Effect of Flow Rate on the Extraction of Volatile Concentrates and Resinoid Compounds from Origanum vulgare L. ssp. virens (Hoffm. et Link) letswaart using Compressed CO2

Abstract

The effect of CO2 flow rate on the extraction of the volatile concentrate and resinoid compounds from Origanum vulgare L. ssp. virens (Hoffm. et Link) letswaart was studied at liquid (300 K, 70 bar) and supercritical (310 K, 100 bar) conditions. The mass flow rate of CO2 was varied between 0.3 kg/h to 0.9 kg/h with 0.2 kg/h increments. The degree and rate of extraction of the volatile concentrate was little dependent on CO2 flow rate and was controlled by intraparticle resistances. An extract slightly richer in volatile concentrate was collected at liquid CO2 conditions. The extraction of the resinoid compounds was mostly dependent on the flow rate of CO2 and was limited by their solubility in compressed CO2.

Key Word Index

Origanum vulgare ssp.virens, Lamiaceae, supercritical CO2 extraction, CO2 flow rate.

Introduction

Extraction using compressed fluids, particularly carbon dioxide, has become an established method for separating volatile concentrates from plants. It is often regarded as an alternative to the more widely used and traditional techniques of steam distillation and organic solvent extraction, though the initial high capital costs often place it beyond the financial means of most processors. A supercritical extract from an aromatic herb is comprised of the volatile concentrate (similar in composition to the essential oil obtained from distillation methods) together with other co-extracted resinoid materials (often referred to as cuticular waxes). This extract generally depends on the herb material and on the process parameters, such as applied pressure and temperature. The attractiveness of a supercritical fluid as a solvent lies in its unique properties: liquid-like densities, high rates of mass transfer and a tunable selectivity, which arises from varying the density by changes in temperature and pressure. In comparison to the traditional techniques, a supercritical extract has a fingerprint closer to that of the natural source (1,2) and is not contaminated with solvent residue.

The effect of the supercritical fluid flow rate on the extraction rates from a plant material can be used to determine whether an extraction is limited primarily by solubility and chromatographic retention of the solute or by the rate of transport of solutes from the matrix to the bulk of the extraction fluid (3). In the first case, the increase of flow rate would result in a clear increase of the extraction rate. Moreover, if the extraction rate is controlled only by solubility, the extraction rate is directly proportional to the solvent flow rate. On the other hand, if intraparticle resistances are the only restriction, the extraction rate is controlled by the kinetics of the intraparticle transport, and consequently, not dependent on the solvent flow rate. Supercritical fluid extraction of volatile concentrates from herbaceous matrices have been reported as being limited by intraparticle resistances [see, for example, Naik et al. (4), Goto et al. (5), Reverchon et al. (6), Reverchon (7), Catchpole and Grey (8)]. However, an efficient pre-treatment of herbaceous matrices is required if acceptable yields and rates of extraction of the volatile concentrates are to be obtained. Communition of an herbaceous matrix prior to extraction has been shown to reduce dramatically the internal resistances; this affects the extraction of volatile concentrates by increasing the amount of solutes accessible to the solvent (9). A significant effect of solvent flow rate on the extraction of the resinoid compounds present within the herb is also to be expected. Resinoid components are present at the surface of the herb matrix, and therefore, their extraction is likely to be controlled by solubility and/or film resistance.

The effect of solvent flow rate on the extraction of volatile concentrates and resinoid compounds from a selected herb was studied under two different conditions, using liquid CO2 at 70 bar and 300 K and supercritical CO2 at 100 bar and 310 K. These two selected conditions also presented an opportunity to illustrate the effect of CO2 state at similar densities (712 and 689 kg/m3 for liquid CO2 and supercritical CO2, respectively) on the extraction rates and yields. The herb used in the present work was Origanum vulgare L. ssp.virens (Hoffm. et Link) letsioaart. Essential oils produced by several species of the Labiatae (L.) family constitute an important group of economic plant products, though this actual species has not achieved economic importance. The herb was selected on grounds of its supply cost and the importance of its family as a whole. Experimental tests were undertaken on a communited sample of the herb with an average bract size of 0.36 mm.

Experimental

Preparation of the herb matrix: The wild herb was collected after the flowering period (late July) from Serra d'Arrabida region (central west Portugal). The sun-dried herbs arrived as intact plants, and a batch was prepared by removing the bracts by hand. Care was taken to discard any stalk-like material from the sample batch. Typically, 500 g of material was prepared for a batch and, after sieving, a mean bract size of 1.55 mm was obtained. The bracts were then comminuted to 0.36 mm in a commercial blender. A prepared batch was kept in an air-tight re-sealable polypropylene bag and stored at 4°C. The essential oil content of a batch was determined prior to compressed CO2 extraction by hydrodistillation, and the composition of the essential oil was analyzed by GC. The hydrodistillation equipment and procedure were based on those detailed in the European Pharmacopoeia (10). An oil yield of 0.67% (w/w) with a standard deviation of 0.04% was obtained. (Main essential oil content: γ-terpinene 20.5%, thymol 12.2%, carvacrol 10.3%, p-cymene 9.3%, linalool 7.2%, α-teqjineol 6.4%, α-pinene 3.8%, β-caryophyllene 4.4%, carophyllene oxide 2.8%.)