Vineyard Fertilization

Nitrogen (N) fertilization in the vineyard has direct influence on the nitrogen contents of the grape berry and the resulting grape must. Excessive fertilization with urea, ammonia and other N-fertilizers in the past is considered partly responsible for generally higher Ethyl Carbamate levels found in wines from traditional wine-producing countries.

Cover Crops

Legumes used as cover crops in the vineyard including vetches (Vichia ssp.), clover (Trifolium ssp.), and pea (Pisum spp.) may be adding a significant amount of nitrogen to their vineyards. If legumes are used as cover crops, soil and vine nitrogen status should be monitored in order to avoid excessive arginine levels in juices.

Cultivars & Rootstocks

Different grape cultivars exhibit variations in nitrogen uptake, with some varieties being generally lower in level of arginine than others. However, low nitrogen status of cultivars is largely related to own-root characteristics and will change with the use of different rootstocks. Total bloom petiole nitrogen can vary by more than 40% on average, while nitrate-nitrogen may even vary 10-12 fold depending on the rootstock-scion combination used. Rootstocks can therefore have a profound impact on vineyard nitrogen status and fertilizer management. Local viticulture farm advisors can provide information on nitrogen uptake by different rootstocks.

2. Juice Nutrient Status/Additions

To correctly determine the nutritional status of an individual grape juice, it may be necessary to measure the level of nitrogen compounds actually available to the yeast for its metabolic activity. Nitrogen status of grapes varies widely with vineyard site, soil, irrigation and fertilization practices, vintage weather, scion and rootstock, and grape maturity. The two major sources for nitrogen in must are ammonia and amino acids. Yeast food preparations may add an unidentified level of yeast-available nitrogen to a juice. It is recommended that winemakers request the supplier to specify the different nitrogen sources. Winemakers should know the nitrogen status of their juices and not over-supplement with diammonium phosphate.

3. Yeast Strains

Wine yeast strains differ in their ability to rapidly catabolize urea during fermentation. When excess urea accumulates in the yeast cytoplasm, it is released into its environment, the must. High urea producing yeasts are those that have a high capacity to degrade arginine to urea, but a low urea metabolizing ability.

4. Lactic Acid Bacteria

Certain wine lactic acid bacteria are capable of forming small amounts of citrulline, a precursor of Ethyl Carbamate, from the amino acid arginine, and excreting this precursor into the wine. Citrulline formation from arginine degradation could result in elevated levels of ethyl carbamate, even at normal temperatures, during prolonged storage. If malolactic fermentation is desired, winemakers should either use a commercial strain that does not produce high levels of citrulline or monitor juice for citrulline content post-fermentation.

5. Urease Application

Since urea is the major precursor for Ethyl Carbamate in wine, enzymatic hydrolysis of urea to ammonia and CO₂ appears to be a suitable way to eliminate formation from this source. Preparations of urease enzyme are commercially available and permitted by BATF for the treatment of wine. However, urease activity is severely limited under normal wine conditions, specifically with respect to low pH and ethanol. Urease is especially inhibited by high concentrations of malic acid, and fluoride residues (from cryolite® application in the vineyard) in excess of 1 mg/L. Any combination of these factors can make it practically impossible to reach the desired low urea levels in a reasonable time, even at a very high enzyme dosage. A complete elimination of Ethyl Carbamate is not possible.

6. Shipment and Storage

The chemical reaction between urea and ethanol increases exponentially with temperature. It is therefore essential that a wine containing elevated levels of urea is not exposed to elevated temperatures (above 100°F) during storage or shipment. This is virtually impossible to guarantee with wine shipments arriving from overseas in container ships or in transport vehicles throughout the United States and Canada.