Analysis of Protein and Total Usable Nitrogen in Beer and Wine Using a Microwell Ninhydrin Assay

J. Inst. Brew. 115(2), 122–127, 2009 In this study we present a ninhydrin based microwell assay that can be utilized in place of the traditional Kjeldahl method for the determination of the protein content of beer or wine. In addition, the assay is ideal for the determination of free amino acids in beer (FAN), a term understood and used by brewers, and yeast assimilable nitrogen (YAN) used by enologists. The assay only measures alpha amino acids and ammonia so other nitrogen sources are not detected, resulting in a 30% reduction in total protein of a variety of beers compared to the Kjeldahl method, which measures nitrogen from all sources. The results also showed that only 25% of the total “protein” in beer is actually derived from peptides larger than 3,500 Kd. Analysis of beer or wine with the microwell assay for total usable nitrogen was compared to the standard FAN and YAN methods and conditions were determined for maximal efficiency and precision. Superior results were obtained with low reaction volumes and a stable sodium acetate buffered ninhydrin reagent at pH 5.5. As an alternative, for use with cuvettes, a reduced volume FAN assay using the same pH 5.5 sodium acetate buffered ninhydrin reagent gave comparable results. The assay is economical, rapid, accurate and applicable to large numbers of samples.