title: Isomerization and degradation kinetics of hop (Humulus lupulus) acids in a model wort-boiling system.
abstract: The rate of isomerization of alpha acids to iso-alpha acids (the compounds contributing bitter taste to beer) was determined across a range of temperatures (90-130 degrees C) to characterize the rate at which iso-alpha acids are formed during kettle boiling. Multiple 12 mL stainless steel vessels were utilized to heat samples (alpha acids in a pH 5.2 buffered aqueous solution) at given temperatures, for varying lengths of time. Concentrations of alpha acids and iso-alpha acids were quantified by high-pressure liquid chromatography (HPLC). The isomerization reaction was found to be first order, with reaction rate varying as a function of temperature. Rate constants were experimentally determined to be k1 = (7.9 x 10(11)) e(-11858/T) for the isomerization reaction of alpha acids to iso-alpha acids, and k2 = (4.1 x 10(12)) e(-12994/T) for the subsequent loss of iso-alpha acids to uncharacterized degradation products. Activation energy was experimentally determined to be 98.6 kJ per mole for isomerization, and 108.0 kJ per mole for degradation. Losses of iso-alpha acids to degradation products were pronounced for cases in which boiling was continued beyond two half-lives of alpha-acid concentration.
authors.affiliation: Department of Food Science and Technology, Oregon State University, 100 Wiegand Hall, Corvallis, Oregon 97331-6602, USA.
authors.forename: Mark G
authors.initials: MG
authors.lastname: Malowicki
authors.forename: Thomas H
authors.initials: TH
authors.lastname: Shellhammer
mesh_terms: D000143:Acids / Q000737:chemistry*; D002851:Chromatography, High Pressure Liquid; D053138:Cyclohexenes; D006358:Hot Temperature; D027582:Humulus / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007536:Isomerism; D007700:Kinetics; D012996:Solutions; D013649:Taste; D013729:Terpenes / Q000737:chemistry; D013816:Thermodynamics
chemical_list: D000143:Acids; D053138:Cyclohexenes; D012996:Solutions; D013729:Terpenes; C007966:humulon