21. Direct ESI-MS quantitation of bittering acids, isomerization, and oxidation products in hops and beer

Andre Venter (1); (1) Western Michigan University, Kalamazoo, MI, U.S.A.

Technical Session 6: Hops I
Sunday, August 14  •  3:30–5:15 p.m.
Plaza Building, Concourse Level, Governor’s Square 15

A new direct analysis mass spectrometric method is demonstrated to simultaneously quantitate the alpha- and beta-acid content, as well as the isomerization and oxidation products of the hop acids in hop or beer samples. The bittering potential of hops can be attributed to isomerization of the hop acids and other non-isomerized hop acid products. The total bitterness in beer is usually measured in international bitterness units (IBU). The bitter non-isomerized hop acid products are typically due to oxidation, which is measured by the hop storage index (HSI). The HSI measures the decrease of the alpha- and beta-acid due to oxidation that occurs during storage, which can reduce the isomerization bittering potential of the hops but also lead to additional bittering due to the bittering potential of oxidized hops when dry-hopping beer. Typically, HSI is measured by UV-VIS as the ratio of absorbencies of two wavelengths corresponding to the oxidized and non-oxidized bittering acids. Standard HPLC-UV methods (e.g., ASBC Hops-14) generally does not measure the hop storage index. Here, we present an alternative method for the quantification alpha- and beta-acids and their oxidation products in hops pellets, bracts and extracts for the calculation of bittering potential and hop storage index. The same method can also be applied to beer samples to measure isomerization products and oxidized alpha-acids, for the calculation of IBUs in beer. The proposed method uses direct analysis by electrospray ionization mass spectrometry of prepared samples to calculate bittering potential, HSI or IBUs and other sample attributes in less than 1 min. Hop standards were force oxidized by UV radiation and aeration for 180 min. Calculation of oxidation product concentrations in the standard was based on the assumption of complete oxidation, as verified by ultra-high–performance liquid chromatography mass spectrometry (UPLC-MS). The alpha-, beta-, isomerized and oxidized bittering acids were measured by loop injections and analyzed by ESI-MS without chromatographic separation. The components were quantitated by external calibration after serial dilution of the forced oxidized standards. The HSIs were calculated by the ratios of the sums of the mass percentages of the bittering acids and oxidation products and compared to the UV-Vis calculated HSI values. Similarly IBUs were calculated based on measured iso-alpha-acid and oxidation product concentrations and compared to standard methods for IBU determinations.

Andre Venter heads the WMU Hops Analysis Laboratory, providing chemical services to hops farmers and microbreweries in Michigan and further afield. He completed both his bachelor’s and master’s degrees and received his Ph.D. degree in 2003 from the University of Pretoria, where he developed a comprehensive multidimensional supercritical fluid and gas chromatography (SFC×GC) method for petrochemical and natural product analyses. He worked as a postdoctoral researcher at Purdue University with Prof. Graham Cooks, West Lafayette, IN. At Purdue University he investigated the fundamentals of desorption electrospray ionization mass spectrometry and other ionization methods. At Western Michigan University, he has been an associate professor since 2014. His research group studies spray ionization mechanisms and ambient surface analysis mass spectrometry.