Testing for Carbohydrates in Foods

Experimental Methods to Identify the Presence of Glucose and Starch

© Adrienne Larocque

Oct 15, 2009

The syllabi for IB Biology and IGCSE Biology and Combined Science explicitly require students to know how to test foods for the presence of dietary carbohydrates. Other secondary curricula include this topic. The experiments described here also could be conducted as demonstrations for primary and middle school classes learning about food and diet.

What are Dietary Carbohydrates?

Carbohydrates are molecules consisting of carbon, hydrogen and oxygen atoms bonded together. The basic building blocks of these complex molecules are glucose and, to a lesser extent, fructose. Both are rings containing 6 carbon atoms. The monosaccharide rings can be joined together through condensation reactions to form disaccharides (sugars) and polysaccharides (starches and fiber). During digestion, sugars and starches break down into glucose (and fructose, if present) and provide the body with energy.

Procedure to Test for Glucose

The presence of reducing sugar like glucose is identified using Benedict’s reagent, a blue solution containing copper sulfate and other salts. The following procedure involves testing a standard solution containing glucose so that students may observe what constitutes a positive result. The best way for students to record their observations in this experiment is for them to construct a table.

1. Fill a test tube one quarter full with glucose solution.
2. Add an equal amount of Benedict’s solution to the test tube.
3. Place the test tube into a 500 ml or 1 L beaker containing boiling water.
4. Allow the contents of the test tube to boil for 2 minutes, agitating gently from time to time.
5. Record the color change observed (the solution should turn orange).

Once students are familiar with what to expect when glucose is present, they may proceed with testing a variety of “unknowns” (food samples) to determine if they contain reducing sugar. Any foods may be chosen; it is worthwhile to include some that do contain a lot of glucose (e.g., soda or pop), some with a small amount of glucose (e.g., processed cheese) and some that do not contain glucose (e.g., cooking oil).

1. Fill a test tube one quarter full with Benedict’s solution. DO NOT add glucose solution to the test tube (this is a common mistake made by students in the lab).
2. For liquid samples, add an equal amount of the liquid to the test tube.
3. For solid samples, use a mortar and pestle to grind about 1 cubic cm of food together with warm water (to extract the glucose). Add it to the test tube containing Benedict’s solution.
4. Place the test tube into a 500 ml or 1 L beaker containing boiling water.
5. Allow the contents of the test tube to boil for 2 minutes.
6. Record any changes observed.

Any foods that contain a lot of glucose should change the Benedict’s solution orange. If a small amount of glucose is present in the food, the solution will turn green. If no glucose is present, the solution will remain blue.

Procedure to Test for Starch

The presence of starch is identified using iodine solution, a yellow-brown mixture of iodine and potassium iodide. The following procedure involves testing a standard solution containing starch so that students may observe what constitutes a positive result.

1. Fill a test tube one quarter full with starch solution.
2. Add 2 drops of iodine solution to the test tube.
3. Agitate the tube gently to mix the constituents.
4. Record the color change (the solution should turn a blue-black color).

Once students are familiar with what to expect when starch is present, they may proceed with testing a variety of food samples to determine if they contain starch. If starch is present, the iodine will stain the food with a blue-black color. Some foods that will yield a positive result include bread, rice and potato.

For liquid samples:

1. Fill a test tube one quarter full with the liquid to be tested. DO NOT add starch solution.
2. Add 2 drops of iodine solution to the test tube.
3. Agitate the tube gently to mix the constituents.
4. Record the color change (the solution should turn a blue-black color).

For solid samples:

1. Use a mortar and pestle to grind about 1 cubic cm of food.
2. Place the ground sample on a small ceramic dish or ceramic tile.
3. Add 1-2 drops of iodine solution to the food.
4. Record any color change. If starch is not present, the iodine solution will remain a yellow-brown color.

Notes to Students and Teachers

The test tube containing Benedict’s solution is placed into boiling water to speed up the reaction. Similarly, cutting up or grinding solid samples of food increases their surface area, thereby increasing the reaction rate.

All monosaccharides and some disaccharides are classified as reducing sugars (sucrose or common table sugar is not a reducing sugar). In the presence of such compounds, copper (II) ions (which give the reagent its blue color) are reduced to copper (I) ions which precipitate as insoluble orange/red copper oxide.

Starch can be separated into two fractions – amylose and amylopectin. Natural starches are mixtures of the two. It is the amylose fraction that causes the formation of the deep blue/blackcolor in the presence of iodine. Iodine solution will also react with glycogen (a starch found in animal cells) to produce a brown color.

Related Articles

What Not to Eat: Give Up These Carbohydrate-Loaded Foods and Lose Weight

Starch Test on Sugar - No Reaction

Starch Test on Bread - Bread Turns Black

Starch Test on Rice - Rice Turns Black

Print Article

View All Articles

How to subscribe to feeds