Development of stable isotope dilution assays for the quantitation of free phenolic acids in wheat and barley and malts produced thereof

Free phenolic acids present in cereal malts serve as precursors of different volatile compounds in beer. To study their generation from wheat and barley on the way to the respective malts, stable isotope dilution assays (SIDAs) were developed using high-performance liquid chromatography–tandem mass spectrometry to quantitate the free phenolic acids caffeic, cinnamic, p-coumaric, ferulic, sinapic, and vanillic acid in the grain as well as in the malts produced thereof. An optimised extraction procedure using methanol instead of water in combination with the newly developed SIDAs enabled their selective and sensitive quantitation with low limits of detection and limits of quantitation as well as good recoveries. In kiln-dried wheat malt, the most abundant free phenolic acid was sinapic acid with 2.30 mg/kg dry mass, while in kiln-dried barley malt ferulic acid predominated with 2.76 mg/kg dry mass. The concentration of the latter in kiln-dried malt is of special interest, because this acid is a well-known precursor of the important wheat beer aroma compound 2-methoxy-4-vinylphenol. The concentration of cinnamic acid, the precursor of the toxicologically relevant vinylbenzene (styrene), in kiln-dried wheat and barley malt were 0.56 and 0.97 mg/kg dry mass, respectively. Generally, kiln-dried barley malt showed higher concentrations of most phenolic acids in comparison to kiln-dried wheat malt.