Elucidation of the Hoppy Aroma in Beers by Stir Bar and Headspace Sorptive Extraction followed by Thermal Desorption - CGC - MS/PFPD

Importance of the Topic

The characteristic hoppy aroma in beer is a key quality parameter in brewing, influencing consumer acceptance and brand differentiation. Reliable analytical methods with high sensitivity and selectivity are essential to correlate sensory perception with chemical composition.

Study Objectives and Overview

This study evaluates two novel sorptive extraction techniques—stir bar sorptive extraction (SBSE) and headspace sorptive extraction (HSSE)—for the enrichment and analysis of hop-derived volatile and semi-volatile compounds in beer. The aim is to overcome limitations of traditional methods such as solid phase microextraction (SPME).

Methodology and Instrumentation

Sample Preparation and Extraction:

• SBSE: A PDMS-coated stir bar is immersed in beer at 700 rpm for 45 min, rinsed, and introduced into a thermal desorption tube.
• HSSE: A similar stir bar is suspended in the headspace above the beer under stirring for 45 min at room temperature.

Thermal Desorption and Chromatography:

• Thermal desorber: Gerstel TDS-2, 20–240 °C ramp, splitless desorption.
• Gas Chromatograph: Agilent 6890 with CIS 4 PTV inlet; Stabilwax column (30 m × 0.25 mm × 0.25 µm).
• Carrier gas: Helium at 1 mL/min.
• Detectors: Parallel OI PFPD in sulphur mode and HP 5973 MSD in scan mode.

Main Results and Discussion

Comparison with SPME showed that both SBSE and HSSE achieve higher recoveries across a range of volatilities (log Kow 0–7), particularly for hydrocarbons, aromatics, esters, and chlorinated compounds. The extract from beer revealed a complex profile, with analytes ranging from acetaldehyde to palmitic acid. Parallel PFPD detection enabled selective identification of sulfur compounds at trace levels, including dimethyl sulfide, methylthioacetate, dimethyldisulfide, and methional. Sesquiterpenes such as humulene (α-caryophyllene) and β-damascone were identified via extracted ion chromatograms (m/z 204). The “sunstruck” off-flavor compound 3-methyl-but-2-ene-thiol was quantified at parts-per-trillion after UV exposure.

Benefits and Practical Applications

The combination of SBSE/HSSE with TD-GC-MS/PFPD provides a robust platform for quality control in breweries, enabling:

• Trace-level monitoring of aroma and off-flavor sulfur compounds.
• Improved sensitivity and phase ratio compared with SPME.
• Streamlined sample preparation for routine QA/QC.

Future Trends and Potential Applications

Advancements may include novel sorbent coatings for broader selectivity, integration with multidimensional GC for enhanced separation, and real-time process monitoring to ensure consistent beer quality.

Conclusion

SBSE and HSSE combined with thermal desorption and parallel PFPD/MS detection offer a sensitive, selective, and practical approach for elucidating the hoppy aroma and sulfur-based off-flavor compounds in beer, addressing challenges in brewery analytics.

Reference

• Baltussen E., Sandra P., David F., Cramers C. J. Microcolumn Separations 1999, 11, 737.
• Tienpont B., David F., Bicchi C., Sandra P. J. Microcolumn Separations 2000, 12, 577.