Five methods for evaluating nitrogen status were compared using 70 Cabernet Sauvignon juice samples: nitrogen by o-phthaldialdehyde (NOPA), arginine NOPA, enzymatic ammonia, Formol, and high-performance liquid chromatography (HPLC). Parallel recovery studies using model solutions of various amino acids and ammonia, presented singly and in combination, were also conducted. The results from two fruit-processing methods were compared using immature and mature berries. NOPA measurements were significantly higher in mature, pressed whole berry-derived samples, compared with homogenized juice. Adjustment of formaldehyde pH prior to analysis was found to be critical to consistency of the Formol method. Average amino acid recoveries for the Formol titration ranged from 82 to 99%. Average recovery for proline was 16.9 ± 0.4%. Ammonium nitrogen was also recovered (84 ± 3%) in the Formol procedure. Formol results trended significantly with NOPA. The correlation coefficient between Formol and NOPA plus NH4+ was 0.87, with Formol values being higher. The average deviation between the Formol and HPLC plus NH4+ and between the NOPA plus NH4+ and HPLC plus NH4+ was 7.3%.
Acknowledgments The authors would like to thank the following agencies for their assistance in funding this research: Virginia Winegrowers Advisory Board, California Agricultural Technology Institute, California Competive Grants Program and the American Vineyard Foundation. Additional thank you to Sandy Brown, Robert Henry, Dan Musso, Jeremy Weintraub, Lawrence Herder, Steve Marko, and John Giannini.
- Copyright 2002 by the American Society for Enology and Viticulture