Polyphenol composition and antioxidant and metal chelating activities of the solid residues from the essential oil industry

Highlights

• Polyphenols and antioxidant and chelating activities of aromatic plants were studied.

• Yield of extraction and total polyphenols largely depended on the method of extraction.

• Significant antioxidant activity of the residues was found, especially T. mastichina extracts.

• C. ladanifer extract was the most effective iron chelator.

• Solid residues from essential oil industry are valuable source of bioactive compounds.

Abstract

Essential oils of aromatic plants have numerous applications in fields like medicine, cosmetic and food industry. Nevertheless, the average yield of essential oil distilled from aromatic plants is below 5% (w/w) and consequently a considerable amount of solid residues is generated. These residues are especially rich in polyphenols that can be exploited as health-promoting compounds in food and feed or as anti-aging ingredients in cosmetic products, and thus to enhance the overall profitability of the aromatic plants. In this work, we have tentatively identified the polyphenols present in solid residues from the steam distillation of Cistus ladanifer, Lavandula × intermedia, Santolina rosmarinifolia and Thymus mastichina, and evaluated their antioxidant and chelating activities by means of several in vitro methods, such as the β-carotene/linoleate model system, reducing power, DPPH radical scavenging, and iron and copper ion chelation. A Soxhlet extraction and an ultrasound-assisted extraction have been used in terms of comparison. Most of the extracts whether they were obtained by Soxhlet system or by ultrasonic method showed good antioxidant activities. Soxhlet ethanolic extract of T. mastichina showed an antioxidant activity comparable to the standard red grape pomace. The influence of the total polyphenol content on the chelating activities was not as important as on the antioxidant activities and the predominant role of certain polyphenols and/or the presence in the extracts of other compounds with a favourable chemical structure to metal complexation can be assumed. This was the case of the ultrasound ethanolic extract of C. ladanifer, which was the most effective in iron chelating activity despite its medium-to-low content of total polyphenols. All these data point to profit from these aromatic plant solid residues as a valuable source of bioactive compounds.

Introduction

Aromatic plants have been widely used from ancient times in medicine, cosmetics and for preserving and improving the flavour of foods. Their beneficial properties have been mainly attributed to the presence of essential oils, which are complex mixtures of compounds belonging to diverse chemical families including terpenes, alcohols, aldehydes, phenolic compounds, esters, ethers and/or ketones. These compounds are well characterized in most of the aromatic plants and numerous publications have been devoted to their antioxidant, antimicrobial, antitumor and bioplaguicide properties (Gould, 1997, Miguel, 2010, Santana et al., 2012).

Essential oils are usually obtained from raw plant materials (flowers, buds, seeds, leaves, twigs, bark, herbs, wood, fruits and roots) by distillation with water and/or steam. The average yield of essential oil distilled from aromatic plants is below 5% (w/w) and consequently a considerable amount of both liquid and solid residues is generated during the industrial processing (Santana-Méridas et al., 2012). These residues may result in environmental concerns if they are not properly managed, but they are also a significant source of bioactive compounds that can increase the overall profitability of the aromatic plants improving the rational utilization of natural resources.

Residues from distillation are especially rich in polyphenols that can be exploited as health-promoting compounds, as antioxidants in food and feed or as anti-aging ingredients in cosmetic products (Navarrete et al., 2011, Torras-Claveria et al., 2007). However, the literature concerning chemical and biological characterization of these residues is scarce especially in the case of solid residues. Torras-Claveria et al. (2007) have detected flavonoids and hydroxycinnamoylquinic acid derivatives with antioxidant activities in lavandin (Lavandula × intermedia Emeric ex Loiseleur) waste and Parejo et al. (2004) have reported the separation and characterization of phenolic compounds in the remaining material after distillation of fennel (Foeniculum vulgare Mill.). More recently, phenolic compounds with antioxidant, antiradical and antiacetylcholinestarase activities have been described in spent flowers and green leaves of oil-bearing rose (Göktürk and Baydar, 2013) and in decoction waters of some aromatics plants (Albano et al., 2012).

Since much of phenolic compounds are not volatile, are not degraded with thermal treatments and remain in the aromatic plants wastes after distillation of raw plant materials (Torras-Claveria et al., 2007). Aerial parts of Cistus ladanifer L. show abundance of hydrolysable tannins derived from gallic and ellagic acids, and flavonoids like quercetin, kaempferol and apigenin derivatives (Barrajón-Catalán et al., 2011, Barros et al., 2013, Chaves et al., 1998, Fernández-Arroyo et al., 2010), whereas Lavandula × intermedia is characterized by the presence of phenolic acids and procyanidins (Blažeković et al., 2010). To our knowledge, the polyphenol composition of aerial parts of Santolina rosmarinifolia L. and Thymus mastichina L. has not been yet investigated. However, Thymus species have been reported as sources of a large variety of phenolic acids and flavonoids, which play important role as antioxidants (Roby et al., 2013) and as taxonomic markers providing distinction of species (Boros et al., 2010, Corticchiato et al., 1995, Marin et al., 2003). Literature on the polyphenol composition of the genus Santolina is very scarce although some flavonoids like apigenin, luteolin and quercetin have been described as components of Santolina oblongifolia Boiss. with anti-inflammatory activity (Silván et al., 1996).

In this work, we have identified the major polyphenols present in solid residues after the steam distillation of C. ladanifer, Lavandula × intermedia, S. rosmarinifolia and T. mastichina, and evaluated their antioxidant and chelating activities by means of several in vitro methods, such as the β-carotene/linoleate model system, reducing power, DPPH radical scavenging, and iron and copper ion chelation. Two methods of extraction have been used: Soxhlet extraction and ultrasound-assisted extraction. The objective is to investigate the potential of such residues as source of antioxidant compounds in order to improve the added value of these aromatic plants.