School Science/Demonstrating the effects of amylases on starch

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This experiment demonstrates the effects of heat and pH on the activity of enzymes.

Materials required[edit | edit source]

Saliva (contains amylases that break down starch), starch, iodine, test tubes, hot plate

Methods[edit | edit source]

Exact amounts should be determined by trial and error.

  1. Collect some saliva (no mucus) in a beaker. Conduct the experiment in test tubes or small beakers and do not put anything into your mouth to mix with saliva.
  2. Mix starch with water and boil until solution becomes clear. Cool. This step is necessary to homogenize the starch solution and increase the surface area of the starch for the enzymes to act on.
  3. Mix starch solution with a small drop of iodine. A positive reaction is indicated by a dark blue/black color.
  4. Mix starch solution with some saliva. Mix well. Keep at body temperature for several minutes (hold in hand). Amylases in the saliva break down the starch.
  5. Mix saliva/starch solution with a small drop of iodine. Reaction should be negative.

Here are some variations

  1. Alternative: Heat up saliva to denature the enzyme before mixing with starch solution. Iodine reaction should be positive for starch.
  2. Alternative: Mix saliva with acids or bases (e.g. lemon juice for safety) before mixing with starch. This can be used to demonstrate pH sensitivity of the amylase.
  3. Alternative: Mix saliva first with iodine. Then use the iodine/saliva mixture to test for starch. The iodine possibly inhibits the amylases in the saliva.
  4. Alternative: Conduct a Benedicts Test to test for simple sugars after starch solution has been treated with saliva. Benedict's test should be positive after addition with saliva. Test results are expected to be negative without addition of saliva or after heat denaturation or pH change of the saliva.

For a briefer demonstration of the enzymatic effect, hold a cracker in your mouth for 60 seconds. The amylases will break down the starch and the cracker will start to taste sweet due to the sugar produced.