Volatile fraction and total extract

Two distinct CO₂ extracts come from the same plant. One is the light and fluid volatile fraction (the easily evaporating portion of the extract), similar to essential oil. The other is the dark and dense total extract. The difference is not in the plant, but in the pressure. Under pressure, carbon dioxide (CO₂) becomes a solvent (a liquid that pulls substances out of the plant). Its solvent power depends on its density. Density is adjusted with pressure and temperature. At low pressure, only light, volatile molecules dissolve. As pressure rises, heavy molecules also emerge: waxes (the waterproof layer coating the leaf), fatty acids, pigments, fat-soluble antioxidants.

Why do two extracts come from the same plant?

CO₂ is a selective solvent under pressure. Above about 31 °C (critical temperature) and 74 bar, CO₂ becomes "supercritical". It enters plant tissue like a gas and dissolves like a liquid [1]. In this state, it takes small, fat-soluble molecules. It does not take large, water-soluble molecules [2]. As pressure increases, CO₂ compresses. Its density and solvent power rise. First, light molecules dissolve. As pressure rises, heavy molecules also join.

The extract obtained at low pressure is rich in aromatic components. When pressure rises, waxes also emerge from the same plant, and the share of aromatic components drops. In citron peel, the heavy aromatic components of the essential oil (sesquiterpenes) were measured in three products. They were 0.6% in distilled oil (the classic essential oil taken from the plant with steam), 5.6% in the low-density CO₂ extract and 12.1% in the high-density extract [3]. Limonene (the main component of citrus aroma) is 60% in distilled oil and 4.5% in high-density extract. Limonene did not vanish. The waxes that came into the extract at high density diluted it [3].

Raising the pressure does not add essential oil, it adds heavy components. In a peppermint leaf study between 100 and 300 bar, the extraction rate of waxes increased distinctly with pressure. The extraction rate of essential oil remained almost constant [4]. Essential oil comes out at low pressure. High pressure adds waxes, fixed oils, and heavy active substances (components that are the primary reason for extraction besides aroma).

Heavy components need higher pressure. For essential oil, 80 to 100 bar is enough. Under this condition, all components of the essential oil dissolve well in CO₂ [5]. Oleoresin carrying the pungent components of ginger (oily extract carrying both the aroma and taste of the spice) was obtained mainly at 150 bar and above. At 80 bar, the yield was lowest, and the product was a clear essential oil extract [6]. For turmeric yellow pigments (curcuminoids), the best result was found at the highest pressure tested, 425 bar. Pressure was considered the most influential variable [7].

Which one is used for what?

The volatile fraction is used for aroma and taste. The total extract is used for heavy components such as bitterness, color, or antioxidants [1].

The volatile fraction is similar to essential oil, but not identical. Steam distillation uses boiling steam. CO₂ extraction works warm and uses no water. Distilled oil contains more of the lightest components. CO₂ extract contains more heavy components [8], [3]. In thyme, the CO₂ extract contained a component not found in distilled oil (thymoquinone) and a small portion of wax [5].

Total extract is similar to oleoresin. Depending on conditions, CO₂ also takes waxes, pigments, fatty acids, and fat-soluble antioxidants such as vitamin E. The product becomes a dark extract resembling oleoresin, not a classic essential oil [1]. If the goal is aroma, conditions are selected to take volatile components and leave the heavy fraction behind. If the goal is an extract carrying active substances for food, cosmetics, or supplements, heavy antioxidants are also desired [1]. Waxes and fatty acids are solid at room temperature. That is why a total extract is often semi-solid.

How are the two products separated?

Separator vessels divide the extract after leaving the extractor. CO₂ leaving the extractor (the pressurized vessel where the plant is washed) passes through one or two separators. In the separator, pressure and temperature are reduced. Solubility decreases and the substance precipitates. If the first separator is kept cold, waxes precipitate there, and the essential oil collects in the second separator [5]. In a clary sage (Salvia sclarea) study, the first separator was cooled to 0 °C, and only waxes accumulated inside [9].

Sequential extraction washes the same raw material twice. First, the volatile fraction is obtained at low density. Then the same material is extracted again at higher density. In the clary sage study, the raw material was not the plant itself, but its hexane pre-extract (concrete). In the first step, only light aromatic components were collected. In the second step at slightly higher density, only sclareol (the heavy, non-volatile aroma component of clary sage) and related substances were collected. When density increased, total yield rose from 6.0% to 9.3% [9]. In rosemary, four separate fractions were collected with four sequential settings. Ethanol was added in small amounts to CO₂ in the last two [10].

Temperature works both ways. When temperature rises, CO₂ density decreases and solvent power drops. At the same time, substances evaporate more easily and pass into CO₂ more readily. Which effect dominates depends on pressure [2]. In ginger, a colder setting was found more efficient at low pressure, and a warmer setting was more efficient at high pressure [6]. Therefore, pressure and temperature are selected together.

What does subcritical CO₂ mean?

Below the critical temperature, CO₂ is liquid. The critical temperature of CO₂ is 31 °C [2]. Below this, under sufficient pressure, it remains in liquid state. This is called subcritical (liquid) CO₂ extraction. Liquid CO₂ is colder and denser than the supercritical state. Its selectivity comes from low temperature, not density. As temperature drops, heavy components evaporate less easily and do not pass into CO₂ [2]. In hops, cold (10 °C) liquid CO₂ selectively extracted the substances that give beer its bitterness (alpha acids) and light aromatic components. Supercritical conditions also dissolved heavier, less volatile components [11]. Low temperature protects heat-sensitive components [2]. Subcritical water extraction is a different method. There the solvent is hot pressurised water, here it is liquid CO₂.

How do you tell which one it is from the documents?

The difference appears in the defining component, not the product name. The names of two products can be identical. A product defined by an aromatic component is a volatile fraction. A product defined by a bitter, pigment, or antioxidant component is a total extract. The lower limit of the same aromatic component may be lower in the total extract because heavy components dilute its share. Technical data sheets (TDS) often do not state pressure. A total extract often includes a "volatile oil content" line. How to read documents is explained on a separate page: TDS, SDS, CoA. When pressure drops, CO₂ becomes gas and evaporates. CO₂ itself does not remain in the extract [1]. If a co-solvent was used, what might remain is explained on a separate page: solvent residues.