LimsPulse
Food & proximate analysis

Kjeldahl digestion & distillation unit

Digests a food sample in hot sulphuric acid to turn its organic nitrogen into ammonium, then distils the ammonia and titrates it. The nitrogen found, multiplied by a conversion factor, gives crude protein.

  • Food & beverage
  • Dairy
  • Animal feed
  • Agriculture
  • Environmental
  • Fertiliser
Photo of a Kjeldahl digestion & distillation unit
Photo: Giancarlo Dessì, CC BY-SA 3.0 (cropped), via Wikimedia Commons
TROUBLESHOOTING

Common problems, causes and fixes

Low nitrogen recovery
Likely causes
  • • Incomplete digestion (too short or too cool)
  • • Nitrogen loss from overheating or too much salt
  • • Ammonia lost through a leak or a dry receiver tip
  • • Not enough alkali added
What to do
  • • Extend digestion and check block temperature
  • • Keep to the acid-to-salt ratio in the method
  • • Check seals and dip the condenser outlet into the boric acid
  • • Check the alkali pump volume; the digest must turn strongly alkaline
High results or high blank
Likely causes
  • • Contaminated reagents or catalyst
  • • Ammonia in the lab air or distilled water
  • • Carry-over from a previous high sample
What to do
  • • Use fresh nitrogen-free reagents and record lot numbers
  • • Use fresh boric acid and keep it covered
  • • Run steam-only rinse distillations between samples
Digest froths over the tube
Likely causes
  • • High fat, sugar or protein content
  • • Heating too fast
What to do
  • • Use a lower start temperature and a ramp programme
  • • Add an antifoam agent or reduce sample mass
Digest stays dark or charred
Likely causes
  • • Not enough acid for the sample mass
  • • Digestion time too short
What to do
  • • Reduce sample mass or add acid as per method
  • • Continue digestion until clear plus the extra time
Digest sets solid on cooling
Likely causes
  • • Too much acid evaporated
  • • Cooled too long before dilution
What to do
  • • Check exhaust suction is not too strong
  • • Add water while still slightly warm (not hot), as the manual describes
Unstable or late titration end point
Likely causes
  • • Old indicator or boric acid
  • • Dirty or slow pH electrode
  • • Titrant concentration wrong
What to do
  • • Prepare fresh receiver solution
  • • Clean and recalibrate the electrode
  • • Restandardise the titrant
In LimsPulse · Calibration Management

Keep every Kjeldahl in control: usage logs, calibration and maintenance schedules, and automatic blocking of overdue instruments.

See how