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.
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