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Clean and Green Technology – Liquid and Supercritical CO2 Extraction for Food Applications

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“You are what you eat” is a phrase that is frequently heard today, and there is a lot of truth behind this. It makes sense, therefore, to carefully control the ingredients that are used in food manufacture, in terms of residues and other undesirable components. That is where the unique properties of supercritical CO2 extraction can be exploited to their full potential.

Extraction with liquid or supercritical CO2 is essentially a simple concept, although specialised equipment and technically skilled operators are needed to bring concept to reality. CO2 can exist in solid, liquid or gaseous phase, in common with all chemical substances. Furthermore, if the liquid phase is taken beyond the so-called critical points of temperature and pressure, a supercritical fluid is formed, which in simple terms can be considered as a dense gas (Figure 1). Both liquid and supercritical CO2 act effectively as solvents. While liquid CO2 is excellent for dissolving relatively non-polar, small molecules (liquid CO2 can be compared to hexane in this regard), supercritical CO2 allows the extraction of larger and more polar compounds. 
 

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Figure 1

Phase diagram for pure CO2, showing the solid, liquid, gaseous and supercritical fluid phases. Obtained with permission from Professor Chris Rayner, Leeds Cleaner Synthesis Group, Department of Chemistry, University of Leeds.

What sets liquid and supercritical CO2 apart from other solvents such as hexane and ethanol are two key properties. Firstly, once the extraction has been effected, the CO2 solvent is released as a gas and recycled in the process, so that a solvent-free extract is produced (Figure 2). This has two immediate benefits – the extract is free of all solvent residues, and importantly so is the extracted material, which can then be further used for processing if required. Secondly, the solvating power of CO2 can be manipulated readily by altering temperature and pressure. This means that extraction can be highly selective, which greatly reduces the need for further downstream purification and refining.

Figure 2

Schematic diagram of a supercritical CO2 extraction circuit. Supercritical CO2 is pumped through the plant material in the extraction columns, where extraction of the desired plant components takes place. After passing through the expansion valve, the extract-laden CO2 is de-pressurised and the extract precipitates out of solution in the separator. The gaseous CO2 can be recycled for further extractions.

Many plants contain compounds that show strong antioxidant properties. There is growing interest in developing applications for these molecules, so as to reduce the need for synthetic chemicals to prevent oxidation problems such as browning of meat, rancidity of fats and oils, etc. Well-known culinary herbs of the Labiatae family, such as rosemary contain powerful antioxidants in their leaves. Rosmarinic acid is suitable for water-based applications, such as beer, wines and soft drinks, whereas carnosic acid provides highly effective protection in oil- and fat-based products. Of course, an important issue is that the plant’s antioxidants should be purified without the strong herbal aroma, since this is not necessarily desired in the final application.

Carnosic acid is readily extracted from culinary herbs by supercritical CO2. The resulting extract contains high levels of carnosic acid in a solvent-free resin, which can be simply de-odourised and blended in a carrier oil if desired. Furthermore, the extracted herb material is also solvent-free, and already de-odourised by virtue of the supercritical CO2 that has removed the non-polar, volatile aroma constituents. It can thus be extracted further by conventional solvents to remove the more polar rosmarinic acid, and so produce a very low odour antioxidant product for aqueous applications.

Omega 3’s and 6’s

The increasing scientific understanding of the importance of a proper dietary balance of omega-3 and omega-6 fatty acids has fuelled the development of omega-3 and omega-6 supplements and fortified foods . As is well known, long-chain omega-3 fatty acids (docosahexanoic acid – DHA, and eicosapentanoic acid – EPA) are found in rich supply in fish oils. Both omega-6 and omega-3 fatty acids can also be extracted from plant sources such as seed oils. Seed oil sources are particularly important for applications where plant derivation is required, such as supplements suitable for vegetarians and vegans. Borage and evening primrose are the most common plant sources of the omega-6 fatty acid gamma-linolenic acid (GLA). Blackcurrant seed oil also contains GLA, together with the omega-3 fatty acids alpha-linolenic acid (ALA) and stearidonic acid (SDA) . SDA is interesting as it is more readily converted by humans to the long-chain omega-3’s than ALA . Echium seed oil is another vegetable source of omega-3 acids, with high levels of SDA, in addition to omega-6 fatty acids, and is of current interest for cosmetic applications.

Oils containing the omega-3’s and 6’s are often extracted with hexane. Supercritical CO2 provides a means of obtaining solvent residue-free extracts rich in fatty acids. Furthermore, the mild extraction conditions coupled with the unique solvent properties of CO2 results in extracts with typically low peroxide values, colour and aroma. Thus post-extraction processing is greatly reduced, and usually only involves de-watering. As with the natural antioxidants described above, the extracted seed source is also left solvent-free. As such, this protein-rich spent material can be used directly as animal feed.

Natural flavours and aromas

Extraction of aromatic herbs and spices has historically been carried out by steam distillation or by solvent extraction but both methods have some technical disadvantages. Steam distillation can induce rearrangement of labile molecules, which can result in undesirable aroma characteristics. Solvent extraction is not selective for aroma molecules and therefore complex mixtures of volatile and non-volatile materials are obtained. A graphic example of the breakdown of labile components is seen in the extraction of essential oil from German chamomile (Matricaria recutita). German chamomile contains an anti-inflammatory sesquiterpene known as matricine. Matricine is a colourless compound, and is readily broken down to chamazulene, which has an intense blue colour. Steam-distillation of German chamomile yields a blue oil containing chamazulene. On the other hand, the use of CO2 eliminates such rearrangement reactions and a yellow oil is obtained, containing matricine and only low levels of chamazuelene. This example shows how a much more natural extract is possible with CO2. Furthermore, the level of non-volatile components can be controlled by the pressure and temperature used. Lower temperatures and pressures will favour the extraction of only the essential oils, whereas higher pressures and temperatures will also extract more non-volatile molecules such as the molecules responsible for the heat in pepper (piperines) and ginger (gingerols).

Hops (Humulus lupulus) provide an example of a plant extract that has made a complete transition from solvent extraction to CO2 extraction in the last 25 years. During the 20th century, a variety of extraction solvents were used for hops, including alcohols, chlorinated solvents and hydrocarbons, but CO2 has been the method of choice since the 1980s. Table 1 shows how the desirable components of hops, namely the alpha- and beta-acids, and the volatile oils, are extracted with higher selectivity when using liquid CO2 as opposed to conventional solvents. In addition, undesirable components such as hard resins, tannins, chlorophyll and residual solvent are absent. The high extraction selectivity achievable with CO2 provides a premium product without the need for further post-extraction purification.

Table 1: Selectivity of Liquid CO2 for the Extraction of Hop Components

A considerable amount of research and development has been performed on CO2-extraction of hops for the brewing industry. CO2 is used to extract the volatile and non-volatile odour and flavour constituents of hop cones, and these can then be formulated into pure hop aroma or bittering products. This provides valuable tools for the brewer to perfect a desired flavour in the final beer, whether to accentuate the characteristics of a particular hop variety, to emphasise a certain aroma characteristic such as floral, citrussy, or spicy notes, or to control bitterness quality.

Natural vanillin from vanilla beans is another good example of the advantages of CO2 extraction. Vanilla beans are a precious commodity and so efficient extraction of a high quality end-product is a necessity. Using supercritical CO2 as a solvent, a light coloured resin containing in the region of 20% vanillin is obtained in a one-step extraction, again with no solvent residues remaining in the product.

Nutraceutical applications and herbal medicines

The health and wellness sectors are of increasing importance. Crucial to the sustained credibility of products in these sectors are a well-defined concentration and quality of the active ingredient. As we have already seen in the examples given above, the solvating power of supercritical CO2 can be easily manipulated by altering pressure and temperature. This means that a large variety of molecules are accessible to CO2 as a solvent, including many constituents of herbal medicines. A study of a herbal medicine textbook will show that most of the active principles include monoterpenes, diterpenes, sesquiterpene lactones, triterpenes, alkaloids, carotenoids or fatty acids.  Many of these compounds can often be extracted directly with CO2, or in the case of less soluble molecules, by addition of a small amount (generally less than 10%) of a co-solvent such as water or ethanol to the CO2. Thus clean extracts with low post-extraction processing requirements are obtained, ready for incorporation into herbal or nutraceutical applications.

Summary

Extraction with liquid or supercritical CO2 is a flexible extraction technique with numerous applications for the food and beverage industry. Key advantages are the fact that solvent-free extracts are produced, and that selectivity of extraction is readily achieved, leading to reduced requirements for post-extraction purification. Mild extraction conditions ensure that product degradation is minimised, even for labile compounds.

Dr Yannick Ford, Research Coordinator
Botanix Ltd, Hop Pocket Lane, Paddock Wood, Kent TN12 6DQ, United Kingdom

Acknowledgements: The author is grateful to Professor Chris Rayner, Leeds Cleaner Synthesis Group, Department of Chemistry, University of Leeds, for the phase diagram in Figure 1.

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