Mechanism of hop-derived terpenes oxidation in beer

Abstract

Terpenes are the main constituents of hops essentiol oil and contribute to the singular sensory properties of beer. However, terpenes are sensitive to oxidation leading to quality loss during beer aging. Herein, the reactivity of terpenes towards 1-hydroxyethyl radical has been determined employing a competitive kinetic approach using the spin-trap α-(4-pyridyl-1-oxide)-N-tert-butylnitrone (4-POBN). The apparent rate constant (kapp) for the reaction of terpenes with 1-hydroxyethyl radical ranges from (3.9 ± 0.2) × 105 to (1.5 ± 0.2) × 107 L mol -1 s-1 for β-pinene and terpinolene, respectively. The reaction involves hydrogen atom transfer from the terpene to 1-hydroxyethyl radical rather than electron-transfer and the rate constant is shown to be dependent on the number of allylic and benzylic hydrogen atoms and on the value of the bond dissociation enthalpy for the weakest C-H bond. The results provide a better understanding on the mechanism behind terpene decomposition in beer brewing and aging process and may further contibute to improve the oxidative stability of the herb-flavored beverages.