The determination of the free amino nitrogen content in beer using photometric measurement of OPA (o-Phthaldialdehyde) and NAC (N-acetyl cysteine). The method is performed manually or automatically at a 340 nm wavelength at 37 °C temperature. In the automated method, the side wavelength of 700 or 750 nm is also measured. The method estimates amino acids by measuring the terminal α-amino nitrogen groups of peptides and proteins. The method is specific for α-amino nitrogen. Proline, ammonia or ammonium are not included.