Direct Assay of S-Methylmethionine Using High-Performance Liquid Chromatography and Fluorescence Techniques

Abstract

A new, rapid method to directly assay the amino acid S-methylmethionine (SM M), one of the dimethyl sulfide (DMS) precursors in beer, is based on the formation of a fluorescent compound between SM M and o-phthaldialdehyde after high-performance liquid chromatography of the sample on an ion-exchange column. This method requires no chemical pretreatment of the sample. Under the present optimized conditions for chromatography, and using aminoguanidinopropionic acid as an internal standard, a quantity as small as 0.2 μg of DMS equivalent/ g of dry malt can be determined. Germination of two two-rowed barley varieties (a winter and a spring barley) was analyzed. In both varieties, the evolution of the SMM content is clearly biphasic, the first phase coinciding with the evolution of the α-amino nitrogen content of the malt. The SMM content was determined for 33 different barley varieties (spring barleys and two- and six-rowed winter varieties). The SMM content varied from 4.6–19 μg DMS equivalent/g of dry malt. The microclimate of the growing area played a critical role in determining the SMM content of the malt. From this limited survey, it was concluded that the choice of barley variety may be a promising means to vary the potential DMS content of malt and consequently of beer.