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.
