In Vivo and in Vitro Investigations of the Synthesis of S -Methylmethionine during Barley Germination

The in vivo and in vitro synthesis of S-methylmethionine (SMM), one of the dimethyl sulfide precursors in beer, was studied during barley germination. SMM was specifically measured by direct assay using high-performance liquid chromatography and fluorescence techniques. The results indicated that SMM synthesis occurred simultaneously and to a similar extent in the rootlets and the germ. SMM levels in these two fractions accounted for more than 80% of the SMM content in the germinating barley kernel. A slight increase in SMM was observed in the starchy endosperm. Our data suggest that this increase is attributable to SMM diffusion from the germ into the endosperm region. Under our malting conditions (laboratory scale) the evolution of SMM in the germinating barley kernel followed the amino nitrogen profile. In vitro data demonstrated the presence of a methyltransferase in the rootlets and the germ extracts that catalyzed the synthesis of SMM from S-adenosyl-L-methionine and L-methionine. An optimum pH of 6 and an optimum temperature around 50° C were obtained for enzyme from both the rootlets and the germ.