Fully automatic quantitation of eight different metabolites in coffee using 1H-NMR spectroscopy and the PULCON methodology

Background

Coffee contains a plethora of constituents with some of them being especially important either due to their physiological effects or as quality markers. As quantitative proton nuclear magnetic resonance spectroscopy (¹H-NMR) has been established as a fast and reliable analytical tool its application was evaluated for the simultaneous quantitation of lactic acid, acetic acid, formic acid, caffeine, caffeoylquinic acid (CQA) isomers, N-methylpyridinium, trigonelline, and 5-hydroxymethylfurfural (HMF) in aqueous extracts of roasted Coffea arabica samples.

Results

Simultaneous quantitative determination was achieved by an automated analysis based on the PULCON methodology (pulse length-based concentration determination). The method was validated regarding linearity, accuracy, precision, limit of detection (LOD), and limit of quantitation (LOQ). Recovery rates were between 76% (CQA) and 116% (HMF), and precision was between 1.7% (caffeine) and 10.3% (HMF). The LOD varied between 0.06 g/kg (HMF) and 1.35 g/kg (caffeine and CQA), with the LOQ being between 0.22 g/kg (HMF) and 4.87 g/kg (CQA). To verify the results of the ¹H-NMR method, caffeine, trigonelline, HMF, 3-CQA, 4-CQA, and 5-CQA were additionally quantitated by HPLC-DAD and the results were compared. The described ¹H-NMR method was additionally applied to coffee samples that contained different coffee defects. Results showed only slight changes in the concentrations of the analytes by adding defective beans to defect-free coffee.

Discussion

The developed 1H-NMR approach was proven to be fast (30 min), reliable, and precise. Thus, it is well suited to analyze several coffee constituents of interest in a large number of samples in, for example, quality control.