Development and Application of a Spectrophotometric Assay for Arginine in Grape Juice.

Ethyl carbamate (EC) is a byproduct of most alcoholic fermentations and may be carcinogenic to humans when consumed in large doses. The discovery of high EC levels in many alcoholic beverages prompted the United States wine industry to set a voluntary limit of 15ug/L of EC in all table wines. Arginine is generally the most abundant a-amino acid in grape juice and is the primary precursor to EC in grape juice fermentations. Studies indicate that juices containing more than 1 g/L of arginine have a high potential for producing wines with EC levels over the voluntary limit. Therefore, the wine industry needs a simple method to measure the arginine concentration in a juice before fermentation, to minimize the EC content in the resulting wine. The arginine assay developed for this thesis project uses ion exchange to isolate arginine from the other amino acids in grape juice, and utilizes a modified version of the NOPA Assay to measure the isolated arginine. The method is accurate (3% error), inexpensive ($2.50/sample), and requires a spectrophotometer, which is already available to most wineries. The utility of the arginine assay was demonstrated by examining the arginine content of grapes during ripening, pressing, and extraction, and the arginine content of several cluster fractions, including the juice, pulp, skins, seeds, and stems. Although the results cannot be applied to other vineyards, due to the limited scope of this project, some of the implications are important enough to warrant verification and further research. The arginine content increased for the first two weeks after veraison, and then remained relatively constant until harvest, with some varieties dropping slightly just before harvest. The press juices were slightly higher in arginine than the free run juices, although the difference is not statistically significant. Extraction doubled the arginine content of the must within 2-4 days, and the arginine appears to be evenly distributed throughout the berry, when measured per weight of cluster fraction.