Filbertone

Filbertone is (E)-5-methylhept-2-en-4-one, also named 5-methyl-2-hepten-4-one. It has been reported as the primary odorant of roasted hazelnuts, with a nutty, roasty, hazelnut-like aroma. It is designated as generally recognised as safe (GRAS) for use in food products.

The molecule was only identified in hazelnuts in 1989. A paper of 1989 reported the isolation, synthesis and absolute configuration of filbertone. A paper of 1991 on chirospecific analysis in flavour and essential oil chemistry carried filbertone in its title as the character impact compound of hazelnuts.

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Chirality

Filbertone is chiral and exists as two mirror-image isomers, the (-)-(E,R) and (+)-(E,S) enantiomers. The enantiomers have been separated by chromatography on a chiral column. The odor threshold of the (S)-enantiomer has been estimated as 10-fold lower than that of the (R)-enantiomer. In nature there is a slight excess of the (S)-isomer, and the amount varies depending upon the source of the filberts. The ratio between the two enantiomers is variable and depends on the origin of the hazelnuts, thermal treatment, analytical sample collection, and technological processing. The enantiomeric excess in the extracted filbertone is greater under mild extraction conditions.

An excess of the (S)-enantiomer was found in both raw and roasted samples, with larger differences in raw samples. The amount of both (R)- and (S)-filbertone increases during roasting. The enantiomeric composition of filbertone proved discriminant for origin authentication.

Natural Occurrence

Filbertone naturally occurs in raw hazelnuts, but its content extensively increases during roasting. In a model experiment, the content increased at 180 °C from 1.4 to 660 μg/kg in 9 minutes, and to 1150 μg/kg in 15 minutes. More than 70 compounds were detected in roasted hazelnuts, among them 5-methyl-2-hepten-4-one.

Filbertone is not the only molecule responsible for the smell of hazelnuts. A paper of 2010 on Italian hazelnuts identified 37 odour-active compounds in the raw nuts. A larger number, 46 aroma-active compounds, was detected in roasted nuts. Filbertone showed significant variations between three Italian cultivars.

Potency and Taste Thresholds

Filbertone is distinguished by a surprisingly low taste threshold. In 3% sucrose solution the perception threshold is at about 5×10⁻⁶ ppm and the recognition threshold at about 30×10⁻⁶ ppm. At the perception threshold the taste is described as soft, buttery, with a full sensation in the mouth. At and above the recognition threshold it is described as nutty, hazelnut, soft, buttery, with a full sensation in the mouth.

Marker in Hazelnut Products

Filbertone was found to be a proper marker for the quality sorting of commercial hazelnut spreads. Spreads with more than 10% hazelnuts had filbertone above 45 μg/kg, and spreads with less than 1% hazelnuts had less than 4 μg/kg. Because of the high variability of filbertone content, it cannot be used as a single authenticity marker or for the precise quantification of hazelnut content.