Application of the Precolumn Derivatization Reagent CIM-C2-NH2 for Labeling Carboxyl Groups in LC-MS/MS Analysis of Primary Organic Acids in Japanese Sake

Japanese sake, a traditional alcoholic beverage, contains several organic acids that may contribute to its sour taste. To identify these, a precolumn derivatization reagent, benzyl 5-(2-aminoethyl)-3-methyl-4-oxoimidazolidine-1-carboxylate (CIM-C2-NH2), developed for labeling carboxyl groups, was synthesized and applied to liquid chromatography–tandem mass spectrometry (LC-MS/MS) analysis of organic acids in six commercial sake samples. The majority primarily contained lactic acid (LA), and dicarboxylic acids, such as succinic acid (SA), malic acid (MA), and citramalic acid (CMA). The organic acid concentrations and compositions in the sake differed among brands. Notably, both l- and d-forms of LA were detected in all samples, while only d-CMA was present. To estimate the total acidic content, neutralization titration with sodium hydroxide was performed. In four of the six samples, titration results closely matched LC-MS/MS data, suggesting that l-LA, d-LA, SA, MA, and d-CMA were the primary contributors for the sour taste in these sakes. The discrepancy between titration and LC-MS/MS data for the other samples was attributed to the presence of other organic acids, which will be investigated in future studies.