(Note: All temperatures below are reported in Fahrenheit measurements).
One of the greatest insults to nourishment in our modern, fast-paced, and processed food culture is the high heat at which so much of our food is cooked. We deep fat fry at 350-450 degrees; we fry on the stovetop in shortening and vegetable oils right up until their smoke points of 375-450 degrees; and we barbecue with gas grills that can reach temperatures of over 1000 degrees! This exposure of food to high heat may be convenient and quick, and it may fill the air with aromas that we savor, but it comes at a definite nutritional cost. Our food just wasn't designed to withstand extremely high temperatures. Neither was the nutrients contained within our food.
Nutritional research is just starting to catch up with the consequences of our high-heat approach. We've learned, for example, that some of the most mutagenic agents formed in cooking are called heterocyclic amines, and they are commonly found in barbecued beef, chicken, and pork cooked at 392 degrees or above. We even know what the basic ingredients are required for these mutagenic agents to be produced: high temperature for more than a few minutes, free amino acids (from protein), creatine (or creatinine) and sugar. Without the high temperature component, the formation of heterocyclic amines does not occur. Direct flame grilling produces another of type of carcinogen called polyocyclic aromatic hydrocarbons, which might be just as bad as the heterocyclic amines.
Researchers at Mt Sinai Medical found that foods cooked at high temperature contain greater levels of compounds called advanced glycation end products (AGEs) that cause more tissue damage and inflammation than foods cooked at lower temperatures. AGEs irritate cells in the body, damaging tissues and increasing your risk of complications from diseases like diabetes and heart disease. Those chemicals can be avoided by cooking meals at lower temperatures through using our Healthy Sauté, Healthy Steaming or Healthy Stir-Fry methods of cooking and also by cooking meats with foods containing antioxidant bioflavonoids, such as garlic, onion and peppers.
Unfortunately, we're not off the hook if we are vegetarian and don't eat beef, chicken, or pork. Very recent research has discovered that a potentially toxic substance called acrylamide, a nerve-damaging compound in humans and clear cancer-causing agent in rodents, may be excessively formed when certain foods are cooked at high temperature. Potato chips are a key target of research interest here, as are some other foods, including flaked breakfast cereals, and roasted nuts. As is the case with heterocyclic amines, acrylamide does not appear to form excessively when high cooking temperatures are absent, provided that lower temperature cooking does not continue for a prolonged period of time (generally involving hours versus minutes).
The problems with high-heat cooking are not restricted to creation of toxic substances, however. High-heat cooking is also problematic when it comes to loss of nutrients. Virtually all nutrients in food are susceptible to damage from heat. Of course, whether a particular nutrient gets damaged depends on the exact nutrient in question, the degree of heat, and the amount of cooking time. But in general, most of the temperatures we cook at in the oven (250-450 degrees) are temperatures at which substantial nutrient loss occurs. And although very short cooking at 212 degrees in boiling water produces relatively little nutrient loss, once boiling goes on for more than a very short period of time (a couple of minutes) the nutrient loss becomes significant. Up to 80% of the folic acid in carrots, for example, can be lost from boiling. Ditto for the amount of vitamin B1 in boiled soybeans. Even the high heats involved with commercial food canning rob foods of vast amounts of nutrients. In canned mixed vegetables, the vitamin C loss can be as high as 67%. In canned tomato juice, up to 70% of the original folic acid can be lost.
We've searched and searched through the nutrition research, and all of the evidence points to the same conclusion: prolonged, high-heat cooking is just not the way to go.
While we cannot find any evidence in favor of high-heat cooking for extended time periods, we've found plenty of evidence in favor of high-heat cooking for very short time periods, or for cooking at various temperatures for relatively short periods of time. For some foods, especially animal foods, cooking temperature and duration are associated with food safety and elimination of potentially disease-causing bacteria. Exposure to heat can actually increase the variety of nutrients found in some foods. For example, heat increases the variety of sulfur-containing compounds found in onions and garlic since it triggers certain chemical reactions that create variations in those sulfur compounds. Flavor, color, and aroma can also be enhanced through cooking. We don't know of any traditional cuisines anywhere in the world that rely exclusively on raw foods. Every healthy food tradition incorporates some aspects of cooking.
Sometimes scientific research just reminds us that we can trust our five senses and our own good judgment. This conclusion seems to apply to high-heat cooking. Almost always, there is some magical point at which our senses begin to dislike the result of the high-heat. It may be a color change in the kale or collards, where the green ceases to become more and more vibrant and begins to take on a duller, grayer shade. It may be a change in air and aroma, as occurs when a vegetable oil starts to smoke. Vegetables oils have unique smoke points that can be more than 200 degrees apart. Although we don't like heating oils much at all, the fact that they smoke is still a common-sense warning that high-heat is doing some damage. If we expose foods to high heat for too long, our taste buds will also let us know.
Throughout our website, we've tried to emphasize the wonderful diversity and uniqueness of food. We've tried to pay attention to all of the little details that make each fruit or vegetable or legume nutritionally special. It should not be surprising that specific foods within any food family must be treated just as uniquely when it comes to cooking. Nevertheless, we've still found it amazing just how sensitive some foods are when it comes to high-heat - especially vegetables!
When it comes to vegetables, sensitivity to high heat has to be measured in matters of minutes! In some foods, like Swiss chard, loss of vitamin C can increase by 15% in a matter of just 4-5 minutes. Swiss chard can't be cooked as an afterthought, while we are talking on the phone, or setting the table, or feeding the cat. Just a few minutes can change the outcome completely! Green beans will steam in 3-7 minutes. During this time, their color will take on a more vibrant green hue. But toward the end of this 7-minute period, a drop in color intensity will begin to occur. By 9 or 10 minutes, the color intensity will have dropped noticeably. Just 2-3 minutes of steaming can make this notable difference.
The optimal timetable for high-heat cooking will vary with a number of factors in addition to the type of vegetable. How the vegetable is sliced, for example, will change the amount of steaming it needs. Finely shredded cabbage requires less steaming than coarsely shredded cabbage. Because more surface of the finely shredded cabbage gets directly exposed to the steam, it takes less time for the cabbage to become tender. If you mix vegetables in the steamer basket, the top-most layers that are more directly exposed to the steam should be the vegetables needing the least steaming. The vegetables requiring longer steaming should be placed on the bottom-most layer. Alternatively, vegetables that need less steaming can be added to the steamer basket later on, after the vegetables that are more coarse and dense have been added.
You may think of steam as a high-heat way of cooking, but in comparison to most other ways, it isn't. Since water boils at 212 degrees and transforms into steam, steam is actually a lower-heat way of cooking than most oven-based approaches and most stove-top ones as well. When compared with boiling, steaming is a better way of avoiding nutrient loss, since the food is surrounded by water dispersed in air, rather than being completely submerged in water alone. The decreased contact of water with the surface of the food results in decreased nutrient loss. If a food is sliced or chopped into sufficiently small sections, steaming can get it into a tender and tasty form long before most other heating methods. Even our butternut squash can be perfectly steamed in under 20 minutes.
It may sound silly, but covering the pot while steaming can help preserve the nutritional quality of our food. When a pot is covered, steam contact with the food is more consistent, allowing the steaming process to be completed in the least amount of time. In addition, light-sensitive nutrients - like vitamin B2 - will not be leeched out of the food so easily. As an added benefit, many water-soluble nutrients will pass out into the steam, and then drop back down into the water below the steamer basket. Save this water! It can be used as a base for soups and sauces, or at the very minimum, allowed to cool and used to water plants in the garden.
There is no area of high-heat cooking that we are more concerned about that the area of vegetable oils. We recognize that several companies have produced very high-quality refined oils, specifically designed for high-heat cooking. The refinement of these oils raises their smoke point, sometimes by as much as 200 degrees. In addition, we recognize that some vegetable oils that would normally be very susceptible to high heat because of their polyunsaturated fat content have been produced in such a way as to lower this polyunsaturated fat content. We're thinking here of a vegetable oil like safflower oil, which ordinarily contains a lot of polyunsaturated fat but relatively little monounsaturated fat. Today a version of safflower oil is available called "high-oleic" safflower oil. (Oleic acid is a monounsaturated fatty acid, and it's less susceptible to heat damage than the polyunsaturated fatty acids normally found in safflower oil). This version of safflower oil can indeed withstand higher heats, and is better for use with high heats than ordinary safflower oil. At the same time, however, we wonder why any oil should be exposed to high heat if there is a low-heat or no-heat alternative. We developed our healthy stir-fry technique for precisely this reason - to avoid unnecessary heating of vegetable oils. (The method we developed uses vegetable broth instead of vegetable oil).
Extra virgin olive oil is our oil of choice at the World's Healthiest Foods, and it deserves special mention in this discussion of high heat. As such we have a whole article dedicated to this, which you can read here.
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