What is the chemical score in protein quality assessment?

• A. Proportion of total protein that is essential amino acids
• B. Ratio of most limiting essential amino acid in a test protein to the amount of that amino acid in egg protein
• C. Digestibility factor of the protein
• D. Calories per gram of protein

Answer: (B)

Chemical score measures protein quality by comparing the amount of the most limiting essential amino acid in a test protein to the amount of that amino acid in a reference protein, typically egg protein. The idea is that the body's ability to use a protein for growth is constrained by the essential amino acid present in the smallest proportion relative to needs. So you take the limiting essential amino acid in the test protein, divide its amount by the amount of that amino acid in egg protein, and that ratio is the chemical score. This is why the best description is the ratio of the most limiting essential amino acid in the test protein to the amount of that amino acid in egg protein. Other options don’t fit because they describe overall essential amino acid proportion, digestibility, or energy content, not the limiting amino acid comparison.