20. The impact of solution-state equilibria on the rate of alpha-acid isomerization

Paul Hughes (1); (1) Oregon State University, Corvallis, OR, 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

The recent revision of the absolute configurations of naturally occurring alpha-acids and their isomerized counterparts in some ways simplifies the rationalized mechanism for the isomerization mechanism. Essentially, the revised stereochemistry invokes conservation of the stereocenter in the alpha-acids, so that the tertiary carbinol retains the same S-configuration and partial racemization at the “5” position, resulting in the formation of cis- and trans-iso-alpha-acids. Nevertheless, there are some unanswered questions regarding alpha-acid isomerization. Firstly, alpha-acids exist as a set of keto-enol tautomers, which influences the electronic distribution around the alpha-acid system. Secondly, alpha-acids are dibasic, so they can effectively exist as free acids, monoanions and dianions, which is presumably important as isomerization is much more efficient at higher pH values. Thirdly, certain metal cations, such as magnesium(II) can accelerate isomerization rates substantially. Here we have carried out a thorough theoretical evaluation of the solution state of alpha-acids and their mono- and dianions, as well as the various possible magnesium chelates, using well-established computational chemistry procedures. We have identified a small population of free acids, monoanions and dianions that form the basis of alpha-acids in solution, and we have further tentatively identified a number of key features in these populations that point to potential opportunities to increase the rate and specificity of the isomerization reaction.

Paul gained bachelor and Ph.D. degrees in chemistry before joining the U.K. Health and Safety Executive in 1988. In 1990 Paul moved to the Brewing Research Foundation, where he worked on and managed a wide range of research projects, including the sensory and foaming properties of hop acids, LOX control in malts and malting and protein–polyphenol interactions. In 1999 Paul moved to Heineken Technical Services in the Netherlands as principal scientist. He was responsible for a number of projects and aspects of product safety and integrity. In 2005 Paul took up the position of professor of brewing at Heriot-Watt University, before assuming the role of director of the International Centre for Brewing and Distilling in 2006 and director of research for the School of Life Sciences in 2008. Today Paul is assistant professor of distilling at Oregon State University, where he leads research, education and outreach on distilled spirits. Paul holds an MBA from the University of Surrey and the IBD diploma in brewing. He is the author of more than 60 research papers and of four patents and is co-author of two textbooks on beer quality and whisky. He is a frequent speaker at international conferences and runs international courses, principally focused on spirit production and quality.