SALTINES , SALIVA, and the STARCH REACTION
turning a" classic" failure into a successful exploration
Introduction to "the myth"
Here's how it goes - give students saltine crackers. Put a drop of iodine on a cracker. See
the cracker turn blue-black. The reaction indicates the presence of starch. Instruct the
students to choose a fresh cracker and chew it, NO SWALLOWING ALLOWED! Chew
and observe taste sensations for at least 5 minutes (maybe ten minutes). Tell them to
notice the taste changing from bland to sweet (it actually happens, but the mush in your
mouth at this point is way too distracting to focus on the subtle change in taste. Spit out
the chewed cracker mush into a dish. Add iodine. Behold! The cracker much is
now……blue black!
The expectation is that the cracker would no longer show the starch reaction with iodine.
This is nearly impossible to see with the "chew a cracker" procedure. What follows is
one example of a successful way to conduct this experiment.
Materials
• Saltine crackers
• Test tubes and racks• Small cups/containers
• Water
• Droppers
• Graduated plastic pipette or teaspoon
• Waxed paper
• Tincture of Iodine (NOTE: To make the iodine go
farther, you may dilute it by half with tap water to make a 50% tincture solution. Store
in light-proof container.
Optional: Benedict’s solution, hot plate
To Do and Notice
Do the chew a cracker experiment described above. Note the taste change (subtle) and
lack of change in the iodine test after chewing the cracker.
Spit into a cup. You must collect at least 5 ml (one teaspoon) of saliva.
Obtain two test tubes. Break off two pieces of cracker approximately the same size -about the size of a small pea. Place the cracker pieces in separate squares of waxed
paper. Fold the waxed paper over the cracker and crush each piece of cracker into
oblivion. Place each crushed cracker bit into separate test tubes.
To one tube add 5 ml of water. To the second tube add 5 ml of saliva. Mix each tube
thoroughly, and allow to sit at room temperature for at least 5 minutes.
Add one drop of iodine to each tube. Mix. What do you see?
Optional: Check for the presence of glucose (see the handout “How Sweet It Is”).
Starch will not give a positive reaction but glucose will.
Karen Kalumuck
Exploratorium Teacher Institute
Copyright 2006
What's Going On?
Starch is a long polymer of glucose molecules. There are three forms of starch - amylose,
amylopectin, and and "animal" starch or glycogen -- (see attached diagram). Amylose is
the form found in crackers. Amylose is a linear chain of 200 - 2000 glucose subunits
bonded together that forms a long helical structure. Iodine molecules fit tightly inside the
helix, and consequent changes in the electron configuration of the iodine molecules cause
the starch/iodine complex to turn the diagnostic blue-black color. This color reaction is
reversible and is specific to the helical forms of glucose polymers found in starch.
Amylase, an enzyme found in saliva, breaks down amylose into, ultimately, it's
individual glucose subunits. As amylose is broken apart and the remaining sections grow
shorter, at some point they can no longer accommodate the iodine molecules. The color
reaction does not occur and iodine remains its normal brownish color (or the color
reverses, if the reaction has already occurred).
As the cracker is chewed, salivary amylase starts breaking down the amylase into shorter
glucose polymers. Some will be converted into individual glucose molecules that may
impart a sweetish taste. (Of note: Glucose is not particularly sweet; fructose very similar
in structure to glucose, and is much sweeter. Table sugar - sucrose - is a disaccharide of
glucose and fructose.) Because there is so much starch in a cracker, there is not enough
amylase in the saliva to break down all of the starch into glucose. There is plenty of
starch left, after chewing, to give a positive reaction with iodine. Using a very tiny piece
of ground up cracker in a large volume of saliva provides enough amylase to digest all of
the starch in a few minutes, thus resulting in a negative reaction in the starch test.
Benedict’s test: In the Benedict’s test reducing sugars (sugars that reduce copper (II)
sulfate to produce a precipitate of copper (I) oxide upon heating) such as glucose and
many other monosaccharides give a positive reaction. For many disaccharides (e.g.
sucrose) and long molecules like starch, the aldehyde is destroyed during the bonding of
the monomers into the polymers. If sufficient time has passed, amylase may break down
enough of the starch into glucose to give a positive reaction in the Benedict’s test.
Cracker alone and Benedict’s will be negative, but cracker with amylase may be positive
(positive is a green to orange preciptate).
Going Further
Amylase is a protein that functions as a biological catalyst (enzyme). Therefore, its
activity can be changed by some simple manipulations. Try the following:
• Vary the ratio of cracker to saliva, and see how enzyme concentration affects enzyme
activity as assessed by the iodine reaction.
• Repeat the experiment at various temperatures - refrigerator, warm water bath, and
assess for changes in reaction rate.
• Dilute the saliva and assess for changes in reaction rate.
Karen Kalumuck
Exploratorium Teacher Institute
Copyright 2006
• Use a 1% starch solution (1 g starch in 100 ml water; corn, potato, or rice starch). Add
a drop of iodine to the starch solution in a test tube. add saliva (with any of the above
variations) to look for reversal of the starch reaction.
• Treat the saliva by boiling or freezing, and assess for amylase activity
• Different people produce different amounts of amylase in saliva. Test other students'
saliva for amylase activity by any of the above methods.
Karen Kalumuck
Exploratorium Teacher Institute
Copyright 2006
All three of the above molecules are composed entirely of glucose monomers,
indicated here by hexagons. All have helical shapes, but amylose is unbranched,
amylopectin is branched, and glycogen (stored in animal tissue) is highly branched.
Karen Kalumuck
Exploratorium Teacher Institute
Copyright 2006