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Toast in a toaster

The Chemistry of Toast – A Food Science Investigation

I have a friend who is a food scientist and when she worked on her graduate degree, she studied the chemical changes which take place when a piece of bread is toasted. You can do a simple investigation of this chemistry of toast with your own children.

Basically, heating the surface of the bread turns the starch into dextrin. Dextrin has a slightly sweet taste, which accounts for why many people prefer the taste of toasted bread.

Iodine in liquid form is available at pharmacies. It can be used to test for theĀ presenceĀ of starch. This is called the iodine test. If applied to a starchy surface, it will appear dark purple color. If there is no starch, you will not notice a color change. If your test area has enough dextrin, it might even turn a darker reddish color.

Be careful handling the iodine. Do not ingest it! Do not eat toast which has iodine on it! Do not get it in your eyes!

The Chemistry of Toast


  • bowl of water
  • tincture of iodine
  • one thick slice of white bread
  • toaster
  • knife
  • aluminum foil


  1. Mix about 5 drops of iodine in one half cup of water.
  2. Toast the white bread.
  3. Cut the toast in half.
  4. Dip one half into the iodine mixture and then place it on the foil.


Look at the bread. On the edge where you cut the bread the interior should be untoasted. Most of the exterior of the bread is toasted. Along the untoasted cut edge, you should see that the iodine turned a darker purplish color, indicating the starch. The toasted surface of the bread should not be the dark purple color though. This is because the starch has turned to dextrin.

Further Study

Try this with different types of bread. Or use lightly toasted bread versus bread which has been toasted to a deeper brown. What differences do you see?

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One Response to The Chemistry of Toast – A Food Science Investigation

  1. John January 17, 2016 at 5:34 pm #

    Just a question. Does the bread have to be taken directly from the freeze and toasted to get the converted starch.

    Or as long as the bread has been frozen and thawed the stretched have still been converted due the breed being freezing?

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