range.
The Health Central website reports that steaming veggies is a good way to preserve their nutrients, since steaming uses only a little water and the cooking time is relatively short. It adds that microwaving is even better, since a microwave requires no water at all and cooks very quickly. As an example, the site points out that broccoli loses 10 - 20 percent of Vitamin C when microwaved, as opposed to a loss of 27 to 62 percent when it's boiled in a lot of water.
Cooking can rob food of anywhere from 25 to 70 percent of a given vitamin, according to the Self Nutrition Data website. Draining your food after cooking can add 10 to 25 percent more vitamin loss, since water-soluble vitamins can be washed away and lost if the cooking stock is discarded. Vitamin C, Thiamin, Folate, Folic Acid and Vitamin B6 are especially prone to be lost when food is cooked and drained. The site notes that nutrients can also be lost when you roast, broil or fry in oil, and then discard the drippings.
Preventing Vitamin Loss
The Health Central website recommends microwaving and steaming veggies as the best way to preserve their nutrients. The site notes that there is a relatively small loss of nutrients when microwaving frozen foods, or reheating. It also recommends that if you choose to boil veggies, use only a little water, put the food in only when the water is strongly boiling and cover the pot to speed up cooking. Broil rather than fry to preserve nutrients, and use very little oil. Cook root vegetables such as carrots or potatoes in their skins to preserve their Vitamin C.