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.
Highlights
Hops (Humulus lupulus L.) are essential for the brewing process in order to confer the singular sensory properties of beer.[1,2] Terpenes are the principal constituents of hops essential oil, which make up to 3% (v/m) of the hop cone.[3]
We report the reactivity of hop-derived terpenes (1-16) toward 1-hydroxyethyl radical (HER), the predominant radical formed by thermal oxidation during beer brewing process, storage and aging.[14]
The apparent second-order rate constants for the reaction among selected terpenes and HER were determined by means of a competitve kinetic approach,[23,24] according to the procedure previously reported in the literature.[15,16]
Summary
Hops (Humulus lupulus L.) are essential for the brewing process in order to confer the singular sensory properties of beer.[1,2] Terpenes are the principal constituents of hops essential oil, which make up to 3% (v/m) of the hop cone.[3]. Terpene hydrocarbons like β-myrcene (1), α-humulene (7), β-caryophyllene (8), and β-farnesene (9) have been shown to be the main components of hop oil,[3,6] the predominant terpenes in finished beer are terpene alcohols due their hydrophilic properties,[6] especially linalool (14) and geraniol (13), which have been found in appreciable concentrations (ranging from 1 to 906 μg L−1) depending on the beer type.[4,7,8,9] several studies have
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