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Supercritical Fluid Extraction of Terpenoids and Steroids from Plants

12 Béla Simándi , Ágnes Kéry

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Introduction

Terpenoids and steroids constitute the largest known group of plant secondary metabolites. The different groups of terpenes are formed from isoprene (C5) units in enzymatic reactions. Head-to-tail condensation of isoprene units results in the formation of monoterpenes (C10), sesquiterpenes (C15), diterpenes (C20), sesterpenes (C25) and polyterpenes. Tail-to-tail coupling of C15 and C20 units leads to the precursors of triterpenes (C30) and carotenes (C40) [1].

Supercritical fluid extraction (SFE) processes have gained increasing interest in production of high-value plant extracts for pharmaceutical, cosmetic and food industries in recent years. A large number of books and review papers have been published [2-5]. There are several reviews of systems, for which high-pressure phase- equilibrium data have been published [6-8]. The comprehensive book of Stahl and coworkers [2] contains a systematic review about the SFE of terpenes.

This chapter describes a recent period of supercritical fluid extraction of botanical feedstocks. The natural terpenoids are discussed subsequently. It is demonstrated that most plants contain a wide spectrum of biologically active compounds which can be dissolved or precipitated selectively. Some of the most important trends affecting the uses of SFE extracts are identified.

Monoterpenes and Sesquiterpenes

Mono- and sesquiterpenes and their oxygen containing derivatives are the major constituents of essential oils. Essential oils are defined as products obtained from a plant starting material, either by steam distillation, or by mechanical pressing (e.g. citrus oils). The essential oil is subsequently seperated from the aqueous phase by physical methods. Various non-terpene components such as phenylpropanoids (C6-C3), aliphatic hydrocarbons, acids, alcohols, esters, nitrogen- or sulfur-containing compounds are also present in essential oils.

As low molecular lipophilic substances with high vapour pressure, the essential oil constituents are well soluble in liquid or supercritical carbon dioxide [9,10]. Hence, essential oil components have been extracted from a wide range of plant materials, and have found applications in many product areas [11]. A comprehensive review of published literature on the yields, major components and major applications for 42 botanical extracts was published by Moyler [12]. Supercritical fluid extraction and fractionation of essential oils were reviewed by Reverchon, recently. The literature in this field covering the period from 1991 to 1995, was also summarized in this paper [5]. Recent publications on SFE of plants containing essential oils, starting from 1996, are summarized in Table 1.

Comparison of SFE extracts with distilled essential oils results that the composition of the products is determined by the isolation methods. Normal steam distillation usually results in chemical changes, loss of the lightest components and loss of certain water-soluble components. Carbon dioxide extracts are closer in composition to the natural essential oils present in the plant materials. Alteration of volatile components during distillation can be recognized by comparing the oils obtained by steam distillation and SFE. The hydrolysis of esters to the corresponding alcohols was observed in lavandin and clary sage oils. The hydrolysis of thymol bound in glycosides resulted higher thymol concentration in thyme essential oils. During the distillation of the oil of chamomile flowers the blue chamazulene is formed from the colourless matricine.

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of waxes and resins are co-extracted with the volatile components. This is generally not a disadvantage in flavouring and perfumery applications.

Removal of waxes can be solved by stagewise precipitation of products using two or more separators in series. The separators are operated at different pressures and temperatures to precipitate the resinous compounds and volatile components in different vessels. This method was suggested by Stahl and co-workers [62]. Recently, this fractional separation technique has been refined by Reverchon and co-workers [63,64] for selective precipitation of cuticular waxes and volatile oils. Fractionation of volatile and fatty oils can be also carried out by this method [29,62].

Department of Chemical Engineering, Budapest University of Technology and Economics, Budapest, H-1521, Hungary Tel: + (36 1) 463 1490 Fax: + (36 1) 463 3197

E-mail: simandi.vmt@chem.bme.hu

Department of Pharmacognosy, Semmelweis University, Budapest, Hungary

Extraction of essential oil components can be carried out with liquid CO2 (60-70 bar, 5-10°C) or with supercritical CO of low densities (0.4-0.5 g/cm3, e.g. at 90 bar and 40°C). At these conditions the light fractions

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