Degradation kinetics of iso-alpha-acids

Hops Session
Mekonnen Gebremariam, Technical University Munich, chair of Brewing and Beverage Technology, Freising-Weihenstephan
Co-Author(s): Yarong Huang, Martin Krottenthaler, and Thomas Becker, Technical University Munich, Chair of Brewing and Beverage Technology, Freising-Weihenstephan, Germany

ABSTRACT: The kinetics of the degradation of iso-alpha-acids has been studied at four pH levels between 4.5 and 6.5 at different boiling temperatures (90–130°C) during a wort boiling time of 0–360 min. This research seeks to discover the kinetics of degradation of iso-alpha-acids as a function of temperature and pH value. The aim of this study was to investigate the free energy of activation and the reaction rate constant (k) of the degradation of iso-alpha-acids. A purified iso-alpha-acids extract in different pH buffered aqueous solution was used. The determination of the reactions order was computed by a program compiled in Matlab R2007a language: first order. We developed a compact 10-min HPLC method for the analysis of the concentration of total iso-alpha-acids, isocohumulone, isohumulone, and isoadhumulone. It was found that increasing the pH value decreases the free energy of activation. A typical plot of the natural logarithm of the iso-alpha-acids concentration in percentage versus time indicates that this reaction follows an Arrhenius equation. The Arrhenius equation demonstrates that the reaction energies of degradation of total iso-alpha-acids, isocohumulone, isohumulone, and isoadhumulone are nearly identical. The acyl side chain at the C2 atom of the iso-alpha-acids has no effect on splitting the isohexanoyl side chain at the C4 atom during the boiling process. Raising pH values from 4.5 to 5.5 and from 5.5 to 6.5 decreased the reaction energy by approximately 20 kJ/mol. We developed a kinetic model for the irreversible consecutive reaction; iso-alpha-acids are isomerized from alpha-acids by the application of heat in solution and, at the same time, iso-alpha-acids are degraded to humulinic acids and other compounds. This kinetic model fit the experimental data and allows the determination of all relevant reaction constants. This kinetic model is useful in calculating the precise point of time to add hops into the boiling kettle.

Mekonnen Melaku Gebremariam received his B.S. degree in chemistry from Debub University, Ethiopia. He began employment with the Ethiopian Ministry of Education in July 2000 as a chemistry teacher in the South Nations and Nationality People Region. He terminated his contract agreement with the Ministry of Education after four years. He next was employed as a chemist in the Federal Micro and Small Enterprises Development Authority. After 18 months with this company, he terminated the contract agreement and joined Addis Ababa University for further studies. He graduated from Addis Ababa University, Ethiopia, in 2007 with an M.S. degree (with great distinction) in food engineering. Immediately after graduation he was employed as a lecturer and researcher by Hawassa University, Ethiopia. After about two-and-a-half years of work at Hawassa University, he went to Germany for his Ph.D. studies with the support of his employer, Hawassa University. Currently he is pursuing his doctoral studies at the Technical University of Munich, Germany.