banner 31. Direct ESI-MS quantitation of bittering acids, isomerization, and oxidation products in hops and beer for calculation of the hop storage index and international bitterness units

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

Technical Session 9 - Hop Flavor and Analytics
Wednesday, June 17
8:30–10:15 a.m.
Fiesta 3,4,6,8

A new direct analysis mass spectrometric method is demonstrated to simultaneously quantitate the α- and β-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 α- and β-acids 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 do not measure the hop storage index. Here, we present an alternative method for the quantification α- and β-acids and their oxidation products in hops pellets, bracts, and extracts for the calculation of bittering potential and HSI. The same method can also be applied to beer samples to measure isomerization products and oxidized α-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, HIS, or IBUs and other sample attributes in less than 1 minute. Hop standards were force oxidized at 80°C for 48 hours or subjected to direct sunlight for up to 21 days. 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 α-, β-, 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 force 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-α-acid and oxidation product concentrations and compared to standard methods for IBU determinations.

Gregg Hasman, Jr. studied chemistry at Michigan Technological University (Houghton, MI), where he received his B.S. degree in chemistry in 2011. He taught Studio Chemistry Lab I and II from 2010 to 2012 under the guidance of Paul Charlesworth at Michigan Technological University. Gregg is currently in his third year of the Analytical Chemistry Ph.D. program at Western Michigan University (Kalamazoo, MI) studying under Andre Venter. In mid-2013, Gregg supervised the start-up of the hops testing facility at Western Michigan University, which offers standardized hop analysis according to the official methods of analysis of the American Society for Brewing Chemists (ASBC). Gregg’s current research involves the development of novel methods of hops analysis using electrospray ionization mass spectrometry to rapidly quantitate and identify characteristic components within various breeds of hops.