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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Rapid Analysis for Available Nitrogen Compounds in Must

A rapid method for the analysis of a-amino acids in must would help winemakers decide in a timely manner whether or not they should supplement nitrogen levels prior to fermentation. Inadequate nitrogen levels can lead to sluggish or stuck fermentations, while excessive nitrogen levels can lead to the formation of ethyl carbamate. Our findings have demonstrated that the concentration of ammonia in the must is not correlated with the concentration of a-amino acids (r2=0.202, N=35) in juices from the 1994 vintage. Thus, ammonia determinations can not be used as a rational indication for the nutritional status of grape juice. The requirement for the determination of a-amino acid concentration (as well as ammonia concentration) has thus been demonstrated. We developed and tested a new assay for analysis of nitrogen in grape must. It is based on photometric determination of amino acids as o-phthalaldehyde (OPA), N-acetyl-L-cysteine (NAC) derivatives. Our results show the test’s advantages over conventional and alternative nitrogen assays used in the wine industry. The test shows reasonable correlation with standard HPLC analysis. The main focus of research is currently fine tuning of the procedure to reduce error. It is insensitive to proline, the most abundant yet for wine yeast not utilizable amino acid in must. It is extremely rapid, works at room temperature, and uses stable and non-toxic reagents. At ‘ present the assay shows reasonable correlation (r2=0.818) with standard HPLC analysis. The findings from our first 6 months suggest that the optimization of the test will be completed by August 1995. Our current focus of research is to increase performance of this assay and to simplify our procedure, to make the test available for industrial application in the 1995 crush.

Investigation of pattern of utilization of nitrogen compounds and urea

A field experiment, fertilized at seven different nitrogen levels was tested. All the samples responded in a similar manner with no significant effect on rates of fermentation or amounts of residual urea. This was the first year of the treatments and for all practical purposes was base line information. No differences were noted between on skin or off skin fermentations due to treatments. The results of ten dessert grape fermentations made from Davis grapes showed that two of the varieties ended up with extremely high urea values of 98 and 134 mg/L. This is due to high nitrogen values in the grape and failure to use all the assimilable amino acids.