Author(s): Christian E Butzke
Yeast assimilable nitrogen status of 1523 clarified musts from Vitis vinifera vineyards on the West Coast of the United States was determined utilizing an o-phthaldialdehyde/N-acetyl-L-cysteine spectrophotometric (NOPA) assay for primary amino acids and ammonium ion analysis. During the 1996 harvest, the overall variation ranged from 40 to 559 mg N/L with an average of 213 mg N/L (primary amino acids: 29 to 370 mg N/ L, average 135 ± 51 mg N/L; ammonium: 5 to 325 mg N/L; average 70 ± 35 mg N/L). 13% contained combined nitrogen below 140 mg/L, a concentration generally considered sufficient for the completion of a low solids grape juice fermentation starting at sugar concentrations found in normal table wine production. There were statistically significant differences between the means of nitrogen status of 968 musts from different varietals. Cabernet franc musts had the lowest average concentration (172 ± 65 mg/L) with 37% of the juices below 140 mg N/L, while Chardonnay and Pinot noir were highest (254 ± 76 and 236 ± 57 mg/L). Only 1.1% of the Pinot noir juices were deficient. No correlation (R2 = 0.086) between primary amino acid nitrogen and ammonium nitrogen was found. This emphasizes the need for analysis of both major sources of yeast assimilable nitrogen in grape must.