Kjeldahl Method
Definition
The reference method for protein determination in food: sulphuric acid digestion converts all nitrogen to ammonium sulfate, which is titrated after distillation. The method measures total nitrogen, not protein specifically, making it susceptible to inflation by any non-protein nitrogen source.
- Reference use
- Standard method for protein determination in food
- Process
- Sulphuric acid digestion, then distillation and titration
- Actually measures
- Total nitrogen, not protein directly
- Known limitation
- Inflated by non-protein nitrogen, e.g. melamine
Common questions
How did the Kjeldahl method's limitation enable the 2008 melamine milk scandal?+
Melamine is nitrogen-rich but is not a protein, so adding it to diluted milk artificially raised the nitrogen reading and made the product appear to meet protein content standards on a Kjeldahl or Dumas test, concealing the dilution until melamine-specific testing was introduced.
What test is used alongside or instead of Kjeldahl to catch this kind of adulteration?+
Targeted chromatographic methods such as HPLC or LC-MS are used to detect melamine and other non-protein nitrogen adulterants directly, since they measure the specific compound rather than total nitrogen and so cannot be fooled the same way.
Related terms
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- Codex Alimentarius
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- LC-MS/MS MRM
- Liquid chromatography with triple-quadrupole tandem mass spectrometry operated in multiple reaction monitoring mode. The most sensitive confirmatory method for melamine (primary MRM...
- Melamine
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- Melamine-Cyanuric Acid Co-Crystal
- A 1:1 insoluble complex formed when melamine and cyanuric acid co-occur in acidic renal urine. The co-crystal precipitates inside renal tubules, causing...