Crude Protein Formula: Protein (%) = Nitrogen (%) × Conversion Factor
Kjeldahl Method: Measures total nitrogen, assumes 16% N in protein (factor = 6.25)
Standard factor = 6.25 (assumes 16% N in protein)
Crude protein is an estimate of the total protein content in food, feed, or biological samples based on total nitrogen analysis. The term "crude" indicates that the method measures all nitrogen-containing compounds, not just true proteins, including amino acids, nucleic acids, and other nitrogenous compounds.
The Kjeldahl method, developed in 1883, remains the official method for protein analysis in food and feed industries worldwide. It's based on the assumption that proteins contain approximately 16% nitrogen, leading to the standard conversion factor of 6.25 (100/16 = 6.25).
Sample is heated with concentrated H₂SO₄ and catalyst (CuSO₄ or Se) at 300-400°C:
Organic-N + H₂SO₄ → (NH₄)₂SO₄ + CO₂ + H₂O
All organic nitrogen is converted to ammonium sulfate
Add excess NaOH to make solution alkaline and distill released ammonia:
(NH₄)₂SO₄ + 2NaOH → 2NH₃ + Na₂SO₄ + 2H₂O
Ammonia is captured in boric acid or standard acid solution
Titrate with standard acid (if using boric acid) or back-titrate with standard base:
NH₃ + H⁺ → NH₄⁺
Calculate nitrogen content from titration volume
Different foods have different nitrogen contents in their proteins. Using specific conversion factors improves accuracy:
| Food Type | Factor | %N in Protein | Reason |
|---|---|---|---|
| General Foods | 6.25 | 16.0% | Average protein composition |
| Wheat/Flour | 5.70 | 17.5% | High glutenin and gliadin content |
| Milk/Dairy | 6.38 | 15.7% | Casein and whey proteins |
| Rice | 5.95 | 16.8% | Oryzenin composition |
| Soy Products | 5.71 | 17.5% | Glycinin and conglycinin |
| Meat/Fish | 6.25 | 16.0% | Muscle proteins (actin, myosin) |
| Gelatin | 5.55 | 18.0% | High glycine content (collagen) |
Note: The factor 6.25 (100/16) assumes 16% nitrogen in protein. Different proteins have different amino acid compositions, leading to different nitrogen contents. Using specific factors improves accuracy for protein quantification.
Problem: A wheat flour sample is analyzed by the Kjeldahl method and found to contain 2.28% nitrogen. Calculate the crude protein content using the appropriate conversion factor.
Step 1: Identify the appropriate factor
For wheat and flour products, the conversion factor is 5.70 (not the general 6.25)
Step 2: Apply the formula
Crude Protein (%) = Nitrogen (%) × Conversion Factor
Crude Protein (%) = 2.28 × 5.70
Crude Protein = 13.0%
This is typical for all-purpose wheat flour
Comparison:
• Using factor 5.70 (correct): 2.28 × 5.70 = 13.0%
• Using factor 6.25 (general): 2.28 × 6.25 = 14.3%
• Difference: 1.3% (10% relative error!)
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