Comparison of analytical methods for prediction of prefermentation nutritional status of grape juice

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%.