The chemical relationship between the scent features of goldenrod (Solidago canadensis L.) flower and its unifloral honey

Abstract

The marker compounds of Solidago origin in goldenrod flower and honey have been investigated. After Likens–Nickerson simultaneous distillation–extraction (SDE) the extracts were dehydrated and concentrated, then thoroughly analysed by GC–MS. Among the common compounds of the flower and honey, germacrene-D proved to be the most suitable for identification of the goldenrod origin. The presence of this constituent creates a unique sesquiterpene spectrum in aroma extracts of the flower and honey, making the recognition more precise and reliable. The aroma structure examinations revealed the characteristic scent pictures of the flower and its unifloral honey and proved that no direct “morphological” similarity/identity exists between the aromaspectra of the two fragrance extracts (honey contra flower), except the presence of the common compounds. The results of the investigations covering the 2005 and 2006 apiarian seasons showed excellent repeatability and reliability.

Introduction

Increasing focus on the health-promoting effects of foods of natural origin has resulted in an abundance of research on characteristics of specific products. Honey has received significant attention in this context (Molan, 2002, Molan, 2006, Cutting, 2007, Bogdanov et al., 2008). While hitherto only “common” (e.g. polyfloral) honeys such as locust (false-acacia, Robinia pseudoacacia L.) or sunflower have been examined in Hungary (Farkas and Zajácz, 2007), rising consumer demand has prompted research on less common unifloral honeys, such as those of lavender, sweet chestnut (Castanea sativa L.), goldenrod (Solidago) or Limonium. Several articles have been published on the composition and features of unifloral honeys, primarily the aspects that characterize the botanical or geographical origin that are collected from different bee-pastures (Bogdanov et al., 2004, Persano Oddo and Piro, 2004a). Volatile and non-volatile substances play an important role in the aroma and nutritional value of honey. Primarily, volatile components create the aroma-structure of a honey, thus aroma investigations aim to determine these compounds. Furthermore, the chemical constituents that characterize the floral scent may also identify the botanical origin of the honey. The price of honey depends on its source, the rarer honey having the higher price. According to the Codex Standard for Honey (Revised Codex Standard for Honey, 2001) and the European Union Council Directive relating to honey (2001) the use of a botanical designation of honey is allowed only if it originates predominately from the indicated floral source. Until recently, however, it was not possible to prove the authenticity of honeys of floral origin (Persano Oddo and Bogdanov, 2004b).

Goldenrods are members of the genus Solidago L. (Compositae: Astereae), which includes about 100 species of North American wildflowers and more than a dozen species inhabiting South America, Europe and Asia. All species are herbaceous and most of them difficult to distinguish from one another. All goldenrods are late bloomers, flowering in late summer into the fall. They are common along the edges of moist forests, roadsides and meadows. S. gigantea was introduced to Europe in the 18th century as an ornamental plant and became invasive some 100 years later. Currently, it is generally considered a harmful invasive plant, and is widely spread in the Dunántúl region of Hungary and across the largest part of county Zala. As an “invasive alien” herb it suppresses the original, indigenous plant cover, pervasively and abundantly pullulating wherever no agricultural activity is occurring. While there are approximately 130 species of goldenrod in two genera, with 3 of these widespread in Hungary, the dominant species in Hungary is Solidago canadensis.

Goldenrod serves as bee-pasture in a period of the year when the majority of the plants have already ceased flowering. Therefore, these plants blooming until September represent the last opportunity for nectar collecting by honeybees. Although Solidago is an inextirpable weed, it might be useful for bee-grazing, similar to Asclepias syriaca (Common milkweed) in spring.

Until now, the most completely examined chemical constituents of goldenrod have mainly been color components, primarily flavonoids (Apáti et al., 2002), and antioxidants for their activity (Skrzypczak et al., 1999, Apáti et al., 2003). Since Solidago is also a medicinal herb (Leuschner, 1995) used for kidney and prostate gland complaints (Wright et al., 2007) and as an antibiotic (Thiem and Goslinska, 2002), some data on its essential/volatile oil composition is available. Kalemba et al. (2001) investigated the volatile oil of Solidago gigantea and identified 95 components. The isolate was prepared from the flowers and leafed stem, since Solidago has a panicled (compound) inflorescence and cannot be analysed without stalk. The main compounds of the volatile oil were (−)-germacrene-D, α-pinene, myrcene, p-cimene, (−)-bornyl-acetate, α- and γ-gurjunene, (−)-ledol, eudesma–4(15)7-diene-1-β-ol and (−)-cyclocolorenone. Solidago has also been described previously as a rich germacrene source, with (−)-germacrene-D considered to be the source of sesquiterpene-biosynthesis, giving rise subsequently to the formation of elemane, guaiane, germacrane, eudesmane, cadinane under the influence of acids, heat or light (Bülow and König, 2000). Researchers assume that (−)-germacrene-D might play the role of precursor of the above substances in nature as well. The previously non-recognized molecules of β-Ylangene structure in Solidago canadensis were identified as 6-epi-α-cubebene and 6-epi-β-cubebene by Kasali et al. (2002), as well as the following components: trans-2-hexenol, α-pinene, camphene, β-pinene, myrcene, limonene, bornyl-acetate, α-copaene, β-elemene, β-caryophyllene, α-humulene, germacrene-D, β-selinene, bicyclo-germacrene, δ-amorphene, γ-cadinene, germacrene-B, 6-epi-cubenol (Kasali et al., 2002).

There are no literature reports on the composition of Solidago honey as a health remedy according to the nature of its parent plant. One goal of this study was therefore to identify aroma compounds of the honey, as well as to seek for marker compounds (i.e. common compounds in flower and honey, or special compounds, that are characteristic of the goldenrod honeys).

Goldenrod honey is considered one of the significant European unifloral honeys (Persano Oddo et al., 2004c). It has a yellowish-gold color, is viscous (flows densely) and is prone to crystallize, and therefore solidifies with a fine particle size. Its taste is aromatic, mild, and characteristic. The honeys of the 3 goldenrod species growing in Hungary (S. gigantea Ait., S. virgaurea L., and S. canadensis L.) are processed together. Solidago species spreading rapidly in the last 20–25 years can be found in the galleries or flood basins of almost all Hungarian rivers, but unifloral honey from them has been collected regularly in only two regions, West-Dunántúl (Győr-Moson-Sopron County, mainly Hanság; Vas County, flood plains of rivers primarily) and South-Dunántúl (Somogy County, mainly Nagy-berek; Baranya County, e.g. Dráva-basin). The importance of goldenrod honey changes from season to season. In good years when the summer nectar flow of other flowers is plentiful, beekeepers do not collect Solidago honey; in bad years when the honey production is not satisfactory, they travel even from distant parts of the country to collect this nectar as a last chance of harvest.

Surprisingly, no literature data on the volatile compounds of Solidago honey are available, although a large quantity is produced in America where the plant originated and where it still grows abundantly. Unripe Solidago honey (i.e. when the maturing activity of bees is not finished) has an unpleasant smell that is recognized by bears and leads them to the hives where they rob the bees (Root and Root, 2005).

Although honey of goldenrod (Solidago canadensis L.) origin is a growing Hungarian speciality due to the fast spread of the plant and the unique taste of the product, little research exists on its scent features, which contribute to identification of its aroma as well as botanical origin. Therefore, our research reported focused on characterization of this honey first by isolation and gas chromatographic separation and detailed mass spectrometric identification of goldenrod flower and honey fragrance compounds, then evaluation of the recorded total ion chromatograms (TICs) with individual identification procedure (manual-mode, applying background compensation of the appropriate kind) for each constituent to search for unique “marker” compounds and construction of relative aromagrams (aromaspectra).