Application of Nitrogen-Purging of Malt Extracts to Measure Two Dimethylsulfide Precursors by Headspace Gas Chromatography

Abstract

Dimethylsulfide (DMS) in beer can arise from S-methylmethionine (SMM) by heat degradation during malt kilning and wort boiling, or from the reduction of dimethylsulfoxide (DMSO) by yeast during fermentation. Both SMM and DMSO originate from malted barley. A gas chromatography (GC) headspace technique was applied to analyze SMM and DMSO in malt. Nitrogen-purging was used to remove free DMS from the malt extract, therefore only SMM was determined following heat treatment in alkaline solution. The accuracy and precision of malt SMM analysis were improved in comparison with the traditional subtraction method, where the SMM value was based on subtracting free DMS in the untreated sample from total DMS in a heat-alkaline-treated sample. Similarly, for DMSO measurement, free DMS and SMM could be removed by nitrogen-purging of the aqueous sample after heat-alkaline treatment. DMSO then was reduced with stannous chloride into DMS for GC analysis. A small amount of DMS was oxidized to DMSO in a sealed system under heat treatment. This indicated that DMSO could be formed from oxidation of DMS during malt kilning. The level of DMSO present in malts was found to be similar to that of SMM.