How can you tell if the food contains starch or not?

December 7, 2025 •

4 min read

Did you know plants store excess energy as starch, a key complex carbohydrate in our diets? Determining its presence is a fundamental aspect of nutrition and food science, and there is a very simple and reliable method to tell if the food contains starch or not using a common household item.

Quick Summary

The iodine test is a reliable method for determining if food contains starch. A color change to blue-black indicates a positive result, while no change means starch is absent in the sample.

Key Points

The Iodine Test is Best: The most common and accurate method involves using a diluted iodine solution, which acts as a chemical indicator for starch.
Blue-Black is a Positive Result: If the food sample turns a dark blue, purple, or black color upon contact with iodine, it contains starch.
Yellow-Brown is a Negative Result: If the iodine solution retains its original yellowish-brown or orange color, the food does not contain starch.
Iodine Reacts with Amylose: The color change occurs because iodine interacts with amylose, the coiled component of starch molecules, to form a colored complex.
Temperature Affects the Test: Heating a sample can cause the blue-black color to temporarily disappear, which is a known property of the starch-iodine complex.
Use Proper Reagents and Safety: For accurate results, use a standard tincture of iodine and avoid povidone-iodine (Betadine). Wear safety goggles and protect surfaces, as iodine can stain.

Understanding Starch: The Plant's Energy Reserve

Starch is a complex carbohydrate, or polysaccharide, made up of long chains of glucose molecules produced by plants for energy storage. It is a white, odorless, and tasteless powder that is insoluble in cold water. When we eat starchy foods, our bodies break down these complex molecules into simple sugars (glucose) to use for energy. Different food sources, such as grains, tubers, and legumes, contain varying amounts and types of starch. While some starchy foods are easy to identify, like potatoes and bread, others, like thickeners in sauces, may be less obvious. Understanding how to detect starch is useful for dietary planning, especially for those managing carbohydrate intake.

The Principle of the Iodine Test

The most common and reliable method for detecting starch is the iodine test. This test relies on a chemical reaction between iodine and amylose, one of the two main polysaccharides that make up natural starch. When iodine comes into contact with amylose, the polyiodide ions slip inside the helical structure of the amylose chain, forming an intense blue-black complex. The original yellowish-brown color of the iodine solution changes dramatically, confirming the presence of starch. Conversely, if no starch is present, there is no helical structure for the iodine to bind to, and the solution retains its original color.

What You'll Need for the Test

Tincture of iodine or Lugol's iodine solution (standard antiseptic Betadine may give unreliable results)
Dropper or pipette
Food samples (solids and liquids)
White plate or petri dish for solid samples
Test tubes or clear containers for liquid samples
Safety goggles
Protective surface (iodine can stain clothing and countertops)

Step-by-Step Instructions for Solid Foods

Prepare your samples: Cut small, thin slices of each food item you wish to test, such as a potato, an apple, a cracker, or a piece of bread. Arrange them on a white plate, leaving enough space between them to prevent cross-contamination.
Add the iodine: Using a dropper, place 1-2 drops of iodine solution directly onto each food sample.
Observe the color change: Wait about a minute and watch for any color changes. If a food item turns dark blue, black, or purple, it contains starch. If it stays yellowish-brown, starch is not present.

Testing Liquid Foods for Starch

Use clear containers: Place a small amount of each liquid sample (e.g., milk, sugar solution, water mixed with cornstarch) into separate, clear test tubes or cups.
Add the iodine: Add 2-3 drops of iodine solution to each liquid sample.
Mix and observe: Gently mix the solution and observe for a color change. A liquid that turns dark blue or black is positive for starch.

Starchy vs. Non-Starchy Foods: A Comparison

To understand the practical application of the iodine test, it helps to compare foods based on their starch content. This is especially useful for meal planning and nutritional awareness.


Feature	Starchy Foods	Non-Starchy Foods
Carbohydrate Type	Complex carbohydrates (polysaccharides)	Simple sugars, fiber, proteins, fats
Examples (Plant-based)	Potatoes, bread, rice, pasta, corn, peas, legumes	Apples, berries, leafy greens, peppers, broccoli, cucumbers
Energy Release	Slower release, provides more sustained energy	Faster release (sugars), can cause blood sugar spikes
Iodine Test Result	Turns blue-black or purple	Remains yellowish-brown or orange
Effect of Cooking	Some starches may become more available after cooking	Less change, though cooking can affect texture and nutrients

Important Considerations and Potential Errors

While the iodine test is simple and reliable, a few factors can influence your results:

Temperature: High temperatures can cause the blue-black starch-iodine complex to dissociate, causing the color to fade or disappear. The color will typically return upon cooling. For best results, perform the test at room temperature.
pH Level: The test is less reliable in highly acidic conditions, which can break down starch molecules.
Iodine Source: It is important to use a proper iodine solution like a tincture. As noted, povidone-iodine (Betadine) should be avoided because its iodine is pre-complexed, making it less effective for this test.
Sample Color: Testing on very dark-colored foods or liquids can make it difficult to observe the color change accurately.

Conclusion: A Simple Tool for Nutritional Insight

In conclusion, the iodine test provides a simple, direct, and effective method to determine if a food contains starch. By observing a distinct color change to blue-black, you can reliably identify starchy foods, even those with hidden starches like some sauces or processed goods. The experiment is a valuable educational tool and a practical skill for anyone interested in better understanding the nutritional composition of their food. Always remember to handle iodine with care due to its staining properties and toxicity.

For more on the chemical properties of starch and how it's used in different foods, you can refer to the detailed explanations available from educational resources like BBC Bitesize.

Frequently Asked Questions

The key chemical used to test for starch is a diluted iodine solution, often in the form of tincture of iodine or Lugol's solution.

Iodine turns blue-black because its molecules get trapped inside the helical (spiral) structure of the amylose component of starch, forming a new complex with a dark color.

It is not recommended to use Betadine (povidone-iodine) for testing starch, as the iodine is already complexed, and it may not produce a strong or reliable color change. Use a standard tincture of iodine instead.

If you heat the starch and iodine complex, the blue-black color will disappear because the high temperature disrupts the molecular structure. The color will reappear upon cooling.

While the iodine test is the most direct and common method, some observations can indicate starch, such as a thick, gelatinous texture when cooked. However, a chemical test is the only reliable way.

Common starchy foods to test include potatoes, rice, bread, pasta, and corn. They will all show a positive blue-black reaction with iodine.

Cooking does not eliminate the presence of starch. For most foods, the test works well on both raw and cooked samples, although cooked starches can absorb the iodine more readily.

The iodine test can be performed by children with adult supervision. It is crucial to wear safety goggles and warn them that iodine is poisonous if ingested and can cause permanent stains.