Objectives

To determine if there is any difference in protein content between whole milk, 0%, 1%, and 2% milk.

Introduction

Milk is sold based on fat content. There is whole milk with most of the milk fat retained, 0% with no fat, and then 1% and 2% milk fat. In this lab, we will investigate if there is any difference in protein content between them. The test that we will be using is called the biuret test. It is a qualitative test that enables us to compare protein quantity based on color. A blue color indicates that no protein is present and purple means that protein is present. However, the outcome may be one of a range of purple shades. The darker the shade, the greater the protein content.  

The purple color in the test occurs due to the binding of copper in the reagent with peptides which changes light absorption and perceived color.  

Biuret reaction. Image source. Wiki commons.

Procedure

  1. Collect 5 clean, and dry test tubes
  2. Label your tubes with the sample name including a “control” with distilled water
  3. Add 1 ml of each test sample and control to the test tubes
  4. Add 2 ml of Biuret reagent to each tube
  5. Shake well and allow to stand for about 10 minutes
  6. Observe and record the color change

Lab Assessment (10 points)

  1. Complete the table below (5 points)
  2. What conclusion(s) can you draw form your results? (5 points)
Food SampleColor
Distilled water (control) 
0% milk 
1% milk 
2% milk 
Whole milk 
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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.