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Publication Title | Mechanism of Hop-Derived Terpenes Oxidation in Beer

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http://dx.doi.org/10.5935/0103-5053.20150231

J. Braz. Chem. Soc., Vol. 26, No. 11, 2362-2368, 2015. Printed in Brazil - ©2015 Sociedade Brasileira de Química 0103 - 5053 $6.00+0.00

Article

Instituto de Química de São Carlos, Universidade de São Paulo, Av. Trabalhador São-Carlense 400, CP 780, 13560-970 São Carlos-SP, Brazil

Mechanism of Hop-Derived Terpenes Oxidation in Beer

Natália E. C. de Almeida, Inara de Aguiar and Daniel R. Cardoso*

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- avored beverages.

Keywords: beer, 1-hydroxyethyl radical, terpenes, kinetics, oxidation

Introduction

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 It is well-known that the composition of hops essential oil depends on the hop genotype, being identi ed over 200 compounds in its essential oil, some of them have been used for distinguishing among different hop varieties.3-5 The major terpenes present in hop and beer basically comprise monoterpenes (C10) and sesquiterpenes (C15), which exhibit strong sensory qualities.5 The chemical structure of the major brewing terpenes is illustrated in Figure 1, which includes typical terpene hydrocarbons (1-9), terpene epoxide (10) and terpene alcohols (11-16).

Although 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 nished 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.4,7-9 Moreover, several studies have

*e-mail: drcardoso@iqsc.usp.br

reported that geraniol (13), nerol (12) and linalool (14) are biotransformed by yeast during the fermentation giving rise to β-citronellol (11) and α-terpineol (15), respectively.5,6,10 These yeast biotransformation products are responsible for the typical hoppy aroma of beer.5,6,10 However, terpenes are sensitive to oxidation,11-13 which would result in the loss of beer sensory quality and could yield oxidation products that may display unpleasant organoleptic properties. In view of that, herein 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

Experimental

Chemicals and materials

Acetonitrile and ethanol were of HPLC grade and purchased from Panreac (Barcelona, Spain). Ferric chloride tetrahydrate (FeCl2.4H2O) and hydrogen peroxide 30% ACS grade were purchased from Merck (Darmstadt, Germany). α-Humulene, (+)-α-pinene, α-(4-pyridyl- 1-oxide)-N-tert-butylnitrone (4-POBN), α-terpineol, β-caryophyllene, β-citronellol, β-myrcene, (−)-β-pinene, p-cymene, 1,4-cineole, cis-3,7-dimethyl-2,6-octadien- 1-ol (nerol), ferrocene, formic acid, geraniol, limonene,

A

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