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How do cooking temperatures affect vegetable nutrients?

In virtually all cuisines throughout the world, you will find enjoyment of both raw and cooked vegetables. Sometimes the balance between raw and cooked vegetables varies with climate. For example, you can often find greater consumption of raw vegetables in tropical versus colder climates. Very often, you will also find that vegetables have been cooked to bring out their special flavors and aromas. You will also find them having been cooked to increase their tenderness, to assist with digestion, or to increase the safety of their consumption.

At WHFoods, we believe there is room for both cooked and uncooked vegetables in most meal plans. But based on our review of the research carried out by food scientists, we also believe that vegetable cooking is not an "all-or-nothing" proposition and that different cooking temperatures can have very different impacts on the nourishment that you get from vegetables.

Understanding stove temperatures

Most stove manufacturers specify temperature settings for two basic sections of the stove: (1) the stovetop and (2) the oven. (Note: many manufacturers also refer to as oven as the broiler.) On the stovetop, temperature settings typically involve a gas or electric burner. In the oven/broiler, temperature settings also typically involve electric elements or a regulated gas flame. The chart below shows common temperature ranges for stovetop burner settings.

Stovetop Burner Temperature Ranges

Stovetop Burner Setting Temperature Range (F)* Temperature Range (C)**
Low up to 275°F up to 135°C
Medium Low 275-300°F 135-149°C
Medium 300-350°F 149-177°C
Medium High 350-375°F 177-191°C
High 375-450°F 191-232°C

*F — Farenheit **C = Celsius

In the oven/broiler, temperature ranges are also typically described as low, medium, or high, but when these descriptions are used for the oven/broiler, they usually refer to higher heats than are used to describe stovetop settings. In the oven/broiler, a "low" setting usually corresponds to a temperature in the general vicinity of 400°F (204°C); a "medium" setting to a temperature of roughly 500°F (260°C); and a "high" setting to a temperature of approximately 600°F (316°C).

As you can see from the above stovetop temperature ranges, the temperature at which water begins to boil and turn into steam—212°F (100°C) —would be classified as a "low" temperature. And in fact, we think about our vegetable steaming and boiling methods at WHFoods as low temperature cooking methods. Additionally, we would add here that an oven setting of 400°F (204°C) for roasting would also be classified as low temperature cooking method based on the above manufacturer settings.

Stove temperatures versus grilling temperatures

While many people think about "grilling" as a single, uniform type of cooking method, there are actually many different ways to grill and the temperatures used in grilling can differ widely. Let's start with the high end of the temperature scale and talk about grilling that involves direct contact between food and an actual flame.

Flames themselves can be surprisingly high temperature! If you look carefully at the flame around a gas burner, you can distinguish a blue zone that typically burns at 520-575°F (271-302°C); an orange/brown zone that burns at approximately 1000-1500°F (538-816°C), and a yellow zone that burns at temperatures near 2000°F (1093°C) or higher. (These temperature differences in the flame typically correspond to varying mixtures of oxygen, hydrogen, water vapor, and carbon dioxide.) As you can see, if a vegetable comes into contact with an open flame, it can be exposed to a much higher temperature than is present on a stovetop or in the oven.

At the other end of the spectrum, however, grilling can involve very low temperatures as well. The temperatures on a covered charcoal grill, for example, usually range from 225-650°F (107-343°C). These temperatures correspond to the full range of temperatures available on a stovetop and oven/broiler. In fact, a charcoal grilling temperature of 225°F (107°C) would be fairly close to a steaming/broiling temperature on a stovetop.

As you can see, the temperatures that can be reached by an open gas flame—upwards of 2000°F (1093°C)—are much greater than the highest temperatures found in a home oven or broiler. But a covered charcoal grill that has been closed to greatly restrict the flow of oxygen can maintain a low cooking temperature that is at or below a stovetop setting of "low."

Much of the research concern about open-flame grilling has focused on the grilling of meats rather than the grilling of vegetables. For this reason, we don't have enough information about open-flame grilling of vegetables to accurately address potential health risks associated with this vegetable cooking method.

However, in the case of meats, we do have increasing evidence about the production of substances that can elevate cancer risk as a result of high-heat, open-flame grilling. These substances fall into two basic categories: (1) HCAs (heterocyclic amines) and (2) PAHs (polycyclic aromatic hydrocarbons). In the case of HCAs, there has also been a focus on a special subgroup of HCAs called HAAs (heterocyclic aromatic amines).

Researchers have determined that HCAs—including HAAs—are increasingly formed as cooking temperatures rise above 400°F (204°C) and an open-flame cooking method is used. Amino acids, simple sugars, and creatine are a potent combination for forming HCAs and HAAs under high-heat, open-flame cooking. PAHs are also increasingly formed with high-heat, open-flame cooking of meats. In the case of PAHs, however, it is the dripping of fats and juices from the meat onto the flame that results in PAH formation and exposure.

The ability of HCAs, HAAs, and PAHs to increase our risk of certain cancers appears to depend on further metabolism of these substances once they have entered our body. Because their impact is metabolism-related, researchers see differences among populations and among population subgroups in terms of cancer risk following exposure to these substances. From our perspective at WHFoods, even though conclusive evidence about the exact risks posed by high-temperature, open-flame cooking of meats is still lacking, there is sufficient cause for concern here and we believe it warrants general avoidance of this cooking method for meats.

Due to the role of creatine in HCA and HAA formation, however, we would not expect to see the same problems with HCA and HAA formation in high-heat, open-flame cooking of vegetables that we would in high-heat, open-flame cooking of meats. (The reason for this difference involves the key role of creatine as an energy-related compound in animals as compared with its absence in plants.) However, as we will discuss later in this article, we believe there are nutrition-based reasons for avoiding high-heat cooking of vegetables, even though these reasons do not focus on the formation of HCAs, HAAs, or PAHs.

One topic of special interest with respect to temperature, cooking methods, and health is formation of acrylamide. Because this topic is somewhat complicated, we created a separate Q & A entitled "What is acrylamide and how is it involved with food and health?":george,260] to address questions and concerns about this topic.

Cooking methods and nutrient stability in vegetables

Nutrients in food are susceptible to damage from five basic sources: (1) heat, (2) air, (3) light, (4) degree of acidity, also referred to as pH, and (5) time. For most foods, none of these five factors can be ignored in evaluation of cooking methods and their impact.

Unfortunately, there is no simply way to determine which nutrients are most susceptible to each of the five potentially damaging factors described above. For example, there is no general rule that applies to water-soluble versus fat-soluble vitamins, or vitamins versus minerals. At the same time, however, there are some general rules that apply to all five factors.

First, many nutrients undergo an increasing risk of damage the longer that they are exposed to any of the potential damaging factors listed above. For example, cooking a vegetable for 30 minutes almost always poses more risk of overall nutrient loss than cooking the same vegetable for 10 minutes. Similarly, subjecting a vegetable to a cooking temperature of 500°F versus 250°F also poses more risk of overall nutrient damage. The same trend would generally hold true for prolonged exposure to light or prolonged exposure to extreme acid conditions.

A second general rule involves a phenomenon that researchers call "surface exposure." For any vegetable, there is an outer portion that can be described as the surface of the vegetable and an inner part that can be described as the interior or non-surface portion. This difference between surface and non-surface is especially pronounced in a vegetable like a beet, potato, carrot, eggplant, or onion where there is a large portion of the vegetable that lies inside and out of view. Conversely, this difference is less obvious in a vegetable like a collard green or a lettuce leaf since there is a lot of natural surface area in these vegetables. But it is nevertheless possible to increase the surface area for any of these vegetables by chopping them up. When you chop up a vegetable, you are increasing its exposed surface area, regardless of whether the vegetable is a carrot, potato, or collard green. The more finely you shred a vegetable, the more exposed surface area that you create.

Creating more surface exposure in a vegetable is not necessarily a bad thing. It can allow more flavors to pass into the vegetable, and it can speed up desirable changes in texture (for example, tenderness). But in general, an increase in exposed surface area also means that the nutrient contents need to be treated more delicately through factors such as shorter cooking times and lower cooking temperatures. In other words, if you decide to shred a vegetable more finely than you usually do prior to steaming, you'll want to shorten the steaming time to compensate for the increased exposed surface area.

A third basic rule—and one that is related to surface exposure—involves water. When the exposed surface area of a vegetable makes direct contact with water that has been heated, there is an increased chance for loss of water-soluble nutrients into the water. Fully submersing a chopped vegetable in water (as would take place during boiling) for a lengthy period of time is very likely to move a substantial amount of water-soluble nutrients out of the vegetable and into the cooking water. (With many colored pigments found in vegetables—for example, the green chlorophylls, or red anthocyanins—you can actually watch this nutrient movement take place.)

This third rule can be very difficult to apply, however, to comparisons of vegetable steaming and vegetable boiling. During boiling, there is more continuous direct contact between the vegetable surfaces and heated water than occurs during steaming. However, steaming exposes the vegetable surfaces to more heated air than occurs during boiling, and the steamed vegetable surfaces still get exposed to water vapor. As a result, we have seen food science studies on relatively short-term steaming and boiling (always less than 10 minutes) that suggest better preservation of some nutrients by short-term boiling, better preservation of other nutrients by short-term steaming, and in the case of yet other nutrients, equal preservation by each method.

A closer look at vegetable cooking temperatures

Research studies on cooking temperature can be difficult to interpret for a number of reasons. First, temperature is seldom the only factor being analyzed in the studies. For example, a study of cooking temperatures above and below 350°F (177°C) might provide very helpful results about temperature differences, but those results might be overshadowed by the fact that the temperatures were studied within the context of a specific cooking method like deep frying and pan frying. In fact, we've seen some studies of deep frying in oil at 338°F (170°C) and pan frying in oil at 356°F (180°C) that show some advantages to the deep frying, possibly due to less air exposure and decreased risk of oxidation in the food being fried (in this case potatoes). Some studies also suggest that fat-soluble nutrients (like carotenoids) undergo greater transfer than water-soluble nutrients during oil-based frying. From our perspective, while these types of studies might tell us something about the impact of different cooking temperatures, they probably tell us a lot more about other aspects of cooking, like exposure to air or the cooking medium involved (like oil versus water).

A second factor that can make research findings difficult to interpret is the relative lack of studies involving high heat cooking. Most research studies investigate vegetable steaming or boiling, and both of these methods involve low temperature cooking. While oven roasting temperatures may involve higher heats between 320-464°F (160-240°C), these temperatures would still be considered low-to-medium by oven standards and the upper end of this range has gotten less attention in research studies than the lower end of the range.

Within the limitations described above, there are still some general conclusions that can be drawn from cooking temperature studies overall. First, if all other cooking factors (air exposure, light exposure, water exposure, degree of acidity, duration of cooking) are identical, high stovetop heats pose a greater risk of nutrient damage than low stovetop heats. This conclusion gives us a helpful cooking principle in theory, and it is one that we always try to keep in mind at WHFoods.

However, we do not believe that this principle is quite as helpful in practice as it is in theory. For starters, it is simply not possible to make all other cooking factors identical and focus on temperature alone. Cooking always needs to be done in some medium, whether it is water, steam, oil, or air; virtually all types of cooking involve some combination of factors, such as water-plus-air, oil-plus-air, or other combinations. So even if time and temperature could be made identical, the cooking medium factor would always come into play in a cooking study and shape the study results.

Additionally, from a practical standpoint, most stovetop cooking methods—whether at WHFoods or in research studies—involve vegetable steaming or vegetable boiling. Both of these methods involve low temperature cooking around 212°F (100°C). For this reason, most of the research on vegetable steaming and vegetable boiling shifts the focus from changes in temperature to length of cooking time at a low temperature. As you might expect, these studies make it clear that despite the potential benefits of low temperature cooking, extended cooking times can still result in greater risk of water-soluble nutrient loss into the cooking water. From a practical standpoint, what we end up with here is a conclusion about the duration of low temperature cooking rather than a conclusion about different levels of heat.

A second general conclusion about cooking temperatures involves the nature of whole foods. In studies on stovetop and oven cooking, whole foods can typically withstand a wider range of heats than processed foods or extracted food components. We'll use extra virgin olive oil (EVOO) as an example here. EVOO is not a whole food but rather an oil that has been separated from its whole food source (olives). Despite its high concentration of antioxidant nutrients (for example, phenols and vitamin E), EVOO is more sensitive to increasing temperatures than olives, and EVOO can lose over half of its phenols and vitamin E in a relatively short period of heating (10 minutes) at a medium to medium-high temperature range of 350-360°F (177-192°C). Interestingly, due to its relatively high concentration of monounsaturated fat, the fatty acids in olive oil can be relatively resistant to heat and damage (thermal oxidation). But the pressing of oil out of the olives makes many of its antioxidants and other phytonutrients in EVOO more susceptible to heat.

A final conclusion about cooking temperatures involves the role of our senses in detecting potentially unwanted changes in food. We can see vegetables becoming charred or burned when exposed to certain high-heat cooking methods. We can also smell them when they are becoming burned. In both cases, our everyday senses are letting us know that major chemical changes are occurring in the food. And in most instances, these chemical changes are associated with nutrient damage and/or formation of substances that may be potentially problematic from a health standpoint. While it is true that some people greatly enjoy the taste and smell of burnt foods (for example, burnt toast), there is no research evidence to suggest that a food can provide us with increased overall nourishment if it is burnt. And at the other end of the spectrum, there is extensive research to suggest that foods have maximum overall nutrient richness in their fresh, uncooked form.

At WHFoods, we believe that appropriate cooking heats tend to be accompanied by enjoyable sensory changes in vegetables (as well as in other foods). Their colors can become especially vibrant, their aromas can become mouth-watering, and their tastes and textures can become exquisite. In a practical sense, lower-heat vegetable cooking methods like steaming and boiling seem to provide us with better overall control of these sensory characteristics and can add to the pleasure of eating in this context.


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