Protein analysis is a fundamental parameter in food quality control and nutritional evaluation. The Kieldahl method remains one of the most reliable and widely accepted techniques for determining total nitrogen content in food samples.
Procedure Overview:
1 Digestion
The sample is digested with concentrated H2S04 in the presence of copper sulphate (catalyst) and sodium sulphate (to raise boiling point) Organic nitrogen is converted into ammonium sulphate, indicated by a clear green solution.
2 Distillatior
After dilution, strong NaOH is added to liberate ammonia. The released ammonia is distilled and trapped in standard 0.1 N H2S04 containing methyl red indicator.
3 Titration
The trapped ammonia is quantified by titating against 0.1 N NaOH.
A blank is performed for accurate calculation.
%Nitrogen= [(B-S)xNx14>100] : [Wx1000]
Where: B= Blank reading (mL) S=Sample reading (mL) N = Normality of NaOH W=Weight of sample (g) 14. = Atomic weight of Nitrogen
% Protein= %Nitrogenx6.25