Onions are a wonderful reservoir of many important nutrients. Notonly are they a concentrated source of vitamin C, vitamin B6, chromiumand the flavonoid quercetin, but they also contain importantphytonutrients known as allyl sulfides. These sulfur-containingphytonutrients are the phytonutrients that give the onions so many oftheir health promoting qualities. Among these phytonutrients are thoseknown as alliin and ajeone as well as the allyl propyl disulfidesincluding allicin, diallyl disulfide and diallyl trisulfide.
The striking difference in flavor and aroma that exists betweencooked and raw onions is largely due to the effect of cooking on theallyl sulfide phytonutrients, since not only do they contribute to theonions's nutritional qualities but they also give them their pungentflavor and aroma. Thanks to scientific research, we have learned tipson how we can best cook onions so as to maximize theirsulfur-containing phytonutrients profile.
Preparation for Cooking
The latest scientific research tells us that slicing, chopping ormincing Onions before cooking will enhance their health-promotingproperties. A sulfur-based compound called S-alk(en)yl-L-cysteinesulfoxide and an enzyme called sulfoxide lyase (also known asalliinase) are separated in the Onion's cell structure when it iswhole. Cutting Onion ruptures the cells and releases these elements,allowing them to come in contact with each other to form a powerful newcompound called thiopropanal sulfoxide. This new compound not onlyincreases Onion's health-promoting properties but is also the culpritbehind their pungent aroma and the cause of the eye irritation thatresults from cutting Onions.
The finer the Onion is cut, the more extensive is thetransformation of the sulfur compounds. The stronger the smell and themore they affect your eyes, the more health-promoting nutrients theycontain. So the next time you cut your Onions, you will have a greaterappreciation of its "irritating" effects, knowing that the pungentsmell that makes you cry will also make you healthy!
So, to get the most health benefits from Onions, let them sitfor a minimum of 5 minutes, and optimally for 10 minutes, aftercutting, before eating or cooking. This is to ensure the maximumsynthesis 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 5-10minutes before cooking to give the enzyme ample opportunity to enhancethe concentration of active phytonutrients in your Onions. Cooking atlow or medium heat for short periods of time (up to 15 minutes)such aswith 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 thevalidity of this practice. When crushed garlic was heated, its abilityto inhibit cancer development in animals was blocked; yet, when theresearchers allowed the chopped garlic to 'stand' for 10 minutes beforeheating, anticancer activity was preserved.
Practical tips
To maximize the amount and breadth of sulfur-containingphytonutrients in onions, chop or slice them 10 minutes before cookingthem. The best cooking technique would be one like the healthy sautémethod that exposes the onions to heat for a minimal amount of time(less than 15 minutes) as opposed to longer cooking methods such asslow roasting.
References
Chen JH, Chen HI, Tsai SJ, Jen CJ. Chronic consumption of raw butnot boiled Welsh onion juice inhibits rat platelet function. J Nutr2000 Jan;130(1):34-7
Ketter, C. A. T. and Randle, W. M. 1998. Pungency Assessment inOnions. in Tested studies for laboratory teaching, Proceedings of the19th Workshop/Conference of the Association for Biology LaboratoryEducation Volume 19: 177-196
Lawson LD, Wang ZJ, Hughes BG. Identification and HPLCquantitation of the sulfides and dialk(en)yl thiosulfinates incommercial onions products.. Planta Med 1991 Aug;57(4):363-70
Song K, Milner JA. The influence of heating on the anticancer properties of onions.
Song K, Milner JA The Influence of Heating on the Anticancer Properties of Garlic. J. Nutr. 2001 131: 1054S-1057S. |
发表于 2010-5-27 22:50:03