Analysis of Coffee Aroma Compounds by Head-space Solid-Phase Microextraction (SPME) GC-MS with the JMS Q1500GC Master-Quad GC-MS
JEOL USA Inc. 11, Dearborn Road, Peabody, MA01960. USA.
First Published: JEOL Analytical and Imaging Solutions for Advanced R&D May 2021
The composition of volatiles from freshly ground roast-ed coffee is complex, with hundreds of chemical compounds contributing to the aroma. Head-space solid-phase micro-extraction was used to sample volatiles from five different coffees for analysis by GC-MS. Chemometric analysis revealed specific differences between coffees from different origins and different preparations
Four coffees purchased from a local coffee roaster were freshly ground for drip-filter preparation in the store. A fifth sample was purchased from a local grocery and ground to the same coarseness using an adjustable burr grinder. Approximately two grams of each ground coffee were placed in 20 mL head-space SPME vials for analysis. Five measurements were made for each sample except the decaffeinated Colombian coffee, for which only four replicates were measured. Table 1 shows the coffees analyzed.
An HTA HT2800T All-in-One autosampler was mounted on an Agilent 7890B GC used with a JEOL JMS-Q1500GC Master-Quad mass spectrometer. The autosampler was operated in SPME mode. Measurement conditions for the autosampler, GC and mass spectrometer are given in Table 2.
Mass spectra measured by JEOL msPrimo software were imported into AnalyzerPro XD (SpectralWorks, UK) for chromatographic deconvolution, database search, and statistical analysis. The data processing parameters are given in Table 3. Column bleed peaks were omitted from the statistical analysis. Characteristic aroma descriptions are taken from the website listed in reference 1.
The total ion current chromatograms for individual replicates of each coffee are shown in Figure 1, with selected compounds labelled on the chromatograms. It is not trivial to determine small differences in the relative concentrations of specific compound by visual inspection alone.
Figure 2 shows the total ion current chromatogram for the third replicate measurement of the Colombian Supremo coffee head-space with compound assignments labelled for the major peaks.Chromatographic deconvolution with AnalyzerPro XD is needed to identify trace peaks and peaks that are not completely separated.
Figure 3 shows deconvolution of 1-Methylethenyl pyrazine, which overlaps with maltol (the larger peaks) in the Indian Malabar French Roast coffee head-space chromatogram. Principal Component Analysis (PCA, Figure 4) shows distinct groupings for each coffee, with the Bali Blue Moon and the Columbian Supremo being the closest-matching clusters.
The data analysis showed distinctive differences be-tween the coffees. For example, both of the Colombian coffees showed lower levels of pyridine than the other coffees (Figures 5 and 6.) The compound 5-methyl-2-furancarboxaldehyde with an aroma de-scribed as “sweet, caramellic, bready brown, coffee” is very abundant in the Colombian Supremo and Bali coffees, but is at a relatively low abundance in the Indian Malabar French Roast.
The software permits easy comparisons for other compounds. For example, caffeine is at a relatively low level in the decaffeinated coffee, and some other compounds (e.g. pyrrole) are also at a reduced level in the decaffeinated coffee, perhaps because of the decaffeination process. The compound 1-methylethenyl pyrazine, which has an aroma described as “caramel, chocolate, nutty, roasted” and a “burnt coffee” flavor, is abundant in the Indian Malabar French Roast and Rwanda coffees, but at a relatively low level in the other coffees.
The presence of that compound is not visually evident in the chromatogram because it is not chromatographically separated from maltol and other compounds, but it is revealed by chromatographic de-convolution.
Head-space SPME GC-MS analysis combined with statistical analysis provides a great deal of information about the chemical compounds contributing to coffee aroma. The JMS-Q1500GC GC-MS system equipped with the HTA HT2800T autosampler is an easy-to-use platform for data measurement, and data analysis with AnalyzerPro XD permits detailed comparisons between samples for specific compounds. It may be noted that some compounds could not be conclusively identified by database search alone. Conclusive identification of these unknowns may require soft ionization and high-resolution mass spectra that can be obtained with the JEOL AccuTOF GC-Alpha GC-HRTOFMS system .