Why does turmeric create such stubborn stains, and how can they be removed?

Turmeric comes from a plant that grows in tropical South Asia and is related to ginger. Its roots can be boiled for several hours and then dried and ground to make the spice, which is a key ingredient of most commercial curry powders. It has antiseptic properties and shows signs of anti inflammatory activity which could help in many diseases. The main chemical ingredient that's useful seems to be curcumin , which is about 5 percent of the dry powder in the bottle in your kitchen. Curcumin is also responsible for the bright yellow color. The very high concentration (most colored substances occur in plant products at levels way below 5 percent) is part of the reason it creates such bad stains .Curcumin does not dissolve in water, but it does in oil and in alcohol. This also explains some of its staining ability in that water won't help much to get rid of it. You can test this for yourself by putting some curcumin in a bottle with water and some in a bottle with alcohol or oil and shaking up both. You'll quickly see the alcohol or oil take on the yellow color that you associate with the stain, while the water will remain fairly clear. This leads to the first thing you can do to try to get curcumin stains out -- try using alcohol or oil. Of course you then need to get the alcohol or oil out which you can do by flushing with water (in the case of alcohol) or washing with soap or detergent (in the case of oil). Curcumin is also fairly unstable in the presence of ultraviolet light, so you can "bleach" out the stain by leaving it in bright sunlight. You may also have some luck with bleach, but if you're worried about damaging colored fabrics, the sunlight trick can be a good one.

Dr. Knowledge is written by physicists Stephen Reucroft and John Swain, both of Northeastern University. E-mail questions to drknowledge@globe.com or write Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.