Objective

To determine if there is any difference in lactose content between whole milk, 1%, 2%, and skimmed milk.

Background

Milk products are marketed based on the amount of fat present i.e.,whole milk, 1%, 2% milk, and skimmed milk. However, people who drink milk give very little attention to the lactose content in milk. Lactose is a reducing sugar. This is a chemistry term that means that it can act as a reducing agent by donating electrons to other molecules. This functionality is important in food processing because reducing sugars like lactose can react with proteins, especially at high temperatures to produce dark colors in a process called Maillard reaction. The more lactose present, the more dark compounds are produced during processing. The brown color you see on the crust of bread is one example of Maillard reaction.

In this lab, the amount of lactose present in milk will be determined using a qualitative method. The method used, will give us a good idea of the relative amount of lactose present. It will not tell use the total lactose in exact amounts. The name of the qualitative method we will be using is the Benedict’s test.

Benedict’s Test: Since lactose is a reducing sugar, it is able to reduce copper (II) ions (a blue color) in Benedicts reagent, to copper (1) (a brick-red color). The range of possible colors depends on the amount of reducing sugar present. A brick red color indicates the highest concentration of reducing sugar.

Reducing sugar from lowest (left) to highest (right) concentration. Image Source: Wikimedia commons

 Procedure

  1. Set a hot water bath to 80oC
  2. Label 5 clean test tubes – WM, 1%, 2%, 0%, and W (for water as your control)
  3. Add 1 ml of each sample to the test tubes
  4. Add 2 ml of Benedict’s reagent to each tube and mix by gentle shaking
  5. Fill a 400 ml beaker to the 300 ml mark with hot water taken from the hot water bath
  6. Add your test tubes to the beaker and set it in the water bath and leave for 15 minutes
  7. Remove the tubes and place them on the test tube rack
  8. Observe and record the final color

Hot water bath

Lab Assessment (12 points)

1) Which one of the following would be the most suitable hypothesis for this experiment? (2 points)

a) Distilled water is lactose free

b) Whole milk is more nutritious than 1% and 2% milk

c) Lactose content vary depending on milk type

d) Lactose intolerant people should not consume any form of dairy milk

2) Complete the table below (5 points)

Food SampleColor Description
Distilled water (control) 
0% milk 
1% milk 
2% milk 
Whole milk 

3) What is the purpose of a control in an experiment? (2 points)

4) Was there any difference in lactose content in the milk samples? If yes, list the milk types in order of decreasing lactose content (3 points)

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