Kids, did you know that most of the natural color of convenience foods is lost during processing? Food manufacturers add artificial coloring to foods to restore their expected color. Red food dyes are added to hotdogs that would otherwise look gray, for example. Natural food colorings (pigments from paprika, beets, carrots, and leaves) fade with time and generally have a low coloring power

Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

A long time ago chemists discovered how to make good dyes from coal tar, and the FDA has certified four - one blue, two reds, and one yellow - as safe to eat. This experiment will let you detect coal tar dyes in food. You'll need colored liquids like tomato or beet juice, soda, tea, maraschino cherries, grenadine, jellies, water from canned and cooked vegetables; powders like paprika, saffron, or turmeric in 1/2 cup water. Also, a small enamel or steel saucepan (aluminum pans won't work!), vinegar (acetic acid), and white wool yard (not acrylic).

Put some of a colored liquid, a few drops of vinegar and a 3-inch piece of the yarn into the pan and heat to boiling. Make sure that an adult is there to supervise this step. Lift out the yarn with a fork and rinse it in cold water. If the yarn is colored (dyed) after this boiling step, then the colored liquid contains an artificial dye made from coal tar.

The protein in wool reacts with coal tar dyes in an acid environment (vinegar) to form a new stable substance via chemical bonding. Natural dyes do not form this bond with the wool and easily wash away with hot water. Now check your results against the food labels.   

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Kathleen Carrado Gregar, PhD, Argonne National Labs 
[email protected]
May 1992