Food Analysis Lab · W5

Protein Kjeldahl Nitrogen Method

Protein — The Kjeldahl Method

Food Analysis Lab · 4-hour session
Digestion · Distillation · Titration · Conversion factor · Melamine scandal · Recovery QC

Conc. H₂SO₄ digestion40% NaOH distillation4% boric acid traps NH₃Factors 5.7/6.25/6.38Melamine scandal

Pre-lab video (2.5 min) (Chinese narration)

Watch before class: how does a single nitrogen atom become the number “26% protein”?

Contents: ① Digestion (conc. H₂SO₄ + catalyst tablet, N → NH₄⁺) → ② Distillation (NaOH; NH₃ is collected in boric acid) → ③ Titration (1 HCl = 1 NH₃ = 1 N) → %N × 6.38 = 26.0%. Melamine takes the same route; pick the right factor. Open on YouTube →

Before you start, watch two factory videos

Does the Kjeldahl method measure nitrogen or protein?

Milk powder factory (16:54–18:08, the melamine scandal: how protein testing was fooled) Flour factory (15:11–16:14, NIR measures protein/moisture/ash together; 8–13% protein decides bread or cake)

Key question for today: The Kjeldahl method (like the Dumas combustion method we will mention later) only measures “total nitrogen”, then uses a conversion factor to turn nitrogen into “protein”. When does this assumption go wrong? The melamine scandal is the most famous case.

Work out one number and keep it in mind: Melamine is ≈ 66.6% N. Mixing in 1 g per 100 g of sample inflates the apparent protein (total N × factor, with non-protein nitrogen not subtracted; using the dairy factor 6.38) by about 4.25 percentage points. We will recalculate it together shortly.

interactive lab

A nitrogen’s journey: Predict → Observe → Explain

Six scenes: Predict → ① Digestion → ② Distillation → ③ Titration → Melamine → Conversion factor. Answer correctly to unlock the next scene. Open the lab in a new tab ↗

Today’s learning goals

After W5 you should be able to…

State the three stages of the Kjeldahl method (digestion, distillation, titration) and the purpose of each
Distinguish protein nitrogen from non-protein nitrogen (NPN) and explain why melamine can fool the Kjeldahl method
Choose the right conversion factor (general 6.25 / wheat and cereals 5.7 / dairy 6.38) and know how large the error is if you choose wrong

Use %N=(V_sample−V_blank)×N_HCl×14.007/W×100 to calculate a sample’s %N and protein %
Explain what the distillation recovery standard (ammonium sulfate) and the digestion efficiency standard (an amino acid) each check
Follow the safety rules for conc. sulfuric acid and NaOH, and complete the 4-hour division of work and station rotation

Theory review: the full principles of protein qualitative/quantitative methods are not repeated in this lesson. See the full content in 📖 Full-principles deck (Nielsen Ch18) →.

Principle: three stages

Digestion → Distillation → Titration

Key point: Boric acid itself does not need to be standardized. It only “traps” NH₃; the quantification is done by the standardized HCl used afterward.
Two roles of the catalyst tablet: K₂SO₄ raises the boiling point of sulfuric acid (so digestion can reach about 420°C); CuSO₄ is the catalyst that speeds up oxidative breakdown of organic matter. 📖 Full-principles deck (Nielsen Ch18) →

Apparatus diagram

Digestion block and steam distillation unit

During distillation, keep the receiving flask’s outlet below the boric acid surface so NH₃ does not escape. When distillation ends, lower the flask so the outlet leaves the liquid before turning off the steam, to prevent suck-back. Schematic only; the actual appearance varies by brand.

Protein = %N × conversion factor

The Kjeldahl method measures “total nitrogen”, not “protein”

Conversion factor	Applies to	Reason (brief)
6.25	General foods (default)	Assumes protein averages 16% nitrogen (1/0.16 = 6.25)
5.7	Wheat / cereals	Cereal gluten proteins have a higher nitrogen fraction, so a smaller factor is needed
6.38	Dairy	Milk proteins such as casein have a lower nitrogen fraction, so a larger factor is needed

The Kjeldahl method (like the Dumas combustion method) measures “total nitrogen”, which includes protein nitrogen and also non-protein nitrogen (NPN), such as melamine, urea, free amino acids and nucleic acids. The conversion factor is an approximation that “assumes all nitrogen comes from protein”. Once a sample contains a nitrogen-rich non-protein substance, the calculated “protein %” is inflated.
Dumas combustion method: the sample is fully combusted at high temperature and the nitrogen gas is measured directly. It is faster than Kjeldahl (minutes vs hours), but it still measures total nitrogen and is still fooled by NPN. We do not run it in this lab; it is included for comparison only.
Note: the 5.7 factor for wheat/cereals is the long-standing traditional factor from Jones/Codex.

Mini-game ①Does this nitrogen count as “protein nitrogen”?0 / 8

Sort 8 nitrogen sources into “Protein nitrogen” or “Non-protein nitrogen (NPN)”.

Reset

The melamine scandal: why was the Kjeldahl method fooled?

Melamine (C₃H₆N₆): MW = 126.12, nitrogen content as high as 66.63%

In normal milk, almost all nitrogen comes from milk proteins such as casein (protein nitrogen)
Unscrupulous producers deliberately added melamine to milk powder that had been diluted with water. It is not protein, but its nitrogen content is very high
The Kjeldahl method only sees “total nitrogen” and cannot tell whether a nitrogen came from protein or from melamine

Let’s calculate: for every 1 g melamine / 100 g sample (i.e., 1% w/w):
Contribution to %N = 1% × 66.63% ≈ 0.6663 percentage points
Using the dairy factor 6.38, the inflated apparent protein ≈ 0.6663 × 6.38 ≈ 4.25 percentage points

In other words, adding only about 1% melamine raises the Kjeldahl “protein %” by ~4.3 percentage points. That is enough to disguise watered-down milk powder with too little protein as “passing”.

Incident reconstruction · illustrative data

Adulteration curve: melamine fraction vs apparent protein

Illustrative data: based on whole milk powder (true protein about 26.0%). The x-axis is the melamine fraction (% w/w, evenly spaced) and the y-axis is the apparent protein % measured by Kjeldahl. The curve rises as a straight line because the Kjeldahl method cannot tell where the nitrogen comes from.

Calibration and quality control (QC)

4 things to confirm before and during the run

⚗️

HCl standardization

The HCl used for titration must first be standardized against a primary standard such as Na₂CO₃ or THAM (Tris) to get its exact concentration before it can be used to calculate %N

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Reagent blank

An empty tube goes through the whole digestion + distillation + titration procedure (no sample) to subtract the trace nitrogen introduced by the reagents themselves

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Distillation recovery standard

Run ammonium sulfate of known nitrogen content (theoretical %N = 21.20%) through distillation + titration to check whether NH₃ is fully distilled out and trapped

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Digestion efficiency standard

Run an organic nitrogen standard such as lysine hydrochloride or tryptophan through the whole procedure to check whether digestion fully converts the organic nitrogen

Calibration and quality control (QC)

Let’s calculate: recovery and end-point reading

Let’s calculate (distillation recovery): Weigh 0.0500 g ammonium sulfate (theoretical %N = 21.20%); distillation + titration gives %N = 21.01%, so recovery = 21.01/21.20×100 ≈ 99.1% (AOAC 991.20: ammonium sulfate ≥99%, lysine hydrochloride ≥98%).
Let’s calculate (digestion efficiency): Lysine hydrochloride has a theoretical %N of 15.34%; the full procedure gives %N = 15.10%, so recovery ≈ 98.4%. This semester the TA demonstrates one tube of these two standards so the whole class understands the principle; they are not in the 8-tube group schedule below.

Consistent indicator end point: The whole class uses the same batch of 4% boric acid + mixed indicator and must read the same end point: after absorbing NH₃ the receiving solution is green; titrate with HCl until the green fades to grayish purple / pale pink, which is the end point. This avoids differences in interpretation between groups.

Samples and station rotation

One Kjeldahl setup (digestion block + distillation unit) holds only 8 tubes

Blind sample: the semester-wide whole milk powder, tested for moisture in W2 and ash in W3 and now for protein. Each of the 5 groups digests 1 tube of the blind sample, giving the class n=5 (one value per group), pooled into mean ± SD
The other 3 tubes are for QC and comparison: 1 reagent blank, 1 distillation recovery standard (ammonium sulfate), and 1 bread flour. One group runs the bread flour tube and the whole class shares the result
The result for cake flour (low-gluten) is provided by the TA (data from a previous run) for comparison with bread flour. The digestion block schedule is already full at 8 tubes, so it cannot be added on the spot
The digestion efficiency standard (lysine hydrochloride) is still demonstrated by the TA as one tube and does not take up class schedule space
Total: 5 (blind sample) + 1 (blank) + 1 (recovery standard) + 1 (bread flour) = 8 tubes

⚠️ TA, please confirm before class: the digestion block and distillation unit can truly hold 8 tubes at once. This schedule uses the full 8-tube capacity with no spare room; if the equipment holds fewer than 8 tubes, adjust the groups or digest in batches beforehand.

Only one setup: all tubes are digested together, then distilled + titrated in turn, about 8–10 minutes per tube.

4-hour lab schedule

Digest together, then rotate through distillation and titration

Time	Contents
0:00–0:20	Briefing + safety: hazards of conc. sulfuric acid and NaOH, and emergency response
0:20–0:50	Each group weighs its sample and adds the catalyst tablet and conc. sulfuric acid (in a fume hood)
0:50–2:20	Digestion (all 8 tubes heated together until clear). Use the wait for the conversion factor discussion, the melamine case, and calculation practice
2:20–2:35	Cool to room temperature, then dilute by adding a small amount of water slowly down the tube wall
2:35–4:00	Distillation + titration rotation: 8 tubes × about 8–10 min each ≈ 65–80 min, so the 85-minute slot is enough. Each group records the titration volume for its own sample

If time runs short: Digested tubes can be capped and stored, and the TA can finish the distillation and titration after class or next week; this does not affect the chemical reactions already completed in digestion.

Safety rules

Conc. sulfuric acid · NaOH · Waste

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Hot conc. sulfuric acid + white fumes

Digestion produces irritating SO₂/SO₃ gases. Work in the fume hood/scrubber throughout; wear a face shield and acid-resistant gloves

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Dilute by adding water slowly down the tube wall

Only after cooling to room temperature, add water slowly and in small amounts down the tube wall. Never add a lot of water quickly, or the violent heat release will splatter and burn

⚠️

40% NaOH is strongly corrosive

The NaOH added for distillation is highly concentrated. If it gets on skin or eyes, rinse immediately with plenty of water and tell the TA

♻️

Collect copper catalyst waste separately

Copper-containing waste after digestion is heavy-metal waste and must be collected separately as required; do not mix it with general waste

Mini-game ②Order the Kjeldahl procedure— / 7

Use ▲▼ to arrange the 7 steps of the Kjeldahl method in the correct order.

Mini-game ③Match each food to its conversion factor0 / 6

Sort 6 foods into the “conversion factor” each should use.

Calculation: from titration volume to % protein

Subtract the blank first, then multiply by the conversion factor

%N = (V_sample − V_blank) × N_HCl × 14.007W_sample(g) × 1000 × 100 Protein % = %N × conversion factor

Each 1 mmol HCl ↔ 1 mmol NH₃ ↔ 1 mmol N = 14.007 mg N; ÷(W×1000) converts mg to g, and ×100 gives %.

Whole milk powder (blind sample) example: 0.5000 g weighed, blank 0.30 mL, titrant used 14.86 mL, 0.1000 N HCl
%N = (14.86−0.30)×0.1000×14.007/(0.5000×1000)×100 ≈ 4.079%
Protein % = 4.079 × 6.38 ≈ 26.02% (dairy factor)

Bread flour example (the tube shared by the whole class): 1.0000 g weighed, blank 0.30 mL, titrant used 16.58 mL, 0.1000 N HCl
%N ≈ 2.280%; protein % (factor 5.7) = 2.280×5.7 ≈ 13.00%

⚠️ The cost of the wrong factor: For the same bread flour %N, using the general factor 6.25 instead of the wheat factor 5.7 changes the protein % from 13.00% to 14.25%, which is about 9.6% too high (relative error). This is a systematic error. Like melamine, it comes from the assumption that “nitrogen equals protein”, only for a different reason (wrong factor vs adulterated sample).
Cake flour (low-gluten) has %N ≈ 1.404% and protein % (factor 5.7) ≈ 8.00%. This time it is provided by the TA (data from a previous run); our class did not run this tube. It is lower than bread flour, which is why “bread flour makes bread and cake flour makes cake” in baking.

Mini-game ④Calculation challenge: protein % of whole milk powderUnanswered

0.5020 g of whole milk powder is weighed, the reagent blank is 0.32 mL, and the titration uses 15.02 mL of 0.1000 N HCl. Find the protein % (dairy conversion factor 6.38). Formula: %N=(V_sample−V_blank)×N×14.007/(W×1000)×100, protein %=%N×6.38.

Mini-game ⑤Kjeldahl principles: live quiz0 / 5

Mini-game ⑥Decision challenge: QC and interpretation0 / 5

Homework

Colab NB05

📥

Load titration records

Load your group’s and the class’s raw titration volumes and sample weights

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Calculate %N and protein %

For both flours and the blind sample: calculate %N and protein %, and pool the class blind-sample results into mean ± SD (n=5)

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Recovery and factor discussion

Discuss the meaning of distillation recovery and the digestion efficiency standard, and the effect of choosing the wrong conversion factor or encountering NPN

Colab NB05: Open NB05 in Colab ↗ · Beginner (no coding): Excel practice workbook EX05 ⬇.
Your report should include: %N and protein % for both flours and the blind sample (blind sample as the class mean ± SD), distillation recovery, and a discussion of the “conversion factor” and “non-protein nitrogen”.
Keep your blind-sample result: this week’s whole milk powder protein % will be reviewed in W8 together with moisture (W2) and ash (W3).

Self-check

By the end of today you should be able to…

✓State the three stages of the Kjeldahl method (digestion, distillation, titration) and the purpose of each
✓Explain why the Kjeldahl method measures “total nitrogen” rather than “protein”, and distinguish protein nitrogen from non-protein nitrogen (NPN)
✓Recalculate the melamine scandal: how much apparent protein a given mix-in fraction adds
✓Choose the right conversion factor (general 6.25 / wheat and cereals 5.7 / dairy 6.38) and state the consequences of a wrong choice
✓Use the %N formula to calculate a sample’s %N and protein % from the titration volume
✓Explain what the distillation recovery standard (ammonium sulfate) and the digestion efficiency standard (an amino acid) each check
✓Complete one group run of the Kjeldahl method (digestion + distillation + titration)
✓Follow the safety rules for conc. sulfuric acid and NaOH

Tap to tick — make sure you really can do it.