Summary
Introduction
Have you ever heard about starchy and non-starchy foods? What is the difference? Starchy foods contain the carbohydrate starch, which is converted to sugar (glucose) inside our body for energy production. How do we know which foods contain starch and which do not? There is a simple chemical test that you can do to detect starch, which involves an iodine solution. The iodine solution turns any food that contains starch dark blue. Try this activity and watch it in action yourself!
Materials
- Iodine tincture or solution (2%), such as the type used in a first aid kit as an antiseptic to treat minor wounds
- Corn starch
- Water
- Cups (2)
- Knife
- Cutting board
- 1/4 teaspoon
- Sheet of aluminum foil
- Pipette or medicine dropper
- Safety glasses
- Optional: An apron
- A variety of foods, such as:
- Potatoes
- Pasta
- Vegetables
- Fruits
- Crackers
- Candy
- Optional: Microwave
- Optional: Refrigerator
- Optional: Liquid foods or drinks such as yogurt, juices, or milk
Prep Work
- Fill both cups about half full of room-temperature or cold water. Label one cup "+" and one cup "-".
- Add 1/4 teaspoon of corn starch into the "+" cup and mix the solution.
How does the solution look after mixing?
Instructions
- Put on your safety glasses and carefully open the iodine solution. Note, that the iodine will stain your countertop or clothes, so be careful when you handle it and try to avoid any spills. You can wear an apron to protect your clothes from potential spills.
- Using the pipette, suck up some of the iodine solution.
What color does the solution have?
- Carefully add a couple of drops of the iodine solution to the cup with just water.
What do you notice? How does the color of the water change?
- Next, add a couple of drops of the iodine solution to the cup with water and corn starch.
What happens when you add the iodine? How is your result different than before?
- Now you will start testing your foods. With the knife, cut off a small piece of every food that you want to test.
- Place the pieces of food next to each other on a sheet of aluminum foil.
- Suck up more of the iodine solution with the pipette.
- Place one drop of the iodine solution on the first food that you want to test. Wait about 1 minute and observe what happens.
Do you notice any color change as a result of adding the iodine solution? Can you explain your results?
- Continue to test the other foods by adding one drop of iodine solution to them.
How can you tell from your results which foods contain starch and which ones do not?
Cleanup
What Happened?
As you might have noticed in this activity, iodine can be used to detect the presence of starch in foods or other objects. Iodine solution has an orange-yellowish color. When you added the iodine solution to the water, the color should have pretty much stayed the same. It might have become a bit lighter due to the dilution, but the water should have still looked orange-yellowish in color. When you made the starch solution in the second cup, it should have looked like a turbid whitish solution. The solution is turbid because corn starch is partially insoluble in water. When you added the iodine to the corn starch solution, you should have noticed that the liquid inside the cup almost immediately turned dark blue or almost blackish in color. The color change happened because the iodine reacted with the starch and formed a dark blue starch/iodine complex.
The same reaction happened when you added the iodine solution to foods that contained starch. You probably noticed that the iodine solution changed color when you put it on foods that contained a lot of starch, such as pasta, potatoes, or certain crackers. Other foods—such as peppers, cucumbers, and strawberries—are non-starchy foods, and the iodine solution should not have changed color when you added it to these types of foods.
If you heated up the corn starch solution with the iodine, you probably noticed that the dark blue color of the starch/iodine complex disappeared with increasing temperature. This is because the starch/iodine complex is not stable at higher temperatures. As the complex falls apart its color disappears.
Digging Deeper
Starch is a carbohydrate and a main ingredient of many foods, such as bread, potatoes, pasta, or starchy vegetables. It consists of two main components called amylose and amylopectin. Amylose is a long, linear chain of glucose molecules that form a spiral that looks similar to a coiled spring. Glucose is a simple sugar that many organisms need to generate energy for themselves. Amylopectin also consists of a chain of glucose units, but the amylopectin structure is not linear as in amylose, but branched. Due to its branched structure, amylopectin is completely insoluble in water, whereas the linear amylose is partially water-soluble. Many plants make starch and store it as granules to use these as an energy source at night by converting the starch into glucose. The biggest application for starch besides making foods such as pasta or cereals, is papermaking. A typical sheet of copy paper, for example, can contain as much as 8% starch.
A specific chemical test or reaction is used to test for the presence of starch in foods or other items. In such a reaction, iodine (I2) is used to detect starch. Iodine in itself is not very water-soluble, but in iodine reagents such as an iodine tincture or solution, iodine is dissolved in water in the presence of potassium iodide. In the presence of potassium iodide, the iodine forms polyiodide ions, such as triiodide (I3-) or I5- complexes, which are water soluble. The color of an iodine solution is usually orange or yellow depending on its concentration. However, when starch is mixed with an iodine solution, an intensely colored dark blue starch/iodine complex is formed. The exact reaction mechanism of this color change is still unknown. What is known is that the iodide ions slip into the amylose coil structure. Then some transfer of charge happens between the starch and the iodide, which results in a change of the energy level spacing of the iodine/starch complex. This, in turn, leads to the molecules being able to absorb visible light at a different spectrum, giving the complex its intense blue color. The reaction does not happen with amylopectin on its own, as the main reaction partner for the iodide seems to be the amylose. Interestingly, the colored starch/iodine complex is not stable at warmer temperatures. If you heat up the reaction mixture, the blue color will disappear as the amylose coil structure will break apart at higher temperatures.
Ask an Expert
For Further Exploration
- Test how the iodine starch reaction changes with temperature. Make three corn starch solutions. Put one microwave-safe cup of water into the microwave and heat it up for about 30 seconds. Put one cup of water in the freezer or refrigerator and keep one cup of water at room temperature. Then add the iodine solution to the different cups. How do your results differ for all three cups? Can you explain your observations?
- Investigate how stable the starch/iodine complex is. Put a microwave-safe cup with the corn starch solution and the iodine into the microwave and heat it up for about 30 seconds. How temperature-sensitive is the starch/iodine complex?
- Find out if liquid foods—such as yogurt, juices, or milk—contain starch by adding some drops of iodine solution to them. But remember, you have to discard any food or drinks that came in contact with the iodine solution!
- How does the starch concentration affect the color change of the starch iodine reaction? Prepare solutions with different corn starch concentrations and do the starch test. What do you observe with different starch concentrations?
