The Kjeldahl method is a reference laboratory procedure for determining protein content by acid digestion, ammonia distillation and titration, with the measured nitrogen converted to protein using a product-specific conversion factor.
In trade practice
Accredited laboratories apply the Kjeldahl method to grains, pulses, nuts and processed ingredients when a specification or nutrition declaration requires a verified protein figure. The conversion factor used, commonly 6.25 for most plant proteins but different for wheat and dairy, must match the factor specified in the applicable standard or agreed in the contract, as an incorrect factor will shift the declared protein value. Near-infrared analysis is often used for rapid screening, but Kjeldahl remains the reference method for dispute resolution and regulatory compliance.
In a specification
A specification should state that protein is determined by the Kjeldahl method, name the conversion factor to be applied and reference the applicable ISO or AOAC standard procedure.