You do your best – purchasing fresh vegetables and trying to eat three or more servings daily. To sneak a few in, you mix them into dishes you’re already cooking – a veggie omelet, stir-fry, or mac and cheese. But what you might not know is that cooking vegetables can change their composition, resulting in you not taking in the nutrients you think you are.

The way you prepare and cook a food plays an important role in the food’s nutrient density, such as the number of antioxidants, vitamins, minerals, and fiber the food can offer.

A 2009 comprehensive study looked at the cooking methods of boiling, microwaving, pressure-cooking, griddling, frying, and baking on the antioxidant and nutrient properties of 20 different vegetables – and the study’s results might change the way you make your next meal.

Cooking methods

Boiling until tender in a liter of water causes the most antioxidant loss across all vegetables. Peas, cauliflower, and zucchini lose more than 50% of their free radical-scavenging capabilities. Spinach, garlic, broccoli, brussels sprouts, leeks, and green beans fair somewhat better, maintaining 30-50% of their antioxidant capacity. Artichokes, onions, and eggplant fare very well, only losing 5-30% of their antioxidants.

The best vegetable to boil? Asparagus. If cooked perfectly, its antioxidant capabilities actually increase, so monitor its time-to-tenderness in hot water.

Pressure cooking vegetables in 300 milliliters of water only takes about five minutes, but it has similar outcomes to boiling. Most vegetables lose 25-50% of their antioxidant capabilities, excepting broad beans and spinach, which lose slightly less. Swiss chard, beetroot, onions, artichokes, and asparagus do better, and eggplant fairs the best (and only takes about six minutes).

Baking in a ceramic dish for 20 minutes or more is time-consuming, but is beneficial for some vegetables.

It can cause 30-50% antioxidant losses in brussels sprouts, leeks, cauliflower, peas, and zucchini, but onion, broad bean, celery, beetroot, and garlic only lose 5-30% of their free radical-scavenging capacity. 

Fortunately, there are plenty of options to make as a main meal or slide into a side dish. Artichokes, asparagus, broccoli, and pepper do well, and green beans, eggplant, corn, Swiss chard, and spinach can increase their antioxidant activity significantly.

Microwaving in a glass dish with no added water takes only a few minutes, and is probably the most common cooking method for busy households. Whereas cauliflower fares the worst, artichoke, asparagus, garlic, onion, and spinach do well.

Consider making dishes that use eggplant, corn, peppers, or Swiss chard – these few are known to increase their antioxidant capacity from being microwaved.

A much less common cooking method for veggies is griddling. Griddling uses a frying-style pan, but with no oil. Brussels sprouts and peas lose the most antioxidant benefits during griddling, but artichokes, beetroot, celery, eggplant, garlic, and corn maintain theirs.

Most of the lower-carbohydrate green vegetables improve with griddling; green beans, Swiss chard, spinach, asparagus, broccoli, and onions significantly increase their free radical-scavenging capacity with this method. Just griddle, top with spices or fresh herbs, and enjoy.

Frying vegetables in 169-degree refined olive oil may make vegetables unrecognizable in taste, shape, and nutrition.

It causes the biggest antioxidant losses in zucchini, although Swiss chard, artichokes, and green beans can maintain their capacity. If you love fried eggplant, you are in luck. The 2009 study found only the eggplant increases its antioxidant activity when fried, compared to its fresh version.

In summary, which vegetables do best?

Across cooking methods, the invincible artichoke maintains its antioxidant capacity the highest through all cooking methods. Beets, green beans, and garlic keep their antioxidant power during most cooking methods, and celery fares well, except when it is boiled.

Which vegetables do poorly?

Cauliflower loses the most antioxidant capacity after boiling and microwaving. Swiss chard and peppers also lose antioxidant properties in all cooking methods. Garlic loses antioxidant properties when cooked, except when it is microwaved.

What is the best cooking method?

Although it depends on what you’re cooking, griddling, microwaving, and baking are the safest bet for vegetables because they result in the lowest losses of antioxidant properties. Methods of cooking that involve water – like pressure cooking and boiling – appear to cause the most loss, with frying vegetables falling somewhere in the middle.

What is it about cooking that changes food properties?

Whether you are cooking produce, starches, or meats, the environment, modality, and other exposures can alter the nutrients in your foods, for better or worse.

1. Hot temps

Heating, while brightening up your plate, can reduce a vegetable’s vitamin content. Heat causes the breakdown of cell membrane structure in vegetables, which releases gas and allows us to see the bright green color emitting from chlorophyll. Similarly, the structure of proteins and fats changes with heat also, usually changing color.

2. Amount of water

Most water-soluble vitamins are lost through cooking in water. Also, cooking in water can decrease the amount of potassium, calcium, and sodium in foods, especially from overcooking or dicing foods into small bites. To avoid excessive loss, use less water when cooking or try steaming. Cut foods in bigger pieces, so there are less surface areas to lose nutrients, or use the water smaller pieces of vegetables are cooked in, like in a soup.

3. Exposure to light and air

Light and air mostly affect vitamin C, but losses can be minimized by cutting up vitamin C-containing foods just before eating. As when cooking in water, reducing the surface area will reduce the damage – or eat as soon as possible after harvesting and preparing.

4. On the barbeque

Grilling is an easy method of cooking, but it presents some health risks because two types of carcinogenic compounds can be produced: HCAs (heterocyclic amines) and PAHs (polycyclic aromatic hydrocarbons). This occurs when fatty meat comes in direct contact with the smoke or flame of the grill. This can be reduced by opting for leaner cuts of meat, with less fat to drip on the grill and produce smoke. Try marinating your favorite meat before grilling, which can serve as a protective barrier against the chemical reaction that produces HCAs; or grill at lower temperatures and avoid flames. Keep an eye on the grill and don’t char your food.

5. Right from the garden

Although raw veggies fresh from the garden are full of nutrients and fiber, there are pros to cooking some produce. Cooking tomatoes and other lycopene-containing foods makes lycopene more accessible. Likewise, cooking or juicing carrots results in greater bioavailability of beta carotene. Cooking can also kill any harmful microbes or at least make it harder for bacteria to grow and multiply.

The amount of nutrients lost during cooking will vary depending on the characteristics of the food, like ripeness, preparation, and cooking methods.

Try to consume your produce raw as often as possible or cook your vegetables in a way that will keep the nutrient loss to a minimum. Because soil is often depleted of minerals, you might need a multi-vitamin/mineral supplement to ensure you are getting 100% or more of the nutrients you need.

Rules of thumb

  • Fat-soluble vitamins (A, D, E, K) tend to be lost when cooking with oils, so either use less oil (or heat) or make sure you consume the oil too.
  • Water-soluble vitamins (B vitamins, vitamin C) are lost when cooking with water, so use less water when possible.
  • Best cooking method: although it depends on the vegetable, when in doubt, microwave or steam with little to no extra water.


Reference
Jiménez-Monreal A, García-Diz L, Martínez-Tomé M, et al. Influence of cooking methods on antioxidant activity of vegetables. J Food Sci 2009;74(3):H97-H103.