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How to Cook to Preserve Nutrients

Updated: Jul 20, 2018

We want to obtain the most nutrients from the food we eat. However, the way we cook our foods can have a significant impact on the nutrient content of foods. Most vitamins from fruits and vegetables are lost during storage and cooking, and therefore buying them from local sources and eating them raw may be the best way to maximise the nutrient intake. However, this is not always the case as other foods such as grains, beans, legumes and most animal products need to be cooked before we can eat them.

Cooking can destroy some nutrients in food to some extent however it can also be beneficial. For example, cooking makes food easier to digest as it helps break down protein and tough fibre. Cooking also helps destroy lectins, the harmful compounds mostly found in beans and legumes. Consuming raw legumes and grains can result in flatulence, nausea, diarrhea and vomiting. Raw kidney beans contain lectin between 20,000 and 70,000 units, whereas fully cooked beans can reduce lectin to 200-400 units. Cooking peas and beans is also effective in removing or reducing anti-nutrients such as tannins and phytate. Beneficially, cooking makes some nutrients in foods such as phytochemicals (i.e. plant bioactive compounds with antioxidant property) become more bioavailable, that is more easily absorbed and used by the body (this is how we really get the benefit from nutrients). An increased concentration of flavonols was observed when onions were submitted to sautéing (7%) and oven baking (25%). The total content of glucosinolates and polyphenol of fresh broccoli increased by steaming methods.

Different cooking methods are used in different types of cuisine to create unique and delicious dishes. For example, frying method is required in Indian cooking. Stir-frying and steaming are used in Chinese cooking. Baking, roasting and barbequing are also popular. But what is the best way of cooking foods that is healthier than others? It is generally recommended that cooking methods that require the least amount of heat, liquid and time help retain more of the nutrients in foods. Also, make sure not to overcook food, especially when frying or grilling food, as this not only results in loss of nutrients but also creates compounds that can be harmful for health. Listed below are different cooking methods and how they can affect the nutrient contents of food.

Bake or Roast - Foods are cooked using hot dry air, usually in an oven. Because no liquid is added to food during cooking, most nutrients remain in the food.

Boil – Foods are cooked in boiling water, usually for a moderate amount of time. Most of the nutrient content leeches out of the food and into the cooking liquid. Rigorous boiling can cause the food to disintegrate. The food is usually drained before eating, and most of the vitamins and minerals will be lost.

Stir-Fry or Sauté - Foods are fried quickly in a little bit of fat/oil. This also helps your body absorb fat soluble vitamins (i.e. vitamins A, D, E, and K) from your food. Some vitamins and minerals may have leached out into the liquid during the cooking process. Quick stir-frying minimises nutrient loss.

Microwave - This is generally a very quick process for water-based foods. Electromagnetic waves stimulate the water molecules in food, making them vibrate and spin, which creates heat. Foods that contain fats and oils heat less evenly and may require more cooking time. Therefore, some foods should not be microwaved, such as high fat foods, dry foods, and especially baby foods as uneven heating or ‘hot spots’ can cause burns. With regard to nutrients, there is evidence that microwave cooking results in reduction in nutrient content of the food compared to boiling or steaming, despite using a smaller amount of cooking time. Microwaving foods has also been shown to change the property of some active compounds in food. In general, it may be wise to limit using microwave method only for mildly re-heating leftover foods for a short period of time. Notably, microwaving foods in plastic containers should be avoided as harmful chemicals can leach out of plastic containers and into your food. Use glass or ceramic containers only.

Slow Cook Foods are cooked gently, usually in a liquid, over low heat for an extended period of time. Because a lot of water-soluble vitamins and minerals will leach into the liquid during cooking, this method is a good choice for soups and stews as the cooking liquid will also be consumed. The best way to maximize the nutrient content is to cook meat (usually tough cut of meat) for an extended time, then add the vegetables during the last hour of cooking.

Steam - Foods are cooked gently over boiling water without touching the water, usually in a steamer. The steam from the boiling water provides heat to cook the food, usually for a short period of time. Because little water from the food is lost, most nutrients remain intact. Steaming seems to be the best method to maintain the nutritional quality of broccoli (glucosinolates, sulforaphane, antioxidant content).

Simmer or Poach - These cooking methods are similar to boiling but use a lower temperature (not boiling temperature). Food that is simmered is heated to 185-200°F (85-93°C). At a simmer, food is moved around enough to allow flavors to mix, but not moved so much that the food is disintegrated. Poaching uses even a lower temperature, approximately 180°F (82°C), and is best for delicate foods like eggs, fish, and fruit.

Deep frying – Food is cooked in a large amount of fats/oils at a high temperature above 180-200°C. The choice of fats/oils play an important role in this cooking method. Foods cooked on high heat should be cooked in fat or oil with a high smoke point (e.g. avocado oil, ghee, refined coconut oil or animal fats). Fats and oils are destroyed and can be harmful if heated beyond their smoke point - that is, the point at which they smoke and burn. As a guideline, refined oils are generally more stable than unrefined oils (known as virgin or extra-virgin oils), so they are better choices for most high-heat cooking. However, these refined oils undergo heavy processing that destroys beneficial properties of oils. Moreover, refined oils such as soybean oil, rapeseed oil, corn oil, grapeseed oil and sunflower oil are also high in omega-6 fatty acids, which promote chronic inflammation in the body. Moreover, frying foods containing high carbohydrate content at high temperature (e.g. french fries, potato chips/crisps and pastries) creates acrylamide. Exposure to acrylamide has been shown to increase risk of DNA damage in animal studies. Reducing cooking temperature and avoiding heavily crisping or browning foods can decrease the acrylamide content in foods.

Barbequing or Grilling - The amount of time and temperature that meat is cooked on the grill makes a difference. Cooking, especially grilling or barbecuing meat (and fish) at high temperatures induces chemical reactions between proteins and sugars leading to the formation of heterocyclic amines (HCAs) and glycotoxins (AGEs). HCAs can damage DNA and may increase risk for developing cancer. In addition, liquid fat dripping into the flame of a barbeque creates smoke filled with polycyclic aromatic hydrocarbons (PAHs), coating the surface of the meat. To minimise these toxins, use lean and smaller pieces of meat to reduce cooking time and remove blackened or charred areas of meat before eating. HCAs and PAHs can be excreted from the body via detoxification processes in the liver so make sure to support your liver health.

In summary, there are benefits to eating both raw and cooked foods. Aim to eat both to help you maximize your intake of beneficial nutrients. Some foods are better eaten cooked than raw. If you want to cook your food, gentle cooking methods that use little heat, liquid and time are generally better to preserve nutrient contents of food. Some cooking methods are better suited to certain kinds of foods and allow ingredients to mix and develop flavour. Now that you know different cooking methods can affect the nutrient content of foods, make sure you choose the right method of cooking your food to maximise the nutrient content. Would you prefer eating raw or cooked foods? Which cooking method do you usually use? Please give your comment below.

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