Beer’s volatile fraction caught by microextraction

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  • Published: Jul 11, 2018
  • Author: Ryan De Vooght-Johnson
  • Copyright: Image: flubydust/Getty Images
  • Channels: Gas Chromatography
thumbnail image: Beer’s volatile fraction caught by microextraction

Accurate methods needed for analysing volatile compounds in beer

Credit: flubydust/Getty Images.

The flavours of the many varieties of beer can differ significantly, partly due to the different volatile organic compounds found in brews. These include higher alcohols, esters and carboxylic acids. GC is an obvious method for the determination of these volatile compounds, but direct injection of diluted beers can lead to columns becoming blocked by non-volatile material. A better method is to use solid-phase microextraction in combination with headspace GC-MS, so avoiding the clogging of the GC by non-volatiles.

The Prague researchers used a central composite experimental design to optimise the extraction conditions for beer volatiles. This design consists of the ‘corners’ seen in a standard 2n design, along with ‘star points’ at extreme high and low values for each variable and also a central point, the latter typically being replicated. It gives information about the curvature of response surfaces, which is lacking in a basic 2n design.

SPME optimised for beer volatiles

Beer samples were pH adjusted to various pH values, sodium chloride was added and the mixture was stirred with the SPME fibre. Four extraction variables were examined: pH, the amount of salt added, the extraction time and the extraction temperature. The central composite design comprised 16 ‘corner points’, eight ‘star points’ and one central point (five replicates), giving 29 experimental runs. In practice, each of these was run in triplicate, giving 87 runs in all. Statistica 12.0 software was used to analyse the results using analysis of variance (ANOVA). It was found that high temperatures (but not more than 50 °C), long extraction times, low pH values and high levels of sodium chloride gave the best results, so the recommended conditions were temperature = 50 °C, time = 45 min, pH = 3.3 and sodium chloride concentration = 26% w/v.

Prior to the central composite runs, four different fibres, all from Supelco, were examined in a separate comparison: polyacrylate (PA), polydimethylsiloxane-divinylbenzene (PDMS-DVB), Carboxen-polydimethylsiloxane (CAR-PDMS) and divinylbenzene-Carboxen-polydimethylsiloxane (DVB-CAR-PDMS). The CAR-PDMS fibre was selected as it absorbed the greatest range of compounds, despite giving somewhat less total absorbance of volatiles compared with the other three fibres.

GC was carried out using a Supelco SPME holder attached to a CTC COMBI PAL autosampler. A 6890N instrument was employed, fitted with an INNOWax capillary column (both Agilent). The carrier gas was helium 6.0 grade, run at a flow rate of 1 mL/min. The temperature was held at 30 °C for 10 min, raised at 2 °C/min to 52 °C, held at 52 °C for 2 min, raised at 2 °C/min to 65 °C, held at 65 °C for 2 min, raised at 5 °C/min to 250 °C and finally held at 250 °C for 3 min. Mass spectrometry used an Agilent 5975B MSD single quadrupole instrument with electron ionisation. Specific ions were found for 19 volatile compounds (higher alcohols, esters and carboxylic acids). Ethyl heptanoate and 3-octanol were used as internal standards.

The method gave good linearity and precision. The limits of detection (LODs) ranged from 0.0002 to 0.2595 mg/L, while the limits of quantification ranged from 0.0004 to 1.2379 mg/L. The new method was tested with six varieties of Czech lager, giving results for the 19 compounds in line with previous work on other beers. Relatively high concentrations of various higher alcohols (particularly 3-methyl-1-butanol), esters and carboxylic acids were noted.

Optimised method suitable for beer volatiles

The carefully optimised method using SPME and GC-MS has been shown to be suitable for the detection of a wide range of volatile compounds in beer. Similar optimisation techniques could be applied to other beverages. Carefully crafted experimental designs, such as the one employed, generally give better results than old-fashioned ‘changing one variable at time’ optimisation.

Related Links

Journal of the Institute of Brewing 2018, 32, 1092-1098. Nešpor et al.: Application of response surface design to optimise the chromatographic analysis of volatile compounds in beer.

Rapid Communication in Mass Spectrometry 2017, 27, 419-429. Gilbert-López et al. "Performance of dielectric barrier discharge ionization mass spectrometry for pesticide testing: a comparison with atmospheric pressure chemical ionization and electrospray ionization."

Journal of Food Science 2011, 76, C205-C211. Charry-Parra et al. "Beer volatile analysis: Optimization of HS/SPME coupled to GC/MS/FID."

Wikipedia. Central Composite Design

Article by Ryan De Vooght-Johnson

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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