Aroma Profile of Hops, Humulus Iupulus, as a Function of Boil Time by GC-TOFMS and GCxGC-TOFMS

Importance of the Topic

– Hops are a key beer ingredient, providing bitterness and aroma flavors such as floral, piney or citrus notes.
– Timing of hop addition during the boil influences isomerization of alpha acids for bitterness vs. preservation of volatile oils for aroma.
– Analytical characterization of hop aroma compounds as a function of boil time guides brewers to optimize flavor profiles.

Objectives and Study Overview

– Aim: to profile volatile and semi-volatile aroma and flavor compounds in hops during controlled boil times (5 to 60 minutes).
– Approach: HS-SPME sampling combined with one-dimensional GC-TOFMS and two-dimensional GCxGC-TOFMS to monitor time-dependent changes.

Methodology

– Sample preparation: 3 g Cascade hop flowers boiled in 0.5 L water, aliquots taken at 5, 10, 20, 40 and 60 min.
– HS-SPME: automated Gerstel MPS2 sampler, 50 °C incubation for 10 min, DVB/CAR/PDMS fiber extraction for 30 min at 50 °C, desorption at 250 °C inlet.

Instrumentation Used

• GC-TOFMS: LECO Pegasus HT with Agilent 6890 GC, Rxi-5Sil MS column (30 m×0.25 mm×0.25 µm), helium 1 mL/min; oven program 35 °C (4 min)→250 °C (10 °C/min)→(4 min).
• GCxGC-TOFMS: LECO Pegasus 4D with secondary oven and dual-stage thermal modulator; primary Rxi-5Sil MS and secondary Stabilwax columns; modulation period 6 s; secondary oven 10 °C above primary.

Main Results and Discussion

– GC-TOFMS detected 607 peaks at 5 min vs. 373 at 60 min; overall signal intensity decreased with prolonged boil.
– Deconvolution (ChromaTOF True Signal) resolved coeluting analytes, e.g. separation of 2-methyl 2-methylpropyl butanoate and octanal.
– GCxGC increased peak capacity: 1057 vs. 607 peaks at 5 min; improved separation enhanced library match scores (e.g. match for benzoic ester rose from 638 to 916).
– 18 target aroma compounds tentatively identified and quantified; most aroma constituents declined sharply after 10 min, falling below 40% by 20 min and often undetectable by 60 min.

Benefits and Practical Applications

– Provides quantitative profiles of hop-derived aroma compounds to inform optimal hop addition schedules in brewing.
– Demonstrates capabilities of HS-SPME coupled with GC-TOFMS/GCxGC-TOFMS for complex flavor matrices.
– Deconvolution and two-dimensional separations enable identification and quantification of low-level and coeluting analytes.

Future Trends and Potential Applications

– Integration with retention index matching and authentic standards for confirmation of compound identities.
– Application of non-targeted fingerprinting and data mining for varietal differentiation and quality control.
– Coupling to sensory analysis to correlate chemical profiles with perceived flavors.
– Extension to other botanical ingredients and real-time process monitoring in production environments.

Conclusion

– A robust HS-SPME GC-TOFMS and GCxGC-TOFMS workflow was established for profiling hop aroma evolution during boil.
– Two-dimensional GC significantly enhanced peak capacity and detection sensitivity, improving identification confidence.
– Data underscore the importance of late hop additions for preserving desirable volatile flavor compounds.

References

No external literature references were provided in the original document.