Objective

To compare digestion of whole wheat bread and white bread

Background

The body breaks down starch in food using an enzyme in the saliva and the small intestine called alpha-amylase. Enzymes are simply proteins that drive specific reactions in the body without being changed themselves. Alpha-amylase breaks down starch to produce glucose. Fiber in food can interfere with the rate at which starch breaks down to glucose in the body. For example, if you eat a sugary donut you are sure to have a large amount of glucose entering your blood rather quickly after eating. However, if the donut is made from whole wheat flour, then you are likely to experience a slower release of glucose to the blood.

Having this slow release of glucose to the blood is especially important for people with type 2 diabetes. Type 2 diabetes prevents the efficient removal of glucose from the blood, causing blood glucose to be elevated. This can result in health problems. In this lab, we will use bread to test the hypothesis that high-fiber foods digest slower than foods with little fiber.  

To determine glucose content, we will be using a qualitative test with the help of Benedict’s reagent. In a qualitative test the result will be reported in the form of a description rather than a number. Tests giving you a number are called quantitative tests. For example, if I say that the percentage glucose is 5%, this result is quantitative. However, if I say that the color of the sample is red, this result is qualitative data. In a Benedict’s test, the Benedict’s reagent will react with the glucose to produce a range of colors from slightly yellow to brick-red. The greater the intensity of the color, the more glucose is present. Hence a brick-red color will suggest more glucose than a yellow color.   

Materials and Equipment

  1. Water
  2. White bread
  3. Whole wheat bread
  4. Alpha-amylase
  5. Beakers (600 ml and 400 ml)
  6. Test tubes (must hold at least 10 ml)
  7. pH paper or pH meter
  8. Lime juice or other acid (e.g. dilute HCl)
  9. Baking soda solution or other base (e.g. NaOH)   
  10. Stirring rod
  11. Hot plate
  12. Benedict’s reagent

Procedure

Pre-lab Preparation

  1. Purchase alpha-amylase enzyme. This will be used for starch conversion to glucose. You can purchase alpha-amylase from a chemical store or Amazon.
  2. Purchase a whole wheat bread and a white bread from the grocery
  3. Collect water in a large beaker or other container and adjust the pH to 6. The amount of water you prepare will depend on the number of students in the class. The pH is adjusted by adding either a dilute acid or base to the water. Verify the pH using a pH meter or pH strip. A cheap acid that you can use is lime or lemon juice. A cheap base that you can use is baking soda.

Lab Procedure (40 minutes)

  1. Collect a slice of whole wheat bread and a slice of white bread
  2. Crumble each to bread-crumb size
  3. Weight 20 grams of each sample and add it to a
    1. A 600 ml beaker labeled “whole wheat bread” and
    1. A 600 ml beaker labeled “white bread”
  4. Add 300 ml of warm water at 37 oC or 99oF
  5. Stir the mixture to make sure that all the bread is properly soaked and dissolved
  6. Add 1 gram of alpha-amylase enzyme and stir
  7. Let the beakers rest for 30 minutes
  8. During this time, bring some water to boil in a 400 ml beaker  
  9. At the end of 30 minutes, stir the contents in each beaker and then use a pipette to remove 1 ml of liquid from each
  10. Transfer the liquid to two labelled test tubes  
  11. Add 2 ml of Benedict’s reagent to each tube
  12. Collect another empty test tube and transfer 1 ml of water and 2 ml of Benedict’s solution to it. This is your negative control (what the result should look like if there is no glucose present)
  13. Collect another empty test tube and transfer 1 ml of 1% sugar solution and 2 ml of Benedict’s solution to it. This is your positive control (what the result should look like if there is glucose present)
  14. Boil all four test tubes for about 2 minutes and observe the color (Note: A brighter orange or brick-red color indicate the presence of more glucose)
Summary of process

Results

Complete the following table (4 points).

Sample Color
Negative control    
Positive control    
White bread    
Whole wheat bread    

Lab Questions

  1. Based on your results, which bread sample produced more glucose? (2 points)
  2. Does your result support your hypothesis? Explain.  (4 points)

Courtney Simons
Administrator
Courtney Simons is a food science professor. He holds a BS degree in food science and a PhD in cereal science from North Dakota State University.
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