DETERMINATION OF METHIONINE IN MALT

Importance of the topic

Sulphur amino acids such as methionine serve as the primary precursors for volatile sulphur compounds, which profoundly influence beer aroma and taste even at low concentrations. Understanding and controlling methionine content in malt is crucial for ensuring consistent sensory quality and minimizing off-flavour formation during brewing.

Objectives and overview

This work aimed to develop, optimize, and validate a gas chromatographic method with flame photometric detection for quantifying free methionine in malt. The study examined six spring barley varieties (Bojos, Jersey, Malz, Prestige, Tolar, Xanadu) grown at two sites (Branišovice, Hrubčice) and monitored methionine levels throughout kilning at temperatures from 50 °C to 220 °C.

Methodology and instrumentation

• Sample preparation: Five grams of milled malt were ultrasonically extracted for 15 min with a methanol–water (1:4) mixture, centrifuged, and the supernatant subjected to derivatization.
• Derivatization: Aqueous extract was treated with propanol–pyridine (4:1) and ethyl chloroformate, followed by chloroform partitioning. Organic phase was dried under nitrogen and reconstituted in methanol.
• Chromatography: Analysis was performed on a Trace GC Ultra equipped with an RTX-5 capillary column (15 m × 0.32 mm i.d., 0.25 µm film) and a flame photometric detector selective for sulphur. A temperature program ramped from 100 °C to 280 °C. Helium was used as the carrier gas.
• Calibration and validation: Methionine standards (0.8–14 mg·ml⁻¹) generated a linear response (R² = 0.99974). Method performance: LOQ 1.6 mg·g⁻¹, RSD 15%.

Main results and discussion

Methionine content across malt samples ranged from 23.0 to 36.0 mg·g⁻¹. Malz, Prestige, and Tolar displayed consistent levels at both locations, whereas Bojos and Sebastian from Hrubčice were notably higher. During kilning, free methionine decreased with higher temperatures, reflecting its conversion into dimethyl sulphide and related volatile sulfur compounds. This trend highlights the importance of temperature control to limit precursor loss and off-flavour development.

Benefits and practical applications

• Provides maltsters and brewers with a reliable assay to monitor sulphur amino acid levels for flavor consistency.
• Supports raw material selection by identifying barley varieties and growing conditions that produce optimal methionine profiles.
• Enables process optimization in kilning to balance malt color development and retention of essential amino acid precursors.

Future trends and applications

• Coupling GC with mass spectrometry to profile a broader range of sulphur compounds in real time.
• Developing on-line monitoring tools for continuous measurement of methionine during malt kilning.
• Expanding analytical scope to include other sulphur amino acids and Maillard reaction products for comprehensive quality control.

Conclusion

A robust GC–FPD method for free methionine in malt was successfully optimized and validated. The study quantified varietal and locational differences and demonstrated how kilning temperature dictates methionine retention and volatile sulphur formation. Implementation of this protocol can enhance flavor stability and process control in brewing.

Reference

1. Perpete P., Gijs L., Collin S.: Methionine: A key amino acid for flavour biosynthesis in beer. Brewing Yeast Fermentation Performance (2nd Ed.), Blackwell Science Ltd, 2003;206–212.
2. Gijes L., Veermeulen C., Collin S.: Výskyt a vznik sirných aromat v pivu. Cerevisia 1, 2003.
3. Hušek P.: Chloroformates in gas chromatography as general purpose derivatizing agents. J. Chromatogr. B 717, 1998;57–91.
4. Hušek P., Matucha P., Vránová A., Šimek P.: Simple plasma work-up for fast chromatographic analysis of homocysteine, cysteine, methionine and aromatic amino acids. J. Chromatogr. B 789, 2003;311–322.
5. Myung S., Kim M., Min H., Yoo E., Kim K.: Determination of homocysteine and its related compounds by SPME/GC/MSD. J. Chromatogr. B 727, 1999;1–8.
6. Barek J. et al.: Metrologická terminologie v chemii. Chem. Listy 94, 2000;439–444.
7. Spektroskopická společnost Jana Marka Marci: Nejistota a nezávaznost výsledků spektroskopických metod. 2001.
8. Alix J.H.: Molecular aspects of the in vivo and in vitro effects of ethionine, an analog of methionine. Microbiol. Rev. 46, 1982;281–295.