Bananas. Ya gotta love’ em. They’re delicious. They’re healthy. They’re among the world’s most commonly consumed fruits. When we found a potential science experiment using our Glucose Test Strips and bananas, we had to jump on it. But before we get started, let’s get to know the banana a little bit better.

Bananas 101

What are bananas actually made of? Unripe bananas are primarily made up of starch: green bananas can contain 70-80% starch. As bananas ripen, the starch is converted into sugars. When completely ripe, bananas contain less than 1% starch. Wowsers! That’s quite the transformation.

The most common types of sugar found in bananas include the usual suspects: sucrose, fructose and glucose. In a ripe banana, sugar content can be over 16% of the fresh weight.
Of course, bananas also contain fiber, vitamins and minerals, and other plant compounds, but those components are less important for the purposes of our experiment.

Taste the Rainbow

We all know bananas taste very different throughout the stages of ripening. Green bananas definitely aren’t as sweet as yellow bananas, and though blackening bananas may be very sweet, they’re also very mushy.

Green bananas tend to ship better from distributors to the stores, since they don’t bruise as easily. Once the stores receive them, they can help them ripen by adding a small amount of ethylene. Ethylene is a hydrocarbon gas that ripening fruits produce and shed into the atmosphere. The ripening of a banana can also be somewhat controlled by how it’s stored. It seems more often than not, what you’ll find at the store are slightly green to yellow bananas.

Now, let’s get to the fun part. This is an experiment you can try at home, and you’ll need access to your washing machine.

Materials

• 2-3 Bananas in different stages of ripening (green, yellow or black)
• 4-6 Plastic cups
• Spoon
• Balance scale
• 2 Test tubes with screw caps
• Distilled water
• 2-3 Pipettes
• Graduated cylinder
Glucose Test Strips

Experiment

glucose test strips, glucose test strip experiment, glucose in ripe bananasFirst, make your hypothesis. Which banana will have the highest sugar content and which will have the lowest? Next, choose your bananas! Now, we will need to isolate the soluble sugars to determine the amount in each banana. Follow the next steps on each banana, separately. Be sure to thoroughly rinse any tools (spoon, pipette, etc.) that will be reused for the different bananas.

1. Weigh out approximately 1g of banana on a top-loading balance in a small plastic cup. Don’t forget to tare the cup before adding the banana!
2. Record the mass of the banana sample in your Laboratory Notebook.
3. Add 6mL of distilled water to the cup, and mash up the banana using a spoon. Be sure your sample is well mashed and mixed.
4. Pour the sample from the cup into a screw-top tube without spilling.
5. Rinse the cup with 6mL of fresh distilled water, and add the rinse to the same tube.
6. Tape the tubes inside a washing machine, and use the spin cycle to centrifuge the tubes. Keep spinning until all of the banana mixture is a solid pellet at the bottom of the tube. Note: If you don’t have access to a washing machine, you can also try taping the tube to a ceiling fan, a salad spinner, or some other similar spinning method. Get creative!
7. Carefully pour any remaining banana liquid into a cup, and label it Supernatant. Make sure the solid part (banana pellet) stays in the tube. If you can’t do this by pouring, use a pipette to transfer the liquid.
8. Wash the solid banana pellet in the tube with 6mL of fresh distilled water by adding the water to the tube and breaking it apart with a pipette.
9. Repeat step 6 by again suspending the tube in the washing machine.
10. Repeat step 7, by again pouring any leftover liquid into the Supernatant cup as before, and set the solid banana pellet aside.
11. Using a graduated cylinder, carefully measure the volume of the liquid in the Supernatant cup. Record this volume in your Laboratory Notebook.
12. Remove on Glucose Test Strip from the bag, and dip it into the Supernatant cup, completely immersing the pad, for 1-2 seconds. As you remove the test strip, draw it against the rim of the cup to remove any excess liquid.
13. Wait 3 minutes before comparing the test pad to the color chart. Record the amount of Glucose in your Laboratory Notebook. If you think the amount of glucose exceeds 3000mg/dL, try diluting the sample with an additional 10mL of distilled water. Mix well and retest.
14. Repeat the steps above for each different banana. Compare your results.

Calculation Hints:
To determine the total grams of glucose, you will need to do some unit conversion. Before you multiply the concentration by the volume, make sure that the volume is in dL, no mL. Record your final answer in grams, not milligrams.