Validation and comparison of analytical methods used to evaluate the nitrogen status of grape juice

Three methods used to evaluate nitrogen grape juice status were studied and compared in this work. HPLC was used as reference in the determination of amino acids. Some validation parameters such as quantification limits, precision and accuracy levels were determined with FAN (free amino nitrogen) and formaldehyde methods. Afterwards, these methods were compared with NOPA (o-phthaldialdehyde/N-acetyl-l-cysteine assay) and with HPLC. Both formaldehyde and FAN methods showed high level of precision and low analytical limits. After validation, the methods were used to determine assimilable nitrogen in 28 grape juices from 10 different grape varieties grown in the Douro Region. The comparison between methods indicates that the formaldehyde procedure yields higher values (p < 0.05) than HPLC, FAN and NOPA methods. Thus, ammonium analysis must be run separately and NH4+ nitrogen values should be added to the figures obtained by HPLC, FAN and NOPA, in this case no significant differences were observed between the four methods.