The world's 125 healthiest foods that appear on this website come naturally packed with nutrients. How can you be sure that you're protecting them when you prepare a meal? The questions and answers below explain how to protect the healing power of foods by using cooking techniques designed for maximum nutrition.
Why is food preparation so important? Don't most of the nutrients in food remain unchanged, regardless of heating, chopping, or processing?
No! How a food is prepared can dramatically change its nutrient composition! During the bleaching and extraction of flour from wheat, for example, over 60% of most vitamins are lost. The cooking of vegetables and fruits for prolonged periods of time (10-20 minutes) can result in a loss of over half of their vitamin C content. If fruits and vegetables are heated, canned, and reheated, less than 1/3 of the original vitamin C content may be left.
Would I get the most nutrients if I ate everything raw?
No, you wouldn't. Although some nutrients are lost whenever a food is cooked, other nutrients can be gained. Cooking helps release certain nutrients that would otherwise be unavailable. In addition, cooking can de-activate certain chemicals that block nutrient absorption.
Are there any foods that should always be cooked, and never eaten raw?
Yes. There are some foods that should always be cooked, and never eaten raw. These foods include:
Dried beans (especially red kidney beans)
In raw form, beans can contain excessively high amounts of a potentially toxic substance called phytohemagglutinin. This substance is classified as a lectin glycoprotein, and in sufficiently high amounts it has been shown to disrupt cellular metabolism.
The amount of this toxin in beans is usually measured in terms of hemagglutinating units, or hau. In their raw form, red kidney beans can contain 20,000 to 70,000 hau. This number drops down to 200 to 400 hau with fully cooked red beans.
White kidney beans start off with about 1/3rd less hemagglutinin than reds. Other beans, like broad beans (also called fava beans) contain only 5 to 10% the amount that red kidney beans contain. Soybeans are also in this lower category of hemagglutinin content.
In some studies of soybean cooking, researchers have actually arrived at optimal cooking time and temperature recommendations based on decreases in hemagglutinin and other potentially toxic substances. In these studies, optimal time and temperature was found to be 120 minutes at 140°C (284°F), or 30 minutes at 160°C (320°F).
The most common cooking method, however, would be overnight soaking of the beans, discarding the water, then immersion in boiling water, followed by simmering for 2-3 hours or until tender.
Phytic acid is a naturally-occurring substance in grains that can partially block the availability of the grain minerals, including iron and zinc. The processing and cooking of grains lowers their phytic acid content, often by more than 50%. The sprouting of raw grains also lowers their phytic acid content. To minimize the mineral-blocking effects of phytic acid and maximize mineral availability, grains should be sprouted or cooked rather than eaten raw.
Two substances in raw eggs have been shown to block nutrient availability. Conalbumin is a protein that can bind together with iron and block its availability. Avidin is a second egg protein that can bind together with biotin (a B-vitamin) and make it unavailable.
The cooking of eggs helps denature both of these proteins, and can increase the availability of both iron and biotin from eggs. Of course, another reason for cooking eggs involves health safety. The U.S. Centers for Disease Control and Prevention estimate that 1 of every 20,000 eggs may be contaminated with the bacterium Salmonella, which is actually passed from the infected hen to the egg before the shell is formed.
Are there any foods that I should always avoid heating?
This question is more difficult to answer with a simple "yes" or "no," but there is increasing research evidence against the heating of fats and oils, particularly at high heats (used in stove-top frying) or prolonged medium heats (like stove-top sautéeing for 15 minutes).
The heating of fats and oils makes them lose their natural properties; they become oxidized and altered in structure. This change in structure involves formation of an increased percentage of trans fatty acids. Both of these events — oxidation and production of trans fatty acids — have been repeatedly linked to formation of plaque in the arteries and a variety of cardiovascular problems.
Aren't most important nutrients, like protein and fiber, fairly stable in food and unaffected by cooking?
No, cooking affects most nutrients, including macronutrients like protein and fiber. While cooking seldom destroys protein or fiber, overcooking (either for too long, or at too high a temperature) can alter the structure of nutrients and decrease their health benefits. In some cases — like heating polyunsaturated oils — helpful macronutrients are actually converted into harmful ones.
Nutrition has traditionally focused on macronutrients like protein and fiber, and micronutrients like vitamins and minerals. But there are also a vast array of other nutrients in food. Plant foods contain thousands of compounds that are collectively known as phytonutrients (phyto=plant).
Although research studies have yet to determine all the effects of cooking on phytonutrients, some evidence of unwanted effects is visible to the naked eye. Just look at the water in your saucepan after boiling a batch of beets. The water has turned red because anthocyanins (flavonoid phytonutrients) have been leeched from the beets into the boiling water.
Is it true that vegetables are particularly sensitive to cooking methods?
Yes! The nutritional and healing properties of vegetables are in special need of your help when it comes to proper cooking technique! Here are our six cardinal rules for preserving the power of vegetables:
For specific vegetable cooking techniques, consult our power-preserving chart below.
Power-Preserving Vegetable Cooking Chart
|Food Group||Food Examples||Cooking Method||Time||Special Instructions|
|Green Vegetables||spinach||steam||3 minutes||stop when the color changes to bright green|
|kale, chard, or collard greens||steam||3-5 minutes||stop when the color changes to bright green|
|green beans||steam||5-8 minutes||stop when the color changes to bright green|
|green peas||steam||3-5 minutes||stop when the color changes to bright green|
|snow peas||steam||3-5 minutes||stop when the color changes to bright green|
|brussels sprouts||steam||5-7 minutes||cut in half first; stop when the color changes to bright green|
|broccoli||steam||5-7 minutes||cut into bite-side florets and stalk slices first; stop when the color changes to bright green|
|asparagus||steam||10 minutes||fork test for softness|
|cabbage||steam||4-6 minutes||cut into shreds first|
|Red, orange and yellow vegetables||sweet potatoes||bake; preheated oven at 400F||20-30 minutes||use covered baking dish; fork test for softness; time is for 1 lb of whole, unsliced sweet potato|
|winter squash||bake||20-30 minutes||use covered baking dish; fork test for softness; time is for squash cut into bite-sized chunks|
|beets||steam||5-10 minutes||cut into slices first; fork test for softness|
|beets||bake in a preheated oven at 400F||45 minutes||use covered baking dish; fork test for softness; time is for 1 lb of whole, unsliced beet|
|carrots||steam||5-10 minutes||cut into slices first; fork test for softness|
|White vegetables||cauliflower||steam||5-7 minutes||cut into bite-side florets and stalk slices first; stop when the color changes to bright green|
|potatoes||bake in a preheated oven at 400F||25-30 minutes||use covered baking dish; fork test for softness|
|onions||steam||10-15 minutes||slice first|
Although research studies are limited in this area, cooking does appear to help inactivate both isoflavones (commonly found in soy foods) and isothiocyanates (commonly found in cruciferous vegetables). These compounds appear to be heat-sensitive, and cooking appears to lower their availability. In the case of isothiocyanates in cruciferous vegetables like broccoli, as much as one third of this goitrogenic substance may be deactivated when broccoli is boiled in water.
Does cooking affect the oxalate content of food?
According to most research studies, not significantly. Oxalates are naturally-occurring substances found in plants, animals, and human beings. When oxalates become too concentrated in body fluids, they can crystallize and cause health problems. Oxalate-containing kidney stones and gall stones are examples of oxalate-related health problems.
Only a small number of foods contain any measurable amount of oxalates. These foods include berries, currants, concord grapes, figs, plums, rhubarb, tangerines, all green leafy vegetables, green beans, okra, rutabagas, summer squash, tomatoes, sweet potatoes, leeks, eggplant, almonds, cashews, peanuts, cocoa and chocolate made from cocoa, black tea, tofu and other soy products, and the bran and germ of wheat.
In the green leafy vegetable category, rhubarb greens and spinach leaves are particularly concentrated sources of oxalates. Rhubarb (Rheum rhubarbarum) can contain up to 11,000 parts per million of oxalic acid in its leaves, while spinach (Spinacia oleracea) can contain up to 6,580 parts per million. Cooking has been shown to have negligible effects on oxalates when they are contained in the root or stalk of the plant. When oxalates are contained in the leaves, cooking has been shown to reduce their concentration, but not dramatically.
Individuals with already existing and untreated kidney or gallbladder problems may want to avoid eating oxalate-containing foods altogether. For individuals falling into this category, the cooking of high-oxalate foods is not likely to be sufficiently helpful. Individuals wanting to protect their calcium status might also want to avoid intake of high oxalate foods, since oxalates can also interfere with the body's absorption of calcium. In this second type of circumstance, however, cooking high oxalate foods — particularly green leafy vegetables like spinach — may be worth it to help increase calcium availability, even though the degree of oxalate reduction through cooking has not been shown to be statistically significant in research studies.
What about cooking onions? I've heard conflicting opinions about the health benefits of cooked versus uncooked onions.
Onions (Allium cepa) are members of the lily family, and like many members of this family, contain many unique sulfur-based molecules. The three basic categories of sulfur-based molecules in onion are: (1) sulfoxides (including molecules called alliins); (2) thiosulfinates (including molecules called allicins); and (3) dithiins (including molecules called ajoenes). Each of these three sulfur-based groups has its own health-related benefits, and the balance between these three groups within an onion is partly determined by the way the onion is prepared.
Slicing, chopping, or crushing an onion permits its sulfoxides (including its alliins) to be converted into thiosulfinates (including alliciins) or dithiins (including ajoenes). In its unsliced, unchopped, or uncrushed form, the onion contains predominantly sulfoxides. The formation of these other sulfur compounds is important, since they are most closely linked to lowering blood cholesterol and preventing heart disease. In addition, only the dithiins in onion have been shown to help prevent thrombosis (formation of a blood clot in an artery or vein).
Slicing, chopping, or crushing onions allows new sulfur molecules to form by activating enzymes within the onion. For example, slicing activates the enzyme alliinase, which converts alliin (a sulfoxide) into allicin (a thiosulfinate). Because this enzymatic conversion takes a little time, we recommend letting chopped onions sit for 10 minutes before working with them further in a recipe or exposing them to heat. This extra 10 minutes will allow the formation of new heart-protective thiosulfinates and dithiins in the onion.
Although we don't have direct evidence from onions, we have evidence from another closely related member of the lily family — garlic (Allium sativa) - that heating creates a much wider variety of sulfur-containing molecules. In the garlic model, two basic categories of sulfur compounds (thiosulfinates and dithiins) get expanded into five categories (allyl sulfides, methylsulfides, diallyl sulfides, thiosulphinates, and thiosulphonates) after the garlic is heated. Several of the molecules in these expanded sulfur categories have been shown to have anti-cancer as well as heart-benefiting activity.
Even in the world of raw onions, there are large differences in total sulfur content. Sweeter onions like Vidalia and Walla Walla are lower in total sulfur compounds than "hotter" onions like Italian Red or Red Torpedo. When it comes to cooking, however, it is prolonged heating (30 minutes or greater) at temperatures above 158°F that significantly reduces total sulfur content. To avoid loss of total sulfur content, but to maximize the variety of sulfur compounds contained in onions, we've developed the world's healthiest onion sautéeing method for use with onion-containing recipes.
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