Fast determination of N-phenylpropenoyl-l-amino acids (NPA) in cocoa samples from different origins by ultra-performance liquid chromatography and capillary electrophoresis

Abstract

N-Phenylpropenoyl-l-amino acids (NPA) are among the key contributors to the astringent taste of cocoa. Two fast and easy to use methods (CE and UPLC®, both with PDA detection) for routine determination of the main NPA were developed. Crude extracts of defatted seeds were analysed by means of capillary electrophoresis leading to separation in less than 30min. Separation by means of UPLC® was much faster (<4min), however, a preceding SPE clean-up abolishes this benefit in time saving. Thus, the CE- and UPLC®-methods are comparable concerning time consumption and provide similar results. Analysis of 18 samples of raw and roasted beans from the global cocoa market originated from 12 countries and 4 continents showed a great variability of NPA content (0.7-3.6mg/g) and qualitative composition of different NPA. Anyway, all samples from cocoa beans showed a comparable NPA pattern. N-[3',4'-dihydroxy-(E)-cinnamoyl]-l-aspartic acid was the most abundant metabolite, followed by N-[4'-hydroxy-(E)-cinnamoyl]-l-aspartic acid and N-[3',4'-dihydroxy-(E)-cinnamoyl]-3-hydroxy-l-tyrosine (clovamide). The analysis of other plant organs (flowers, leaves, fruits) revealed an entirely different situation. NPA were detected in all parts of the fruit, with husk and pulp being clearly dominated by clovamide. In flowers and leaves no NPA were detected; 2-O-caffeoyltartaric acid was shown to be the major caffeic acid metabolite in leaves.