The purpose of eating food is to obtain the nutrients our bodies need to stay alive and perform all our daily tasks. The process of breaking down food into the nutrients our bodies can use is called digestion.
Even though digestion takes place inside our bodies, we can do this simple digestion experiment to see the first steps of digestion in action.
The First Steps Of Digestion
When we first put food into our mouths, we use our teeth to chew it into smaller pieces. This is called physical digestion. This process makes it easier for the next steps of digestion to take place.
As we chew, the food is mixed with saliva. Saliva contains chemicals and enzymes that break down food into simpler pieces to be broken down further in the stomach and small intestine. When enzymes and chemicals break down food, this is called chemical digestion.
Seeing Digestion At Work
We can do a simple test to see physical and chemical digestion at work.
- saltine or oyster crackers
- 2 bowls
Step 1: Crumble half of a saltine cracker or a couple oyster crackers into a bowl.
Step 2: Add 1 or 2 tablespoons of water to the cracker crumbs and lightly stir to mix the water and the crumbs.
Step 3: Chew the other half of the saltine cracker or 2 or 3 oyster crackers in your mouth very well so that it mixes with the saliva in your mouth. Spit the cracker and saliva mix into the second bowl and add 1 or 2 tablespoons of water to the bowl. I know. Gross. But, it’s for science, so it’s okay!
Step 4: Note the difference between the 2 bowls.
Step 5: Add 2 drops of iodine to each bowl, swirling to spread the iodine. Note what you see.
What Is Happening?
Before the iodine is added to the bowls, we can see physical changes in the crackers in the bowls. In the first bowl, you can still tell that it is a cracker, just in smaller pieces and somewhat soggy. This is what happens in physical digestion. Food is broken into smaller pieces, but the chemical composition isn’t changed.
In the second bowl, the cracker mixed with saliva may not look like crackers. It may be more like a paste. The enzymes and other chemicals in saliva are already breaking down the cracker at the chemical level. This is the beginning of chemical digestion.
What happened when the iodine was added? When starch is present, iodine will turn a dark blue color. Crackers contain lots of starch. The crackers in the first bowl should be a dark color from the added iodine.
Saliva contains the enzyme amylase that breaks down starch into simpler sugars such as maltose and dextrin. Your body cannot use large starch molecules so they must be broken down by enzymes into smaller sugar molecules so they can be used by the body.
When the iodine is added to the second bowl, there is not as much starch present due to it being broken down by the amylase. The color of the second bowl is much lighter than in the first bowl.
Try this at home, or use iodine to test for the presence of starch in other foods.
Find more about the digestive system and more experiments like this one in Apologia’s General Science.