Determination of Short Chain Fatty Acids Isomers, the sensory active products of hops aging, in Beer

Significance of the Topic

The formation of short-chain fatty acids (SCFAs) isomers in beer, especially C4 and C5 derivatives originating from hops aging, contributes to off-flavors and stale aftertastes. Accurate determination of these trace compounds is crucial for quality control in brewing, sensory evaluation and shelf-life prediction.

Objectives and Study Overview

This study aimed to develop a sensitive, selective analytical protocol to quantify isobutyric (2-methylpropionic), isovaleric (3-methylbutyric) and 2-methylbutyric acids in beer. The performance of the new method was compared with a conventional GC-FID approach, and the impact of hops age and brewing procedure (infusion vs. decoction) on SCFA profiles was evaluated.

Methodology and Used Instrumentation

• Sample preparation by solid-phase extraction (SPE) using LiChrolut EN cartridges.
• Internal standard: heptanoic acid; acidified beer (1 M HCl) was loaded, washed, and eluted with chloroform.
• Chromatographic separation: TRACE GC with Tr WaxMS capillary column (30 m × 0.25 mm, 0.25 μm film).
• Detection: quadrupole mass spectrometer (DSQ II) operating in selected ion monitoring (SIM) for SCFA isomers, total ion current (TIC) for other fatty acids.
• Comparison method: GC-FID for total fatty acid profiling.

Main Results and Discussion

• GC-MS in SIM mode resolved C4/C5 isomer quantification despite close retention times, outperforming GC-FID in specificity and sensitivity.
• Good agreement was observed between GC-FID and GC-MS for most fatty acids, except for capric acid and SCFA isomers, where FID overestimated due to coelution.
• Beers brewed with aged hops showed a moderate increase in isobutyric and isovaleric acid levels, approaching sensory thresholds (30 mg/kg for isobutyric, 1.5 mg/kg or lower for isovaleric).
• No consistent influence of infusion vs. decoction mashing on SCFA content was found; variations likely reflect batch-to-batch process differences.

Practical Benefits of the Method

The optimized SPE–GC-MS protocol enables brewers and quality laboratories to:

• Precisely monitor hop aging products that influence beer flavor stability.
• Identify brewing steps contributing to off-flavor precursors.
• Improve shelf-life predictions and sensory quality control.

Future Trends and Potential Applications

• Integration with high-throughput SPE automation for routine QC in breweries.
• Extension to other off-flavor markers such as aldehydes or ketones.
• Application in malt and wort screening to optimize raw material storage and handling.

Conclusions

A SPE–GC-MS method operating in SIM mode was developed to accurately quantify SCFA isomers in beer. It surpasses GC-FID in specificity for isobutyric and isovaleric acids, providing reliable data even with limited chromatographic separation. The method demonstrated that hops aging elevates SCFA levels, informing strategies to control beer flavor stability.

References

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