Supercritical fluid-based extraction/processing: then and now
By Jerry W. King
The term “supercritical fluid extraction” (SFE) in the field of oils and fats processing often brings to mind a technology that emerged in the early 1980s as a potential replacement for hexane extraction of commodity seed oils . Despite promising laboratory and pilot plant-scale tests, the technology’s application to seed oils languished owing to high capitalization costs and the inability to develop a consistent solids feed system comparable to those used in conventional organic solvent-based processes. Industrial adoption waned in ensuing years even though several novel approaches to feeding solids into high-pressure extractors, such as high-pressure lock hoppers or auger-based screw conveyors, were developed .
Despite this setback, SFE is alive and well and being applied to many substrates that contain lipid matter. If a “production plant” is defined as a process that produces a product for sale, then it has been estimated that there are over 125 plants worldwide using critical fluids. Most of these facilities use supercritical carbon dioxide (SC-CO2) since CO2 is environmentally benign and readily available from the atmosphere, geological deposits, and fermentation processes (such as bioethanol production). Indeed, in this author’s 40 years of association with this field, the concept of locating an SFE-based processing facility next to a bioethanol production facility has often been mentioned.
Both sub- and supercritical propane have also been used to produce food products, particularly those with a lowered fat content. SC-CO2 and propane are GRAS-approved (i.e., are Generally Recognized As Safe by the US Food and Drug Administration) for contact with foodstuffs; and the use of food-compatible co-solvents with SC-CO2, such as ethanol, extends the range of SFE for processing substrates that can contain polar lipids (such as phospholipids) and related compounds .
Figure 1 illustrates the spectrum of critical fluid-based solvents that can be applied to lipophilic substrates to produce food and nutraceutical ingredients from lipid-containing feedstocks. In applying SFE to foods and agricultural materials processing, issues such as the degree of comminution and substrate water content have a significant effect on the resultant extract and must be controlled to ensure the desired result. A prime example is the often-cited SFE of water-tempered coffee beans and teas that have been used to produce commercial products for some time now using SC-CO2 extraction.
Expanding the role of supercritical fluids
It is interesting to examine the developmental eras of critical fluid-based technology (Fig. 2) and their relevance to lipids processing. From the early 1980s, the processing technology expanded into modes other than just SFE. The rise of supercritical fluid fractionation (SFF) methodology, embracing columnar-based schemes using distillation-type towers, as well as production-scale supercritical fluid chromatography (SFC), is documented in the literature , but these techniques have faced stiff competition from technologies such as molecular distillation and high-performance liquid chromatography (HPLC). Columnar fractionation-based methods, based on longitudinal temperature gradients along the column, can be operated in either concurrent or countercurrent modes with respect to substrate feeds vs. flow of the critical fluid. Such an approach can enrich lipid-containing mixtures with respect to their concentration in the natural oil matrix, resulting in extracts or raffinates with potential value as nutraceutical ingredients or functional food additives. Toward that end, production plants have evolved to concentrate lipids such as tocopherols, pigments, and sterols.
During the late 1980s and early 1990s, critical fluid-based techniques were gradually adopted by analytical chemists. Regulatory pressures to reduce the use of organic solvents in laboratory environments made SFE attractive to chemists because SC-CO2 is a virtually solventless extraction medium. Considerable effort was made to develop extraction and cleanup techniques . With respect to lipid matter and food analysis, three main themes emerged: (i) application for total fat/oil content as a replacement for Soxhlet-based methods, (ii) extraction of trace analytes followed by fractionation of lipid-containing material, and later (iii) assays compatible with the demands of the US Nutritional Labeling and Education Act of 1990. Although many sophisticated methods and types of instrumentation were developed during this period, competition from reduced solvent-based techniques lessened the use and impact of SFE in analytical chemistry. Nevertheless, many SFE techniques became standard operating procedures both in the United States and abroad, and these instrumental approaches continue to be used in support of process research and development.
Likewise, supercritical fluid chromatography (SFC) has found a particular niche in the pharmaceutical industry for the rapid characterization of chiral compounds. From the perspective of current applied lipid analysis, SFC is a very complementary and perhaps preferred technique to gas chromatography and HPLC for the rapid characterization of the major groups in biodiesel synthesis, as demonstrated in the early 1990s . Within 30 minutes or less, SFC can break down the major functional lipid groups such as fatty acids, mono-, di-, and triglycerides found in biodiesel so they can be quantified.
Reactions of interest to the oils-fats-lipids community have been developed using sub- and supercritical fluid media. Although there are exceptions, most reaction scenarios fall into one of three categories: (i) enzymatic-initiated reactions, (ii) hydrogenation of oils and oleophilic derivatives, and (iii) hydrolysis-based transformations produced in the presence of subcritical water and SC-CO2—both neat and in the presence of various catalysts. Initial studies using primarily lipases in the presence of predominantly SC-CO2 appeared in the early 1990s . Enzymatic catalysis in the presence of supercritical fluids may not be practical for large-scale transformation of the lipids because of the sensitivity of enzymes to the presence of water, their denaturation at high temperatures and pressures, and their cost. However, niche applications in the synthesis of specialty chemicals or aroma compounds, in which avoidance of organic solvents is desired and consumer safety a concern, appear more feasible.
Hydrogenation in the presence of supercritical fluids offers some interesting possibilities since the kinetics for such reactions can be accelerated by 500–1000 times and offer benefits in product selectivity and postreaction isolation of products . The synthesis of oleochemical derivatives such as fatty alcohols, using either SC-CO2, propane, or n-butane, can be more environmentally benign, as demonstrated by van den Hark et al.  on a pilot plant scale, and by others.
Hydrolysis of fats or oils—traditionally called “fat splitting”—has been accomplished since the 1940s in the presence of subcritical water above its boiling point but under pressure as illustrated in Figure 3. Although traditional fat splitting is often recognized as a hydrolysis process, it actually is using water in its subcritical state, and actually offers a “green,” catalyst-free method for the hydrolysis of oils-fats and related materials. By exploiting the temperature dependence of water’s hydrolysis constant (Kw), acid-base chemistry becomes possible even at modestly elevated pressures and temperatures, in contrast to the higher pressures required in supercritical fluid-based processes. In terms of biofuels, the treatment of recycled synthetic and bio-derived wastes using subcritical water for hydrolysis has been demonstrated as an appropriate method for the conversion of waste fats into biodiesel, or for the production of methane and methanol fuels from mixed biowaste streams, and it is appropriate for integration in a sequential manner with a liquid or SC-CO2-based unit process .
One large-scale example of a subcritical water-based processing plant is shown in Figure 4, located in “Eco-Town” in Osaka, Japan, which is capable of processing industrial and biowaste into useful energy-related products, such as methane, methanol, and fatty acids for conversion to biodiesel.
CO2 and water for lipids?
The mixing of CO2 both at low and high pressures with water as well as the addition of CO2 to organic liquids has fostered some unique chemistry that is green and sustainable and that often results in superior products. The addition of CO2 to water, of course, results in its acidification below a pH~3.0, depending on the CO2 pressure and temperature of the aqueous solution. As noted previously, these conditions can be exploited for acid hydrolysis of lipid moieties. Even oil and fat mixtures containing low levels of moisture are not immune to an autocatalytic effect in the presence of CO2. The glycerolysis or methanolysis of oil-/fat-containing materials can be accomplished in the mere presence of CO2 without resorting to the addition of a catalyst to initiate the reactions. Such an approach is very attractive from a green synthesis perspective. The addition of pressurized CO2 to some conventional organic liquids results in a gas-expanded fluid that exhibits properties intermediate between those of a gas and liquid. Wyatt and Haas  as well as others have demonstrated the applicability of this approach for the methanolysis of oils-fats to methyl esters.
SFE redux and a plethora of products
There has been considerable progress in developing CO2-assisted expellers that permit continuous SFE. Though this concept dates back to the mid-1980s when an auger-type screw press was used to assist in the SFE of oils from seeds , it has only recently been realized on a plant production scale. In this hybrid system, the SC-CO2 contacts the oil seed matrix in the expeller barrel. The supercritical fluid phase is created by the hydraulic compression of the feed matrix–fluid mixture, which results in an increase in the temperature and pressure in the expeller barrel. This compression process also enhances the fluidity of liquids, thereby enhancing the extraction of oil as the seeds are expelled. This concept is being tested for the extraction of oil from seeds by several US companies on a pilot scale. This commercial system, as offered by Crown Iron Works, is known as the HIPLEX process and is due largely due to the efforts of Foidl . High oil yields have been obtained from soybean and canola oilseed. These have produced functional meals for incorporation into foods; further de-oiling of press cakes is also possible using this approach. More details on CO2-assisted expelling can be found in the chapter by Srinivas and King in Functional Food Product Development .
SFE continues to find applications too numerous to mention in this update. As judged from the variety of lipophilic extracts available as commercial products, including those that explicitly state they are “supercritical” in origin, the future continues to look bright for SFE in the high-value lipophilic extractives marketplace. Specialty products such as polyunsaturated fatty acid esters derived from fish oils, neat and roasted sesame oil, cranberry seed-based oils, oils high in n-3- and n-6 fatty acid content, pumpkin seed and sea-buckthorn seed-derived extracts, mustard seed oil, SC-CO2-derived chia seed oil, and fiber for nutraceutical use join the traditional SFE-derived products, including decaffeinated coffee, hops extract, ginseng, and spice and antioxidant extracts. The lipophilic extract, using predominantly SC-CO2, from sawtooth palmetto berry is a proven treatment for prostate problems. Often these extracts are advertised as “hexane-free,” as are the expeller-based products. Newer niche products have emerged in the past two years such as ayurvedic medicine extractives that are produced by extraction with SC-CO2 followed by a hydro-ethanolic pressurized solvent extraction.
An array of critical fluid-based processing operations can be applied to a common agricultural commodity such as rice or rice bran. Hence, SFE can be applied to derive the oil, which can then be fractionated using SC-CO2 or hydrolyzed into its constituent fatty acids by using subcritical water. Several plant-scale facilities now exist in Asia for treating rice to retard the development of rancidity and extend the rice product’s shelf life, as described by King et al. .
In the late 1980s, the cosmetic industry paid little attention to lipophilic extractives derived by SC-CO2 extraction. This has changed somewhat with the incorporation of SC-CO2-derived jojoba extracts, lutein esters for topical applications, and SFE-derived Job’s tears extracts for the Asian cosmetics marketplace. There is even an SC-CO2-derived “organic sexual lubricant” called “Nude.”
These new marketplace developments are important since current schemes for processing algae-derived oils for biodiesel use advocate the removal of higher-value lipophilic components such as antioxidants and pigments prior to conversion to methyl esters for use as biodiesel.
The most recent knowledge in this field will be presented at the 10th International Symposium on Supercritical Fluids (ISSF), to be held May 13–16, 2012, in San Francisco, California, USA. This series of ISSF symposia highlights the latest developments in the field with topical sessions on biomass and energy-related conversions, reactions in critical fluids, natural products/nutraceuticals/food-related materials, industrial applications of critical fluids, green chemistry/engineering, supercritical fluids, and more. For information, visit issf2012.com.
A member of inform’s editorial advisory committee, Jerry W. King currently holds the Ansel & Virginia Condray Endowed Professorship in Biochemical and Chemical Separations in the Department of Chemical Engineering at the University of Arkansas (Fayetteville, USA). He has over 40 years of experience in supercritical fluid technology and separation science, is the author of 240 publications, holds three patents, and has received numerous scientific awards. He is chairman of the 10th International Symposium on Supercritical Fluids being held in San Francisco, California, USA, May 13–16, 2012.
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Foidl, N., Device and process for the production of oils or other extractable substances, US Patent 5,939,571, August 17, 1999.
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