How Sweet It Is! Measuring Glucose in Your Food

Abstract

You know that sugar makes food sweet. But did you know that there are different kinds of sugar? Sucrose is the granulated sugar that you usually use for baking. Another kind of sugar, which is found in honey and in many fruits, is glucose. In this science fair project, you will measure the concentration of glucose in a variety of foods. You will use special strips that change color in response to glucose, to measure the glucose concentration in different foods.

Objective

The objective of this science fair project is to measure the concentration of glucose in a variety of common fruits and juices.

Credits

David Whyte, PhD, Science Buddies

Last edit date: 2013-02-15

Introduction

Fresh pineapple, chilled watermelon, and an icy cold soft drink on a hot summer's day. What do these things have in common? They all taste so good because of the sweetness the sugar in them provides. When you think of sugar, you probably picture the white granules you put in cookies, that your parents put in their coffee, or that you put on your cereal. Actually, this is just one kind of sugar, called sucrose, which is extracted from sugar cane or sugar beets. Technically, sugar is a carbohydrate that occurs naturally in every fruit and vegetable. It is the major product of photosynthesis, the process by which plants transform the Sun's energy into food.

Glucose is another type of sugar and is a very important biochemical. For one thing, glucose is the only fuel used by brain cells. Glucose is also an important source of energy for muscles and other tissues in the body.

The glucose in your blood comes from the food you eat. Complex carbohydrates, found in pasta or cereal, for example, are long chains of sugar molecules that are broken down by enzymes to simple sugars, such as glucose. Sucrose, or table sugar, is also broken down to form glucose. Because carbohydrates and sucrose in food are broken down to form glucose, the level of glucose in your blood goes up after you eat.

Like most of the chemicals in your blood, the level of glucose must be tightly controlled. The level of glucose in your blood is controlled by insulin, a hormone made in the pancreas. Too little glucose, and your brain and other organs will not have the energy they need to function. Too much glucose in the blood can cause diabetes, which is a serious and growing health problem in the United States.

In this science fair project, you will investigate the concentration of glucose in common fruits and juices. In order to measure the glucose concentration, you will use glucose strips. These strips were developed to help people with diabetes maintain a healthy level of blood glucose. When you dip the test strip into a liquid, such as orange juice, it changes color if glucose is present. The degree of color change depends on the concentration of glucose.

Terms and Concepts

• Sucrose
• Carbohydrate
• Glucose
• Biochemical
• Complex carbohydrate
• Enzyme
• Insulin
• Hormone
• Pancreas
• Control
• Negative control
• Positive control
• Dilution series

Questions

• Put these five food sugars in order, from the sweetest to the least sweet: glucose, fructose, sucrose, lactose, and maltose.
• What is the concentration range of glucose in healthy people? What is the level in diabetics?
• Which foods have a naturally high level of glucose?
• Which fast foods have the highest glucose levels? (Hint: Look at the nutrition data on the restaurants' websites.)
• Why do certain foods have high natural sugars? (Hint: Fruits contain seeds, and the plant's chances of spreading are better if the seeds are dispersed).
• How many calories are in 1 gram (g) of glucose?
• How does the glucose test strip work?

Bibliography

• HowStuffWorks.com. (2008). How Food Works. Retrieved September 11, 2008, from http://recipes.howstuffworks.com/food2.htm
• Nutrition Data. (2008). Foods highest in Glucose. Retrieved September 11, 2008, from http://www.nutritiondata.com/foods-000010000000000000000-1.html
• Novo Nordisk. (2012) Facts About Diabetes. Retrieved October 10, 2012, from http://www.novonordisk.com/diabetes_care/facts_about_diabetes/default.asp

Materials and Equipment

• Glucose test strips; available at most drug stores. The Experimental Procedure is based on Bayer's Diastix reagent test strips for glucose determination in urine. These can be dipped in a liquid to read the glucose concentration. Do NOT use the kind of test strip designed to test glucose in blood.
• Glucose tablets with 4 g of glucose per tablet; available at most drug stores
• Liquid measuring cup
• Foods and juices to test. Categorize as fresh or processed. Some examples are listed below.
  • Fruit juices: orange juice, lemon juice (you will be surprised!)
  • Fresh fruit (sliced): apple, pear, pineapple, cucumber, tomato
  • Processed foods: soft drinks, diet soft drinks, salad dressing, baby food, vinegar, peanut butter, sauces used on fast food hamburgers
  • Miscellaneous: honey, sugar water (sucrose mixed with tap water)
• Stopwatch or clock with a second hand
• Several small cups to hold liquids for testing
• Adult helper
• Lab notebook

Experimental Procedure

1. To start this science fair project, you should first collect all of the foods and juices that you plan to test.
2. Based on your research, predict which foods will have the highest glucose levels. List the foods in your lab notebook in order from highest to lowest predicted glucose concentration.
   1. For processed food, the labels on most products list total sugar, which is a mixture of sucrose, fructose, and glucose. The test strip only measures glucose concentration. The actual level of glucose is usually available online, at the company's website, or at sites such as www.nutritiondata.com.

Making the Positive and Negative Controls

Controls are samples with known ingredients that should give clear results. They are used to test the procedure. In a negative control, there should be no "signal." In a positive control, there should be a clear signal, showing that the glucose strips are working properly. Tap water is a suitable negative control (because it has no glucose). There should be no change in the test strip when it is dipped into the negative control.

1. To make the negative control, add tap water to one cup and label it as the negative control. Test it by inserting a test strip. There should be no change.
2. Make a dilution series as the positive control to test your strips. For example, make a dilution series with the following concentrations by performing sequential two-fold dilutions: 4%, 2%, 1%, 0.5%, 0.25%, 0.125% and 0.0625%.
   1. Label seven cups 1–7.
   2. Add 8 g of glucose to 200 mL water in cup #1 to make the 4% solution.
   3. Add 100 mL of water to each of the six remaining cups (2–7).
   4. Add 100 mL of the 4% solution to cup #2 to make a 2% solution.
   5. Then add 100 mL of the 2% solution to cup #3 to make a 1% solution.
   6. Repeat for the remaining dilutions. Make sure to rinse the container you are using to transfer the 100 mL volumes between each dilution.
3. Dip a test strip into each of the positive control solutions. Watch the test strip for 30 seconds (or the time recommended in the test strip instructions) and match the color of the test strip to the color on the bottle. Do the colors match what you would expect? Write down your observations in your lab notebook.
4. There should be a clear positive signal matching 1 percent glucose. If you do not get a positive response, repeat the procedure; if it still does not work, buy some new test strips.

Testing the Foods for Glucose Concentration

1. Pour a small amount of liquid that you plan to test into a cup.
2. Get ready to start the stopwatch.
3. Dip the test strip into the liquid.
   1. For the fresh fruits and vegetables, press the test strip against a freshly cut slice until the test strip is thoroughly wet.
   2. For very high-glucose liquids, such as honey, or viscous substances, such as peanut butter, dilute the samples in water prior to testing. For example, add 1 tbsp. of honey to 5 ounces (oz.) of water makes a 1-to-10 dilution (2 tbsps make 1 ounce). Multiply the concentration of glucose in the diluted solution by 10 to obtain the concentration of glucose in the original sample.
4. Start the stopwatch as soon as the test strip has been dipped.
5. Wait for the amount of time specified on the test strip directions, usually 30 seconds.
6. Compare the color on the test strip with the color on the side of the container to determine the glucose concentration. The Bayer Diastix brand has color matches for the following concentrations, in % glucose: 0.1, 0.25, 0.5, 1, and 2. Also, compare the colors with the dilution series you made.
   1. If the color changes to the maximum range (2 percent) before 30 seconds, list it as greater than 2 percent, "> 2%."
   2. To determine the actual percent of glucose in samples with over 2 percent, dilute the sample in water to bring the glucose level down within the range of the test strips. For example, 1 Tbsp. of juice in a final volume of 5 oz. is a dilution of 1 to 10 (2 Tbsp. make 1 oz.).
   3. Test the diluted sample. If it has 1 percent glucose, then the glucose in the sample is really 10 percent, because you diluted it 1 part in 10.
7. Repeat steps 1–6, of this section, for all of your foods and juices.
8. Make a data table of your results in your lab notebook. The data table below is an example, with a "predicted" column for what you expected to find, and an "experimental" column for your actual results.
9. Use < for="" "medium,"="" and="" "low,"="" 1%–2%="" 5%="">2% for "high."
10. Did your results match your predictions?
11. Repeat the glucose measurements for a total of at least three trials.
12. Graph your results. Put the type of drink on the x-axis and the glucose concentration on the y-axis.

Type of Food	Glucose Level: Predicted	Glucose Level: Experimental	Notes
Mixed fruit drink	High	10% (1% in diluted sample)	Diluted 1:10 (example)
Orange juice	Low	1%	Tropicana

Variations

• People with diabetes might occasionally experience low blood sugar. When this occurs, they need to eat or drink something with glucose in it right away. Calculate how much of each food you tested would need to be consumed to provide 4 g of glucose.
• Do different honeys have different glucose levels?
• Compare fresh vs. frozen foods for glucose levels.

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