SOP: Protein Analysis by Kjeldahl Method

The Kjeldahl method has measured protein for over a century and remains the reference against which faster instruments are calibrated. It converts organic nitrogen to ammonia, and from nitrogen you calculate protein. Every step here follows the classical workflow still used in food laboratories worldwide.

Chemical analysis converts invisible composition into numbers that specifications, labels, and safety decisions depend on. Consistent technique matters as much as good instruments: small variations in timing, temperature, or reagent preparation shift results enough to fail a batch that should pass, or pass one that should fail.

Purpose

To determine total nitrogen in food and calculate crude protein content for nutrition labeling, specification compliance, and formulation control.

Scope

Applies to meat, dairy, cereals, legumes, animal feeds, and most solid or liquid foods. Not suitable for products containing non-protein nitrogen that must be excluded (such as urea-adulterated samples) without correction.

Equipment and Materials

• Kjeldahl digestion unit with fume extraction
• Distillation unit (manual or automated)
• Kjeldahl flasks or digestion tubes, 500 mL
• Concentrated sulfuric acid, reagent grade
• Catalyst tablets (potassium sulfate with copper sulfate, or selenium-based)
• Sodium hydroxide solution, 40 percent w/v
• Boric acid receiving solution, 4 percent with mixed indicator
• Standardized hydrochloric acid, 0.1 N
• Analytical balance

Procedure

1. Weigh 0.5 to 2.0 g of homogenized sample (containing roughly 10 to 200 mg nitrogen) into a Kjeldahl flask. Record the exact weight.
2. Add two catalyst tablets and 20 to 25 mL of concentrated sulfuric acid. Swirl gently to wet the sample.
3. Digest on the heating block, starting low and raising to 420 C. Continue until the digest is clear green-blue and fumes subside, typically 60 to 90 minutes after clearing.
4. Cool the flask, then cautiously add 50 mL of deionized water down the flask wall to dissolve the cake.
5. Transfer to the distillation unit. Add 50 mL of 40 percent sodium hydroxide through the addition funnel.
6. Distill into 25 mL of 4 percent boric acid receiving solution. Collect at least 150 mL of distillate over 5 to 7 minutes. The receiver turns green as ammonia is trapped.
7. Titrate the distillate with standardized 0.1 N hydrochloric acid to the grey-pink endpoint. Record the volume.
8. Run a reagent blank with every batch and subtract its titration volume.
9. Run a recovery check (ammonium sulfate or glycine) with each batch; recovery must be 98 to 102 percent.

Recording Results

Percent nitrogen equals (sample titration minus blank) times normality of acid times 1.4007, divided by sample weight in grams. Percent protein equals percent nitrogen times the appropriate factor (6.25 general, 6.38 dairy, 5.70 wheat). Report protein to two decimal places.

Quality Control

Run a reagent blank with every batch and subtract it from sample results. Analyze every sample in duplicate; the relative percent difference between duplicates must not exceed 10 percent for concentrations well above the detection limit. Include a certified reference material or a validated in-house control sample with each batch and plot the result on a Shewhart control chart. Investigate any point outside two standard deviations and reject the batch if a point falls outside three standard deviations. Verify the calibration of volumetric glassware annually against Class A tolerances, and check the analytical balance daily with certified weights. Standardize titrants monthly and label with the expiry date. Document all quality control results in the batch record; sample results must not be reported when associated QC fails.

Troubleshooting

• Low recovery on check standard: incomplete digestion or ammonia loss. Extend digestion time and check distillation seals.
• Dark digest that never clears: high-fat sample. Increase acid volume and digestion time.
• Bumping during distillation: add boiling chips or anti-foam, and control the heating rate.
• Faint endpoint: the mixed indicator may be old. Prepare fresh boric acid indicator solution weekly.

Safety Notes

• Concentrated sulfuric acid and 40 percent sodium hydroxide are corrosive. Wear face shield, acid-resistant gloves, and lab coat. Work in the fume hood.
• Digestion produces sulfur trioxide fumes; ensure extraction is running before heating.
• Add water to cooled digests slowly; the dilution is exothermic.
• Neutralize waste digest before disposal according to local regulations.