Short Note #46: Nitrogen & Protein Determination in Juice and Lassi

Buchi IR and Block Digestion Products

Buchi Digestion 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 juice and lassi according to the Kjeldahl method, as described in the Swiss Food Manual 22/4.1 – 4.2
and other regulations, is introduced in this study. Please contact Buchi to request this study and review our various Kjeldahl products.

The samples, for this study we used fruit juice, beetroot juice and India lassi, require digestion with sulfuric acid the SpeedDigester K-436 or K-439 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 with the KjelFlex K-360.

The nitrogen content is multiplied by a factor (general factor for food: 6.25) to obtain the protein content.

Buchi Distillation & Titration Products

Buchi Distillation & Titration Products

5 – 8 g of the samples were added directly into a sample tube. A portion of 20 ml of sulfuric acid and 2 Kjeldahl tablets were added.

After digestion the ammonia of the sample was distilled into a boric acid solution by steam distillation and titrated with sulfuric acid. The method was verified by using 0.18 g tryptophan as the reference substance.

The determination of protein contents in juice and lassi according to Kjeldahl using SpeedDigester K-436, K-439 and KjelFlex K-360 provides reliable and reproducible results with low relative standard deviations.