Possibilities of Utilization of Modern Sample Preparation Methods for Gas Chromatographic Analyses in Beverage and Namely Brewing Analytics. Part III. – Solid-Phase Microextraction and Stir Bar Sorptive Extraction in Fatty Acids Analysis in Beer

Importance of the Topic

Free fatty acids influence key quality attributes of beer, including sensory profile, yeast vitality and foam stability. Medium-chain acids formed during fermentation can produce off-flavours even below individual sensory thresholds, while unsaturated long-chain acids contribute to ageing character through oxidative degradation. Rapid, solvent-minimized analytical methods are therefore essential for routine quality control in brewing.

Objectives and Overview of the Study

This work (Part III) demonstrates the application of two modern sample-preparation microextraction techniques—headspace solid-phase microextraction (SPME) and stir-bar sorptive extraction (SBSE)—for determination of free fatty acids in beer. The study compares analytical performance, advantages and limitations of both approaches, and discusses their suitability for medium-chain (C6–C12) and long-chain (C14–C18) fatty acids.

Methodology and Used Instrumentation

Extraction procedures were performed on commercial pilsner-type beers and model samples (5 % ethanol in water) spiked with internal standards (heptanoic and undecanoic acids, 1.3 mg/L each). Key instrumentation and reagents included:

• Gas chromatograph Chrompack CP 9001 with split/splitless injector and flame ionization detector
• Phenomenex ZB-WAX capillary column (30 m × 0.32 mm i.d., 0.25 μm film)
• SPME fibers (65 μm Carbowax-Divinylbenzene; manual holder)
• Gerstel Twister SBSE stir bar (PDMS, 0.5 mm coating)
• Carrier gas: helium 5.0; calibration grade hydrogen and synthetic air for FID
• Reagents: ammonium sulfate, BF₃–methanol, dichloromethane/hexane solvent mix

SPME headspace extractions were carried out at ambient temperature for 30 min with salting out; fibers were thermally desorbed in the injector at 250 °C. SBSE employed magnetic stirring at 1 000 rpm for 60 min, followed by solvent back-extraction (dichloromethane/hexane) for 40 min; medium-chain acids were directly injected, while long-chain acids required methylation (BF₃ in methanol) and hexane extraction.

Main Results and Discussion

Calibration curves (0.0015–8 mg/L) showed high linearity (r = 0.995–0.999) for both SPME and SBSE; quadratic fits further improved correlation. SPME recoveries for C6–C12 ranged from 95 % to 110 % (RSD ≤ 5 %), whereas SBSE recoveries for medium-chain acids were lower (57 %–89 %, RSD 4 %–7 %), due to limited extraction of polar compounds (log Kₒ/w for caproic acid = 1.88). SBSE achieved excellent recoveries (94 %–110 %) for long-chain acids (log Kₒ/w > 6), with RSD up to 16 % reflecting derivatization and back-extraction steps.

Benefits and Practical Applications of the Method

• Minimal or no solvent consumption
• Good repeatability and accuracy
• SPME: rapid (35 min overall), ideal for routine control of medium-chain fatty acids
• SBSE: robust stir bar handling, broad coverage including long-chain acids

Future Trends and Application Opportunities

Advances may include optimized fiber coatings and stir-bar sorbents for enhanced polarity range, on-fiber derivatization to streamline long-chain acid analysis, full automation, and transfer of methods to other beverages or complex matrices for multi-residue screening.

Conclusion

SPME and SBSE represent complementary, user-friendly, and solvent-efficient sample-preparation techniques for gas-chromatographic determination of free fatty acids in beer. SPME excels in speed and precision for medium-chain acids, while SBSE provides comprehensive profiling including long-chain acids at the expense of longer processing time.

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