Volatile norisoprenoids as markers of botanical origin of Sardinian strawberry-tree (Arbutus unedo L.) honey: Characterisation of aroma compounds by dynamic headspace extraction and gas chromatography–mass spectrometry

Abstract

In order to characterize and authenticate the aromatic profile of strawberry-tree (Arbutus unedo L.) honey, a dynamic headspace (DHS) extraction, followed by gas chromatography–mass spectrometry (GC–MS) analysis, was performed on 10 Sardinian strawberry-tree (Arbutus unedo L.) honey samples. A total of 28 aroma compounds were identified, but only norisoprenoid compounds such as α-isophorone, β-isophorone and 4-oxoisophorone, were recognized as specific floral origin markers of the strawberry-tree honey. The α-isophorone/β-isophorone ratio varied from 4 to 8, whereas the α-isophorone/4-oxoisophorone ratio was found to range from 11 to 20. The DHS extraction method was proposed as a valid alternative to pollen analysis for floral source detection, especially for products like strawberry-tree honey, characterized by a low pollen content.

Introduction

Assessment of the botanical origin of honey is of great concern in food analysis, since authenticity guarantees the quality of the product, prevents overpayment and helps to identify frauds. In addition, European Community legislation concerning honey i.e. Directive EEC/74/409 amended by the Proposal COM/95/0722 (1996) requires labelling of the floral origin.

Usually the determination of the botanical origin of honey is carried out by melissopalynological analysis, which is based on the identification of pollen by microscopic examination (Louveaux, Maurizio, & Vorwohl, 1978; Sawyer, 1975).

This analysis is time-consuming, requires a very skilful analyst for data interpretation and sometimes results do not allow a reliable identification of the botanical origin, especially in the case of honey characterized by low amounts of pollen, as is the case with strawberry-tree honey. In addition, for this kind of product, assessment of the botanical origin on the basis of pollen analysis is particularly difficult, due to the presence of secondary pollens, since flowers are in an upside-down position. Consequently, due to difficulties in melissopalynological analysis, great attention has been paid to finding specific chemical markers for proving the botanical source of this honey.

Strawberry-tree honey is a typical product of Sardinia, an insular region of Italy; owing to its distinct fragrance and bitter aftertaste, this product is particularly appreciated. Its high economic value, about six times the price of monofloral honeys of other botanical origins, determined by its peculiar organoleptic characteristics and the limited production, makes this product particularly susceptible to sophistication.

On the basis of these observations, chemical analysis of strawberry-tree honey could offer greater reliability in tracing and differentiating botanical origins. Among the non volatile components of strawberry-tree honey, a phenolic compound, i.e. homogentisic acid, has been previously proposed as a marker of the botanical origin of strawberry-tree (Arbutus unedo L.) honey (Cabras et al., 1999).

Since one of the most typical features of honey is its aroma profile, it is used to characterize volatile marker compounds specific to a given botanical origin (Overton & Manura, 1994; Radovic et al., 2001). In particular, the gas chromatographic pattern can be considered as a chemical “fingerprint”, since the nature and relative amount of the compounds present in the volatile fraction are a distinctive feature of different floral sources.

Although honeys of different botanical origin, namely chestnut, eucalyptus, lavender, lime and orange, have previously been characterized on the basis of their volatile fraction (Bonaga & Giumanini, 1986; Bouseta, Collin, & Dufour, 1992), to our knowledge only one previous study regarding the semi-volatile aroma fraction of strawberry-tree honey has been published (Dalla Serra et al., 1999).

In this work, the aromatic fraction of 10 Sardinian strawberry-tree (Arbutus unedo L.) honey samples was extracted using the dynamic headspace (DHS) technique, followed by gas chromatography–mass spectrometry (GC–MS) analysis. The aim of this study was to obtain a “fingerprint” of the aroma of strawberry-tree (Arbutus unedo L.) honey and to find volatile compounds as possible markers of the floral source of this product.