The digestion phase is the most time-consuming step of the Kjeldahl analysis. Speeding up the process ensuring a proper outcome is of fundamental importance when it comes to the accuracy of the results and efficiency in the lab.
In the digestion phase of the Kjeldahl analysis, the goal is to break down the bonds that hold the polypeptides together and convert them into simpler molecules (such as water, carbon dioxide and ammonium sulfate). These reactions can be speeded up by the temperature used during digestion and by the presence of acid, salt and catalysts (selenium, copper, mercury, titanium).
After having weighed the sample, the latter must be placed into a glass test tube along with concentrated sulfuric acid and the correct variety of catalyst tablets according to the Official Methods requirements. As a matter of fact, catalyst tablets containing copper are satisfactory but very slow to react, while selenium catalysts usually react very fast and are typically used for substances that are resistant to digestion (e.g. fats and oils).
For low nitrogen content samples, such as water and beverages, the analysis requires a high sample volume. As a consequence, foam formation and violent bumping may occur during the digestion, causing acid splash on the test tube wall. This can lead to sample loss and, ultimately, to inaccurate results that underestimate the nitrogen content.