Kinetic Modeling of Hop Acids during Wort Boiling

The thermal degradation of iso-alpha-acids wasinvestigated at four pH levels between 4.5 and 6.5 at differentboiling temperatures (90-130 °C) during the wort boiling time(0-360 min). The free energy of activation and the reaction rateconstant (k) of the degradation of iso-alpha-acids werecalculated. Raising pH value from 4.5 to 5.5 and from 5.5 to 6.5decreased the reaction energy about 20 kJ/mol. The relativeproportion of isocohumulone stays almost constant. Wedeveloped a kinetic model for the irreversible consecutivereation; iso-alpha-acids are isomerized form of alpha-acids bythe application of heat in solution and at the same timeiso-alpha-acids are degraded to humulinic acids and othercompounds. This kinetic model is useful to calculate time toadded hops into boiling kettle.