Short Note #35: Nitrogen & Protein Determination in Soy Products Using Kjeldahl

Buchi Kjeldahl, Distillation, Titration and Digestion Solutions

Buchi Kjeldahl Solution Products

The determination of protein in food is a routine procedure for quality assurance and labeling. A simple and fast procedure for protein determination in soy products is introduced in this study as described in the AOAC 945.39.

The sample is digested with sulfuric acid using the Buchi SpeedDigester K-436 or K-439, followed by distillation and titration with the KjelFlex K-360 products. Visit Buchi to request this study and for information on the various Buchi Kjeldahl products.

Protein determination is one of the key analyses performed in the food industry. The samples require digestion with sulfuric acid to convert nitrogen into ammonium sulfate.

After conversion to ammonia through the alkalinization with sodium hydroxide, the sample is distilled into a boric acid receiver by steam distillation, followed by a titration with sulfuric acid solution. The nitrogen content is multiplied by a sample-specific factor (6.25 according to the EU-directive on labeling for foodstuffs) to obtain the protein content.

The determination of protein contents in soy products according to Kjeldahl using SpeedDigester K-436, K-439, and KjelFlex K-360 provides reliable and reproducible results that correspond to the expected values with low relative standard deviations. The total digestion time is approx. 85 min (K-439) or 100 min (K-436).

AOAC 945.39