What is Starch?
Starch is a white, granular and organic substance produced by plants. It is a soft and tasteless powder which is not soluble in alcohol, cold water and other solvents. The basic chemical formula of any molecule of starch is C6H10O5.
A simple test for starch performed at a lab or home can allow students to understand the type of carbohydrate their foods contain.
Production of Starch
Starch is produced within the green leaves of plants from the excess glucose created during photosynthesis. The primary purpose of starch is to act as a reserve food supply for plants.
Furthermore, plants store starch in chloroplasts as granules and other organs such as roots, stem, seeds, tuber, etc. Some prominent examples of foods that contain starch are potatoes, corn, wheat, rice, cassava, etc.
Additionally, this starch is disintegrated with the help of various enzymes and water. This process converts starch into monomer glucose units. It diffuses from the cells and nourishes plant tissues. On the other hand, in the case of humans and other animals, starch is broken down into sugar molecules. It supplies the necessary energy.
Before moving onto the topic of how to test for starch, here are some common questions for students to solve –
What is starch?
What is the chemical formula of starch?
Where do plants store starch?
What is the purpose of starch?
Chemical Test for Starch
A simple chemical test containing iodine is often performed to determine the presence of carbohydrate in a food item. Food items consumed by individuals contain different types of carbohydrate, and starch is one of them. This simple iodine test for starch can assist in this purpose.
Test 1: Test for Starch in a Potato
In this test, students can check the amount of starch in a potato. However, for this test, they need some equipment. These are potatoes, a knife, a clean pot, and iodine solution for starch test.
First, peel the potato and cut it into small pieces. After that, place these small pieces inside the clean pot and add 2-3 drops of iodine solution. Then keep the bowl undisturbed for a couple of minutes to see the change.
Furthermore, if followed correctly, the iodine test for starch results will show that the slices of potatoes have developed a black-blue color.
Test 2: Liquid Test for Starch Iodine Test
This simple test for starch can be successfully performed using any liquid food sample. The equipment and ingredients required for this test are test tubes, liquid food sample, and iodine sample.
In the beginning, pour around 10 ml of food samples in a clean test tube. After that, add around 4-5 drops of iodine solution into the liquid. Then wait to observe the results, if it turns blue then that food item contains starch.
These food tests for starch are simple, and students can take it up as a DIY project.
Test 3: Leaf Testing for Starch
Similar to the tests mentioned above, this test will also indicate the presence of starch. For this test students need a fresh green leaf, around 250ml of water, a boiling pot and a test tube, ethanol and iodine solution.
To begin with, students should boil the water first and then place the leaf into it for two minutes. After that, remove the leaf from there and place it in a test tube containing 90% ethanol. Now, boil this solution for 10 minutes and rinse the leaf in cold water afterwards.
After that, place this leaf on a clean tile and add a few drops of iodine solution. If it turns blue-black, it contains starch.
For more information on other topics of biology besides this simple test for starch, students can go through the rest of the website. The app allows students to access more study materials, and they can join online live classes as well. The subject experts are covering a new topic every day.
FAQs on Test for Starch
1. What is the primary purpose of the iodine test in biology?
The primary purpose of the iodine test is to chemically detect the presence of starch in a substance. It is a common experiment used to identify starch in food items or to verify that photosynthesis has occurred in plant leaves by testing for the starch produced.
2. How do you perform the test for starch in a food sample like a potato slice?
To test for starch in a food sample like a potato, follow these simple steps:
- Take a fresh, clean slice of the potato.
- Using a dropper, add 2-3 drops of a dilute iodine solution directly onto the potato's surface.
- Observe the area for any colour change. A change to a blue-black colour indicates the presence of starch.
3. What result indicates a positive test for starch, and what does a negative test look like?
A positive test for starch is indicated by a distinct blue-black colour appearing on the sample after iodine solution is added. If no starch is present, there will be no colour change, and the iodine solution will remain its original brownish-yellow colour. This is considered a negative result.
4. Why is it necessary to decolourise a green leaf before testing it for starch?
It is essential to decolourise a green leaf before the starch test because the green pigment, chlorophyll, can mask the blue-black colour change. The leaf is typically boiled in ethanol (or methylated spirits) to remove the chlorophyll, leaving the leaf pale. This ensures that the result of the iodine test is clearly visible.
5. What is the chemical reason that iodine turns blue-black when it reacts with starch?
The blue-black colour results from a chemical interaction between iodine and a component of starch called amylose. Amylose has a helical (spiral) structure. When iodine solution is added, polyiodide ions (I₃⁻ and I₅⁻) slip inside this helix, forming a starch-iodine complex. This complex absorbs light differently than iodine alone, producing the intense blue-black hue.
6. Can the iodine test be used to detect simple sugars like glucose or table sugar (sucrose)?
No, the iodine test is specific for starch and will not work for simple sugars like glucose or sucrose. These sugars have a different, non-helical molecular structure and cannot trap the iodine ions to form the blue-black complex. Other chemical tests, such as Benedict's test, are required to detect such sugars.
7. What are some key precautions to follow while conducting the iodine test for starch in a school laboratory?
When conducting the starch test, it's important to follow safety precautions:
- Use a dilute iodine solution, as concentrated forms can be corrosive.
- If using ethanol to decolourise a leaf, heat it using a water bath, not a direct flame, as ethanol is highly flammable.
- Ensure proper ventilation, especially when boiling ethanol.
- Handle all glassware with care to prevent breakage and injury.
