Which method is considered the reference method for total protein measurement by nitrogen content?

Which method is considered the reference method for total protein measurement by nitrogen content?

• A. Kjeldahl
• B. Biuret
• C. Lowry
• D. Bradford

Answer: (A)

The reference method for determining total protein by nitrogen content is the Kjeldahl method. It directly quantifies total nitrogen in a sample by digesting the material in sulfuric acid with a catalyst, converting organic nitrogen to ammonium sulfate, then neutralizing and distilling the ammonia into a receiver and titrating to measure how much nitrogen is present. From that nitrogen amount, protein content is estimated using a conversion factor (commonly around 6.25, based on the typical nitrogen percentage in proteins). This approach measures all nitrogen in the sample, making it independent of protein type or amino acid composition, which is why it’s regarded as the standard for protein determination by nitrogen content. Other methods like Biuret, Lowry, and Bradford rely on colorimetric reactions with specific reagents and amino acid residues, and they can be influenced by sample composition and interfering substances, so they aren’t used as references for total protein by nitrogen.

The reference method for determining total protein by nitrogen content is the Kjeldahl method. It directly quantifies total nitrogen in a sample by digesting the material in sulfuric acid with a catalyst, converting organic nitrogen to ammonium sulfate, then neutralizing and distilling the ammonia into a receiver and titrating to measure how much nitrogen is present. From that nitrogen amount, protein content is estimated using a conversion factor (commonly around 6.25, based on the typical nitrogen percentage in proteins). This approach measures all nitrogen in the sample, making it independent of protein type or amino acid composition, which is why it’s regarded as the standard for protein determination by nitrogen content. Other methods like Biuret, Lowry, and Bradford rely on colorimetric reactions with specific reagents and amino acid residues, and they can be influenced by sample composition and interfering substances, so they aren’t used as references for total protein by nitrogen.