Determination of Alpha Amino Nitrogen in Musts and Wines by TNBS Method

A procedure based on the quantitative reaction of primary amino groups with trinitrobenzene sulfonic acid (TNBS) was developed for analysis of grape musts and wines. The resultant trinitrophenylated amine complex was detected at 420 nm (Field9s Methods in Enzymology, 25: 464-68, 1972). A standard curve of absorbance versus alpha amine nitrogen was prepared using arginine. Proteins and large polypeptides appeared to be removed from must and wine samples more efficiently by trichloroacetic acid precipitation than by alcohol or heat treatment. The method gave reasonable and reproducible results when applied to the analysis of Pinot Chardonnay and Pinot noir musts and the wines made from them. Reproducibility between replicates was within 1-2%. Comparisons of analyses done with an HPLC amino acid system on wine and on model solutions and fermented model solutions indicate the method is best for grape juice and less appropriate for wine.