All juice analyses were conducted using fruit from mature (10-year-old) Cabernet Sauvignon (Vitis vinifera L.) vines

325 Rapid, accurate, and precise analytical methods for assimilable nitrogen involving simple sample preparation, minimal waste products, and minimal instrumentation would be valuable tools for winemakers. Currently, several analytical methods are in common use. The Formol titration is a simple and rapid determination of assimilable nitrogen. This method involves the addition of neutralized formaldehyde to liberate protons that are titrated directly with NaOH to a pH 8.0 end point [12]. The analysis provides an approximate, but useful, index of the nutritional status of juice. The more recent nitrogen by o-phthaldialdehyde (NOPA) procedure [3] has been used to obtain free α-amino acid nitrogen (FAN) by derivitization of primary amino groups with o-phthaldialdehyde (OPA). The resulting isoindole derivatives can be measured spectrophotometrically at 335 nm. As a result of the specificity of this reaction to primary amino acids, the imino acid proline cannot form a derivative and, therefore, does not contribute to the results. Ammonium ion, the second main source of assimilable nitrogen, is not measured by the NOPA procedure. A modification of NOPA, ARGOPA [1] allows for selective determination of arginine, quantitatively the most significant contributor to yeast nutrition and potential ethyl carbamate formation. This procedure uses cation exchange to isolate arginine from other amino acids prior to analysis by NOPA.