How to Test for Protein in Food

Protein is an essential nutrient that builds muscle in the body. It's also easy to test for. Here's how:

Protein Test Materials

• Calcium oxide (sold as quicklime in building supply stores)
• Red litmus paper (or another method to test pH) 
• Water
• Candle, burner, or another heat source
• Eye-dropper
• Test tube
• Milk or other foods to test

Procedure

Because milk contains casein and other proteins, it's a good food to start your testing with. Once you understand what to expect from testing milk, you can examine other foods.

1. Add a small amount of calcium oxide and five drops of milk to a test tube.
2. Add three drops of water.
3. Dampen the litmus paper with water. Water has a neutral pH, so it should not change the color of the paper. If the paper does change color, start again using distilled water rather than tap water.
4. Carefully heat the test tube over a flame. Hold the damp litmus paper over the mouth of the test tube and observe any color change.
5. If protein is present in a food, the litmus paper will change color from red to blue. Also, smell the test tube: If protein is present, you should be able to detect the odor of ammonia. Both of these indicate a positive test for protein. If protein is not present in the test sample (or is in insufficient concentration to produce adequate ammonia during testing), the litmus paper will not turn blue, resulting in a negative test for protein.

Notes About the Protein Test

• Calcium oxide reacts with protein to break it down into ammonia. The ammonia changes the acidity of the sample, causing a pH change. If your food is already very alkaline, you won't be able to use this test to detect protein. Test the pH of food to see if it changes the litmus paper prior to performing the protein test.
• Milk is an easy food to test because it's a liquid. To test solids, such as meat, cheese, or vegetables, you must first grind the food by hand or by using a blender. You may need to mix the food with some water to make a sample you can test.
• The test registers a change in pH, which is the concentration of hydrogen ions in an aqueous or water-based solution. Most foods contain water, so they work fine for the test. However, oily foods may not work as well. You can't test pure vegetable oil, for example, because it doesn't contain any water. If you test greasy foods, such as french fries or potato chips, you'll need to mash them up and mix them with a bit of water first.