Acid digestion is a central step in the Kjeldahl method, where a food sample is digested in concentrated sulphuric acid. The nitrogen from proteins in the food sample is turned into ammonium ions, whereas the carbon and oxygen end up as the gas carbon dioxide and water. The acid digestion frees up the nitrogen from the other elements in the food sample, making it possible to detect it quantitatively in the later steps of the Kjeldahl method.