WHY AVOID HIGH TEMPERATURE COOKING?

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°F (177–232°C); we fry on the stovetop in shortening and vegetable oils right up to their smoke points of 375–450°F (191–232°C); and we barbecue with gas grills that can reach temperatures of over 1000°F (538°C)! 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. Neither the foods nor the nutrients they contain were designed to withstand extremely high temperatures. How you prepare the foods you eat can be just as important to your health as what you eat.

Unwanted Consequences of High-Heat Cooking 

LOSS OF NUTRIENTS 

The problems with high-heat cooking are not restricted to the creation of toxic substances. 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°F/120–230°C) are temperatures at which substantial nutrient loss occurs, except for roasting turkey because it takes a long time for the heat to penetrate the meat and damage the nutrients. And although very short cooking at 212°F (100°C) in boiling water produces relatively little nutrient loss, once boiling goes on for anything more than a very short period of time (1–3 minutes), the nutrient loss becomes significant.

I’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.

Our Senses Can Tell Us a Lot

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 collard greens, 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, but the fact that they smoke is still a nice 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.

Vegetables and High-Heat Cooking 

I’ve tried to emphasize the wonderful diversity and uniqueness of food. I’ve tried to pay attention to all of the little details that make each fruit, 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, I’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 a matter 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, setting the table or feeding the cat. Just a few minutes can change the outcome completely! Green beans will steam in about 5 minutes. During this time, their color will take on a more vibrant green hue. If you cook it longer you will notice a change in color. For example, after 7 minutes 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 extra steaming time 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 (and less heat) 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 topmost layers that are less directly exposed to the steam should be the vegetables needing the least steaming. The vegetables requiring longer steaming should be placed on the bottommost layer. Alternatively, vegetables that need less steaming can be added to the steamer basket later on, after the vegetables that are thicker and more dense have been added.

HETEROCYCLIC AMINES (HCAs)

Nutritional research is just starting to catch up with the consequences of high-heat methods of cooking. We’ve learned, for example, that some of the most mutagenic agents formed in cooking, heterocyclic amines (HCAs), are commonly found in barbecued beef, chicken, and pork cooked at 392°F (200°C) or above. We even know what basic ingredients are required for these mutagenic agents to be produced: high temperature for more than a few minutes’ duration, free amino acids (from protein), creatine (or creatinine) and sugar. Without the high temperature component, the formation of HCAs does not occur. Direct flame grilling produces another type of carcinogen called polycyclic aromatic hydrocarbons (PAHs), which might be just as harmful as the HCAs.

ADVANCED GLYCATION END PRODUCTS (AGEs) 

Researchers at Mt. Sinai Medical Hospital 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. These chemicals can be avoided by cooking meals at lower temperatures through “Healthy Steaming,” “Healthy Sautéing” or poaching and also by cooking meats with foods containing antioxidant bioflavonoids, such as garlic, onion and pepper.

ACRYLAMIDE 

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, which is a carcinogenic nerve-damaging compound, is also 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 HCAs, acrylamide does not appear to form when high cooking temperatures are absent.

Food Safety

Heat is important in cooking because it can kill bacteria that can contaminate foods, especially meats and fish. The time required to kill bacteria decreases as the temperature increases. Eradication of bacteria starts at about 165°F (74°C), but as you move upward from this temperature, the time it takes to kill bacteria shortens (see table below). For some foods, like thick cuts of meat or fish, it is important to cook for however long it takes to produce a certain internal core temperature. In general, however, it takes much less than 5 minutes to kill potentially harmful bacteria on most plant foods.

Guidelines for Final Cooking Temperatures

The table below provides the minimum recommended intermal temperature needed to destroy harmful microorganisms in food that is cooked by conventional methods (i.e., heat source other than a microwave).

The chart below indicates the temperatures that correspond to various cooking methods.

Here are questions that I received from readers of whfoods.org about high temperature cooking:

Q. What are the problems with grilling food on high heat? 

A. There are documented health risks associated with the char-broiling and gas grilling of foods. In general, these risks are associated with the formation of heterocyclic amines (HCAs). Most HCAs are well documented carcinogens, and keeping their levels to a minimum in your diet can decrease your cancer risk.

HCAs form most easily at high temperatures. Under 325°F, the formation of these compounds is very low. As temperatures increase above 400°F, the formation of HCAs can increase by 700%–1000%. Gas and charcoal grilling often (but not always) involve higher temperatures.

The longer a food is exposed to high heat, the greater the HCA formation. When a food like a hamburger is grilled for 10 minutes versus 6 minutes, for example, the HCA levels in the hamburger may increase by 25–30%.

Meat, fish and poultry are more likely to give rise to HCA formation because HCA formation requires the presence of amino acids (from protein) as well as the nitrogen-containing substances, creatine or creatinine. Both of these substances are plentiful in most animal foods. Since most of the research on HCAs has been done on meat, it is uncertain that the grilling of vegetables and fruits may have the same level of outcomes, notably because some of the phytonutrients found in vegetables, such as the sulforaphane in broccoli, have been found to reduce the carcinogenic effect of the HCAs in research studies.

Q. Will roasting nuts at high temperatures affect their nutritional value? 

A  Roasting nuts at a temperature higher than 170°F (77°C); which is usually the temperature above which they are commercially roasted, will cause a breakdown of their fats and the production of free radicals. When nuts roasted at the high temperatures used commercially are consumed, the free radicals they contain can cause lipid per-oxidation—the oxidizing of fats in your bloodstream that can trigger tiny injuries in artery walls—a first step in the buildup of plaque and cardiovascular disease.

Therefore, I don’t suggest purchasing preroasted nuts. Rather if you like the taste of roasted (versus raw) nuts, I suggest that you roast them at low temperatures at home since it is safe to roast nuts if done at a low temperature—typically a 160–170°F (70–75°C) oven (at higher temperatures than this, research clearly shows damage to nuts’ delicate fats) for 15–20 minutes will do the trick. To do so, place nuts on a cookie sheet in a single layer. To enhance the “roasted” flavor, try putting a little liquid aminos or soy sauce into a spray bottle and misting the nuts before roasting.

Because of their high content of delicate polyunsaturated fats, all nuts, whether roasted or raw, are susceptible to going rancid quickly. It is therefore important to either purchase nuts in their protective shell or, if unshelled, from a store with high turnover to ensure freshness. Store nuts in your refrigerator or freezer. Generally, if stored in the refrigerator or freezer, nuts will remain fresh for 6 to 12 months. For more detail on how long specific nuts can be stored, please check the profile for the nut of your choice that is provided on the World’s Healthiest Foods website.

If nuts have been stored longer than recommended, it’s a good idea to throw them out. Rancidity sets in long before they smell or taste “off” or not fresh. Like oils damaged by exposure to high cooking temperatures, rancid oils contain free radicals that can cause cell damage in your body. To protect your body, avoid damaged oils of all types.






By George Mateljan in " The World's Healthiest Foods", George Mateljan Foundation, Seattle USA, 2006, excerpts p.57-63. Adapted and illustrated to be posted by Leopoldo Costa.

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