Cooking causes the loss of some nutrients in vegetables. Here is an overview of how different cooking methods affect the nutrients in vegetables, and how the loss can be minimized.
Boiling
Boiling is the cooking method that causes the greatest loss of nutrients in vegetables. Many of the water-soluble vitamins as well as other nutrients are transferred into the water and lost if the vegetables are drained and the water is thrown away rather than reserved and consumed as soup or broth.
The amount of nutrients lost into the water depends on how long the vegetables are boiled, as well as the surface-to-volume ratio. Cutting the vegetables into large pieces or cooking them whole will aid in retaining nutrients, while mincing them into small pieces before cooking will result in the greatest nutrient loss. It is best to cook root vegetables including sweet potatoes, carrots, and potatoes whole and unpeeled.
Boiling and draining vegetables results in a loss of 75% of the vitamin C and folate, 70% of the thiamine and potassium, 65% of the vitamin B6, 55% of the niacin and sodium, 50% of the vitamin B12, 45% of the riboflavin and copper, 40% of the iron and magnesium, and 35% of the vitamin A and phosphorus.
Baking
Dry heat from baking destroys certain vitamins and other nutrients, including vitamins C, B1 and lysine. The effect is worse if the heat is too high or the vegetables are baked for longer than necessary.
Blanching
It is recommended to blanch vegetables before freezing them. Blanching is the process of scalding the vegetables by placing them in boiling water for a short period of time, usually about 2-3 minutes. This process kills bacteria and inactivates certain enzymes that can impair the color, flavor and texture of vegetables when they are frozen. However, blanching even for a short period of time still causes the loss of some nutrients. To minimize nutrient loss, vegetables should be placed into ice water immediately after blanching.
Steaming
Steaming is generally considered to be the best method to cook vegetables in order to preserve their nutritional content. Steaming uses a minimal amount of water and the vegetables are not submerged. In addition, steaming is a relatively quick method of cooking, so the vegetables are only exposed to heat for a short time.
Microwaving
Since microwaving is fast and only requires a small amount of water, it is often recommended as a preferred method to prepare vegetables to avoid nutrient loss. However, one study found that microwaving drastically reduces the amount of antioxidants in broccoli. Flavonoids in broccoli were decreased by 97% after microwaving, compared to 66% after boiling and only 11% when the broccoli was steamed. On the other hand, another study found more flavonoids were retained in potatoes and tomatoes by microwaving than by boiling.
Cooking Tips to Preserve Nutrients
No matter what cooking method is used, the most nutrients will be preserved by following these guidelines:
- Cut vegetables into large pieces or cook whole
- Put a lid on the pot to retain the steam
- Minimize the amount of water used
- Have the water boiling before adding the vegetables
- Minimize the cooking time
