Unique properties of carbon dioxide-expanded lipids
By Bernhard Seifried and Feral Ternelli
Stricter environmental laws related to the use of organic solvents in many areas of the fats and oils industry have stimulated the search for sustainable technologies for lipid processing. Furthermore, as consumers demand "natural" products, the use of potentially toxic solvents in various production processes is being scrutinized. The application of pressurized carbon dioxide (CO2) in lipid processing may offer new opportunities to reduce the amount of organic solvents needed. Lipids saturated with CO2 under moderate pressure expand in volume, and their physical properties change substantially (Seifried and Temelli, 2009). In this short review, the unique properties of CO2-expanded (CX) lipids are discussed, and their relevance for promising new applications is outlined.
Properties of CX Lipids
Volumetric expansion. Lipids, such as triglycerides (TG) and fatty acid ethyl esters (FAEE), can dissolve large amounts of CO2 when exposed to pressurized CO2. As a result, the lipids expand in volume, and concomitantly their physical properties also change with temperature and pressure. The volumetric expansion of fish oil TG and FAEE at a temperature of 40°C is illustrated in Figure 1. At 40°C, the volumetric expansion of FAEE and TG is very pronounced with an increase in pressure to about 10 MPa, reaching a fairly constant expanded volume at higher pressures ranging up to about 20 MPa. The volumetric expansion of CX TG and FAEE is nearly 40 and 70%, respectively. This increase is caused by the uptake of CO2, which amounts to about 30 and 55% by weight at 15 MPa for TG and FAEE, respectively.
Density, interfacial tension, and viscosity. The dissolution of large amounts of CO2 in lipids at elevated pressures leads to both volumetric expansion and a pronounced change in physical properties, such as density, interfacial tension (IFT), and viscosity. The changes in these physical properties with pressure and temperature at CO2 pressures of up to about 25 MPa for CX TG and FAEE of fish oil are illustrated in Figure 2 and Figure 3, respectively. In contrast to the pronounced volumetric expansion with CO2 pressure of up to 10 MPa, the density of CX lipids only increases moderately, by about 5%. However, the increase in density following the dissolution of pressurized CO2 seems surprising, since the density of pure CO2 and that of pure lipids under all pressure and temperature conditions shown (Figs. 2, 3) are lower than that of the mixture.
It has been suggested that CO2 dissolved in the lipid phase facilitates the compression of the bulky lipid molecules, acting like a lubricant between them. Furthermore, CO2, being a rather small molecule compared to the lipids, may be solubilized in the cavities of the bulky lipid molecules, thus existing in a more "condensed" form in the liquid phase. The density of CX lipids is lowered by increasing temperature, as illustrated in figures 2 and 3.
The influence of CO2 pressure on IFT of lipids in contact with CO2 is notable for both TG and FAEE (Figs. 2, 3). IFT decreases sharply with an increase in CO2 pressure, from ambient up to pressures of 10 and 15 MPa at 40 and 70°C, respectively. In the case of TG, this decrease amounts to about an order of magnitude at 40°C: IFT decreases from around 28 mN/m at ambient pressure to a fairly constant level close to 2.5 mN/m at 25 MPa. FAEE exhibit a higher mutual solubility with CO2; therefore, the IFT reaches ultra-low levels and eventually vanishes at elevated pressures, where the phases become miscible. Furthermore, at pressures above approximately 2.5 MPa, the IFT for both CX TG and FAEE increases with temperature. The change in IFT for CX lipid systems is related to several mechanisms contributing to interactions between the lipid molecules and the surrounding dense CO2 phase. The mechanisms taking place in the bulk CO2 phase, the interphase, and the lipid phase are affected by pressure and temperature. In the low-pressure range of up to 2.5 MPa, the driving force is most likely the dissolution of CO2 into the lipid phase, which increases with pressure and decreases with temperature. As pressure is further increased, adsorption of CO2 at the interphase increases due to the higher CO2 density, thereby leading to more interactions in the vicinity of the interphase. Additionally, with increasing pressure the solubility of lipids in CO2 is enhanced, which is also linked to temperature and the density of the CO2 phase.
The effect of temperature is twofold: First, an increase in temperature causes an increase in vapor pressure, thus enhancing solubility. Second, both temperature and pressure affect the density of CO2, especially close to the critical point (Pc = 7.3 MPa, Tc = 31°C). While an increase in temperature close to the critical point has a very strong lowering effect on CO2 density, this effect is less pronounced at higher pressures. Furthermore, around the critical point, the density of CO2 is extremely sensitive to pressure, where it increases substantially from gas-like to liquid-like densities. For example, the density of CO2 increases from about 250 kg/m3 at 7 MPa to 615 kg/m3 at 7.5 MPa, which has a great impact on the solubility of lipids in CO2 as well as the solubility of CO2 in lipids. Therefore, both temperature and pressure effects impact the IFT of lipids in contact with pressurized CO2.
The trend for viscosity with increasing pressure is most striking for TG at 40°C, where the viscosity of CX TG decreases by about an order of magnitude from initially around 25 mPa·s to 3 mPa·s, with an increase in pressure to about 12 MPa (Fig. 2). In the same pressure range at 40°C, the viscosity of CX FAEE decreases from about 3 to 1 mPa·s (Fig. 3). The CO2 molecules reduce the interactions between large molecules inside the bulk lipid phase by expanding the lipids, thereby reducing viscosity and resistance to flow. The impact of temperature on the viscosity of CX lipids may seem surprising. At atmospheric conditions, viscosity declines with increasing temperature, but the opposite change occurs at elevated CO2 pressures for CX lipids. Since an increase in temperature lowers the amount of CO2 dissolved in the lipids, an increase in viscosity at elevated pressures above 5 MPa can be observed.
Potential of CX lipids for fats and oils processing
The properties of CX lipids can be easily tuned by adjusting the temperature and pressure of CO2. Therefore, many potential applications for fats and oils processing could take advantage of the pronounced expansion of lipids saturated with CO2. The pressure required to induce substantial changes in the physical properties of CX lipids, such as viscosity and interfacial tension, ranges from 10 to 15 MPa, which is a technically feasible range. In recent years, several novel processes have been investigated that profit from the properties of CX lipids. Gas-assisted mechanical expression (GAME) and gas-assisted oilseed pressing benefit from the volumetric expansion and reduced viscosity of CX lipids, thus leading to significantly higher yields in pressing experiments using rapeseed, soybean, sesame, linseed, palm kernel, and jatropha (Voges et al., 2008; Willems et al., 2008). The reduced viscosity of CX lipids could also be beneficial in processes involving reactions, such as transesterification. The reduced viscosity could accelerate the reaction rates in heterogeneously catalyzed reactions, especially in the case of highly viscous oils (Pomier et al., 2007). Furthermore, enhanced flowability of highly viscous liquids by injecting CO2 due to viscosity reduction has been applied to improve filtration of used motor oils and might be an option for filtration of used frying oils as well. Finally, reduced interfacial tension and reduced viscosity may be advantageous for applications involving formation of droplets in spray processes aiming at increasing the interphase, which may be desirable in lipid extraction and purification using pressurized CO2.
The application of pressurized CO2 in fats and oils processing may be beneficial for the development of novel processes ranging from oil expression and extraction of oilseeds to reactions and separation of lipid components. Jessop and Subramaniam (2007) have reviewed the potential of gas-expanded lipids and liquids, and the latest developments point in the right direction, such as using gas-expanded liquids in the production of biodiesel (Wyatt and Haas, 2009).
Bernhard Seifried received his M.Sc. in chemical engineering from Graz University of Technology in Austria. He is currently a Ph.D. candidate at the University of Alberta, Edmonton, in the field of bioresource and food engineering working on novel process development for marine lipids using supercritical fluids and gas-expanded liquids. He may be contacted at firstname.lastname@example.org . Feral Temelli is a professor of food process engineering at the Department of Agricultural, Food and Nutritional Science at the University of Alberta. Her research program focuses on the use of separation technologies for the value-added processing of crops. Her program on supercritical fluid technology involves the use of supercritical carbon dioxide for extraction, fractionation, reactions, and particle formation of lipids and nutraceuticals. She may be contacted at Feral.Temelli@ualberta.ca
For further reading:
Jessop, P.G., and B. Subramaniam, Gas-expanded liquids, Chemical Reviews 107:2666-2694 (2007).
Pomier, E., N. Delebecque, D. Paolucci-Jeanjean, M. Pina, S. Sarrade, and G.M. Rios, Effect of working conditions on vegetable oil transformation in an enzymatic reactor combining membrane and supercritical CO2, Journal of Supercritical Fluids 41:380-385 (2007).
Seifried, B., and F. Temelli, Density of marine lipids in equilibrium with carbon dioxide, Journal of Supercritical Fluids 50:97-104 (2009).
Voges, S., R. Eggers, and A. Pietsch, Gas assisted oilseed pressing, Separation and Purification Technology 63:1-14 (2008).
Willems, P., N.J.M. Kuipers, and A.B. de Haan, Gas assisted mechanical expression of oilseeds: Influence of process parameters on oil yield, Journal of Supercritical Fluids 45:298-305 (2008).
Wyatt, V.T., and M.J. Haas, Production of fatty acid methyl esters via the in situ transesterification of soybean oil in carbon dioxide-expanded methanol, Journal of the American Oil Chemists' Society 86:1009-1016 (2009).