How To Test for Protein in Food

Easy Protein Test Using Calcium Oxide

Protein in food
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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
  • dropper
  • test tube
  • milk or other foods to test


First, test for milk, which contains casein and other proteins. Once you understand what to expect from the test, you can examine other foods.

  1. Add a small amount of calcium oxide and 5 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 in 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 (positive test for protein), the litmus paper will change color from red to blue. Also, if you smell the test tube, you should be able to detect the odor of ammonia if protein was present. If protein is not present in food or is in insufficient concentration to produce enough ammonia for the test (negative test for protein), the litmus paper will not turn blue.

Notes About the Protein Test

  • The 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 your food before performing the test to see if it changes the litmus paper.
  • Milk is an easy food to test because it is a liquid. To test solids, like meat or vegetables, first, grind the food by hand or using a blender. You may need to mix it 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, like french fries or potato chips, you'll need to mash them up and mix them with a bit of water.