Question

How can you test the presence of proteins in a given food item?

Answer 1

Hint: Almost all reactions involved in testing the presence of proteins in a given food item are chromogenic assays (i.e. colored products are formed).

Complete answer:
The presence of proteins in given foodstuff can be tested as follows:
Method 1: To test protein in the food we need a solution of copper sulfate and caustic soda.
- First we grinded the food sample finely (for example almonds).
- Then, we put it in a clean test tube followed by the addition of 10 drops of water.
- Added 2 drops of copper sulfate solution and shook it well.
- Added 10 drops of caustic soda solution and shook it well.
- The Violet color indicates the presence of proteins.

Fig: Copper sulphate and caustic soda test
Method 2: Biuret test –
- First we added the given sample food.
- We then added aqueous copper sulfate.
- The violet coloration confirms the presence of protein.

Fig: Biuret test
Method 3: Xanthoproteic test –
- First we added the given sample food.
- We then added the nitric acid.
- The yellow color solution confirms the presence of proteins.
Method 4: Millions test –
- First we added the given sample food.
- We then added the mercuric sulfate in the presence of sodium nitrite and sulfuric acid.
- The brick red color solution confirms the presence of proteins.
Method 5: Ninhydrin test –
- First we added the given sample food.
- We then added the pyridine solution of ninhydrin
- A Violet color solution confirms the presence of proteins.

Note: Biuret reagent solution can be prepared from copper sulfate, sodium/potassium hydroxide, and sodium tartrate. The above mentioned tests can be used for the different food materials and ease of the process.