Effects of Selected Vineyard Management Practices on Nitrogen Status of Grape Juice

The research team has established relationships with seven vineyards, Virginia, as well as the central coast, central and southern San Joaquin Valley and the Lodi area of California, with respect to vineyard trials, sampling and small lot wine production.

A total of 528 berry samples (excluding duplicates) corresponding to crop load and canopy orientation and multiple sample periods from veraison to harvest have been collected and analyzed by Formol, OPA and ArOPA procedures for FAN and NH3. Comparison of the analytical methods by crop load level and canopy orientation indicated no statistically significant differences (T-test, p<0.05) between high and low crop load levels for that season. There were significant differences in OPA amino acids and OPA arginine values between east and west sides of the canopy. Higher levels were observed on the west side.

A preliminary comparison of Formol values with summed OPA and ammonia from 1999 Virginia Cabernet Sauvignon samples indicated that the Formol method produced significantly higher values for assimilable nitrogen than did OPA + ammonia for the same sample (p <0.05).

Initially, two methods were evaluated for expression of juice from collected fruit. A comparison of blender and stomacher processing methods was performed to evaluate possible effects of seed breakage or analyte stratification on analytical results. Early season and post-harvest samples of 1999 Virginia Cabernet Sauvignon fruit were tested. No significant differences (p <0.05) in Formol values were observed. For the OPA measurements, no significant differences were observed in the early season sample set, but the stomacher method was found to produce higher OPA nitrogen values for the late season samples.

In conjunction with Formol titrations, spectrometric (OPA) analyses for general total amounts of the alpha amino acids, arginine specific spectrometric (ARGOPA) analyses, and ammonia, we are attempting to also conduct HPLC analyses for all the amino acids present. The purpose for the latter analyses is to allow us to look at proline/arginine ratios, and specific amounts of cysteine, methionine, and other specific amino acids, as a function of vineyard or winemaking study. Currently we are investigating the use of phenylisothiocyanate, dimethylamino-azobenzene sulfonyl, and dimethylaminoazobenzene thiohydantoin amino acid derivatizations to determine which of these procedures will work best with our juice and wine samples. A Hewlett-Packard model 1050 HPLC with diode array detection is being used in this work. To date the instrument operational conditions have been established for the derivatives being run and chromatograms of some of the derivatized amino acids run. Analysis of fermentation volatiles (fusel alcohols and esters) has begun.

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