Untargeted metabolomic profiling of 100% malt beers versus those containing barley adjunct

Abstract

Why was the work done: The incorporation of unmalted barley at high proportion in the grist can introduce unfavourable bitter and astringent characteristics to beer, resulting in an upper limit on the proportion used. The flavour active compounds from raw barley which contribute these characteristics to the beer remain to be identified. How was the work done: This study used non-targeted metabolomics to determine non-volatile metabolites which could contribute to flavour differences when brewing with barley. Three beers were analysed using ultra-high performance liquid chromatography (UHPLC) coupled to a quadrupole time-of-flight mass spectrometer (qTOF) with an electro-spray ionisation source (ESI).  One beer was produced using 100% malt and two beers with a grist of 15% barley and 85% malt (beers A and B).  The barley was used untreated (beer A) or treated using a proprietary process (beer B).  The metabolomic profiles of the three beers were compared and statistically different molecular features were annotated via analysis of MS2 spectra. What are the main findings: Several of the main differential molecular features were nitrogenous peptides and purine derivatives. This was attributed to the lack of the malting process and associated proteolytic enzyme activity reducing the extent of protein and peptide breakdown in the unmalted barley. Several of the identified peptides had amino acid residues which are known to cause bitter and kokumi (rich) taste in beer, which could explain the bitterness when brewing with unmalted barley. Why is the work important: A non-targeted approach offers new insights into non-volatile molecular features in beer that have not been previously identified with targeted analyses. Accordingly, this work identifies metabolites and groups of compounds which have not been previously considered when investigating the unfavourable bitterness and astringency associated with the use of unmalted barley