APPLICATION OF SOME MODERN SAMPLE PREPARATION PROCEDURES FOR QUANTITATIVE DETERMINATION OF VICINAL DIKETONES IN BEER

Significance of the Topic

Vicinal diketones, particularly diacetyl (2,3-butanedione) and 2,3-pentanedione, are key off-flavour compounds in beer. Concentrations above sensory thresholds (0.05–0.10 mg/L for diacetyl; ≈1 mg/L for pentanedione) lead to undesirable buttery or honey-like aromas. Accurate quantification is essential for quality control in brewing to ensure consistent product flavour.

Objectives and Study Overview

This work evaluates and optimizes stir bar sorptive extraction (SBSE) for quantitative determination of vicinal diketones in beer. Analytical performance is compared with solid phase microextraction (SPME), and detection is carried out by gas chromatography with electron capture detection (GC-ECD).

Methodology and Instrumentation

• Sample Preparation (SBSE): 10 mL beer or 5 % V/V ethanol model spiked with 2,3-hexanedione (40 µg/L), 2 g NaCl, stirred with a 10 mm×0.5 mm PDMS Twister at 800 rpm, room temperature, 20 min. Back-extraction in 200 µL hexane for 20 min.
• Sample Preparation (SPME): Headspace extraction from 3 mL sample with 1.5 g NaCl using CAR/DVB 65 µm fiber, 30 min at room temperature; thermal desorption in GC inlet.
• Chromatography: DB-624 column (60 m×0.32 mm i.d.×1.8 µm); oven 75 °C (10 min)→120 °C at 5 °C/min (1 min); splitless injector 220 °C; ECD at 180 °C; He carrier (150 kPa at 75 °C), N₂ make-up.

Main Results and Discussion

• Detector Temperature: Optimum ECD temperature was 180 °C, balancing sensitivity and electrode cleanliness.
• Salting-Out Effect: Addition of 2 g NaCl per 10 mL sample provided ~3.5-fold signal enhancement; increasing to 4 g gave minor gains (~2 %).
• Extraction Kinetics: SBSE equilibrated by 20 min; extending to 40 min improved total response by only 4 %. Back-extraction was complete within 20 min.
• Calibration: Quadratic calibration yielded correlation coefficients >0.997 for both analytes; SBSE linear range for diacetyl was 0.100–0.400 mg/L; SPME extended to lower levels.
• Repeatability: SBSE RSD of peak-area ratio was 20 % for diacetyl and 5.6 % for pentanedione; SPME achieved RSD ≤7 % for both.
• Robustness: Varying ethanol content (0–8 % V/V) produced RSD <5.1 % (SBSE) and <3.9 % (SPME) for response ratios.

Benefits and Practical Applications of the Method

SBSE with solvent back-extraction offers a low-cost, simple alternative to SPME and classical steam-distillation methods when analyte concentrations exceed 0.100 mg/L. It avoids sample heating, reducing artefact formation, and is suitable for routine brewery quality control.

Future Trends and Opportunities

Advances may include automated SBSE systems, direct thermal desorption to GC–MS for enhanced selectivity, and extension of SBSE to other beer flavour compounds. Development of in-line monitoring and miniaturized extraction would support real-time process control.

Conclusion

The optimized SBSE protocol enables reliable quantification of diacetyl and 2,3-pentanedione in beer with straightforward handling and satisfactory precision at moderate concentration levels. SPME remains preferred for trace-level detection.

Instrumentation

• Twister PDMS (10 mm×0.5 mm) – Gerstel
• SPME fiber CAR/DVB 65 µm – Supelco
• Gas Chromatographs: CP-9001 (Chrompack), HRGC 5300 Mega (Carlo Erba)
• Autosampler ASG 40 – Labio

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