The World's Healthiest Foods

What is the best way to cook onions so as to maximize the benefit of their sulfur-containing phytonutrients?

Onions are a wonderful reservoir of many important nutrients. Not only are they a concentrated source of vitamin C, vitamin B6, chromium and the flavonoid quercetin, but they also contain important phytonutrients known as allyl sulfides. These sulfur-containing phytonutrients are the phytonutrients that give the onions so many of their health promoting qualities. Among these phytonutrients are those known as alliin and ajeone as well as the allyl propyl disulfides including allicin, diallyl disulfide and diallyl trisulfide.

The striking difference in flavor and aroma that exists between cooked and raw onions is largely due to the effect of cooking on the allyl sulfide phytonutrients, since not only do they contribute to the onions's nutritional qualities but they also give them their pungent flavor and aroma. Thanks to scientific research, we have learned tips on how we can best cook onions so as to maximize their sulfur-containing phytonutrients profile.

Preparation for Cooking

The latest scientific research tells us that slicing, chopping or mincing Onions before cooking will enhance their health-promoting properties. A sulfur-based compound called S-alk(en)yl-L-cysteine sulfoxide and an enzyme called sulfoxide lyase (also known as alliinase) are separated in the Onion's cell structure when it is whole. Cutting Onion ruptures the cells and releases these elements, allowing them to come in contact with each other to form a powerful new compound called thiopropanal sulfoxide. This new compound not only increases Onion's health-promoting properties but is also the culprit behind their pungent aroma and the cause of the eye irritation that results from cutting Onions.

The finer the Onion is cut, the more extensive is the transformation of the sulfur compounds. The stronger the smell and the more they affect your eyes, the more health-promoting nutrients they contain. So the next time you cut your Onions, you will have a greater appreciation of its "irritating" effects, knowing that the pungent smell that makes you cry will also make you healthy!

So, to get the most health benefits from Onions, let them sit for a minimum of 5 minutes, and optimally for 10 minutes, after cutting, before eating or cooking. This is to ensure the maximum synthesis of the sulfur compounds.

Heat will inactivate the effect of thiopropanal sulfoxide, which is why it is important to allow the Onions to sit for 5-10 minutes before cooking to give the enzyme ample opportunity to enhance the concentration of active phytonutrients in your Onions. Cooking at low or medium heat for short periods of time (up to 15 minutes)such as with methods like healthy sauté should not destroy the active phytonutrients since once they are formed, they are fairly stable.

Research on garlic, a close relative of Onion, reinforces the validity of this practice. When crushed garlic was heated, its ability to inhibit cancer development in animals was blocked; yet, when the researchers allowed the chopped garlic to 'stand' for 10 minutes before heating, anticancer activity was preserved.

Practical tips

To maximize the amount and breadth of sulfur-containing phytonutrients in onions, chop or slice them 10 minutes before cooking them. The best cooking technique would be one like the healthy sauté method that exposes the onions to heat for a minimal amount of time (less than 15 minutes) as opposed to longer cooking methods such as slow roasting.


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