Supercritical CO2 extraction of fatty oil from flaxseed and comparison with screw press expression and solvent extraction processes

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Abstract

Flax oil is commonly used in food due to high percentage of omega-3-fatty acid and omega-6-fatty acid. In the present work the flax seed was extracted using green solvent viz. supercritical CO2 and compared with soxhlet and mechanical screw press methods. The chemical compositions of the oils were determined by CHNS analyser, GC-FID, GC/MS and 1H NMR. The supercritical CO2 process selectively extracted the fatty oils with high percentage of omega-3-fatty acid and omega-6-fatty acids. The chemical composition of screw press oil is close to that of supercritical CO2 extracted oil, whereas the yield is nearly 27% less in comparison to the supercritical CO2 method.

Introduction

Flaxseed or linseed (Linum usitatissimum L.), which is a member of the Linaceae family is an important oilseed crop in the world. The plant is not a new crop and native to West Asia and the Mediterranean region. It is mainly grown in Canada, Argentina, America, China and India (Wang et al., 2007). Flax is the Canada’s third major oilseed crop after canola and soybean. Flax is an economically important oilseed crop, especially for Canada, which produced about 40% of the world’s flaxseed and is the world’s largest exporter, representing about 75% of the global flax trade (Oomah and Mazza, 1999). The world demand for flaxseed is currently dominated by the industrial use of flaxseed oil. However, flaxseed is making great strides in the world’s food supply, and demand for human food and livestock markets is expected to increase owing to the unique properties of this ancient crop (Oomah, 2001).

Flaxseed is rich in fat, protein and dietary fibre. The compositions of flaxseed averaged 30–40% fat, 20–25% protein, 20–28% total dietary fibre, 4–8% moisture and 3–4% ash, and the oil contains vitamins A, B, D and E, minerals, and amino acids (Bhatty, 1997). Traditionally, flaxseed has been grown for its oil, which is used in the manufacture of paints, varnishes and linoleum, because of its drying and hardening properties when exposed to the air and sunlight. Flaxseed oil is used as a purgative for sheep and horses. There is a market for flaxseed meal as both animal feeding, human nutrition and also as poultry feed since it increases levels of omega-3-fatty acid in eggs (Rebolé et al., 2002). It is the most prominent oilseed studied to date as a functional food, as it is a leading source of the omega-3-fatty acid, which is known as α-linolenic acid (ALA) (52% of total fatty acids) (Oomah and Mazza, 1998).

Commercial production of vegetable oils is based on mechanical pressing and extraction. The mechanical expression of oil from oilseeds is one of the method mostly used in the removal of oil from oil-bearing materials. This method which offers the possibility of using the cake residue has relatively low initial and operational costs and produces uncontaminated oil (Fasina and Ajibola, 1989). However, mechanical oil-expression equipment and processes presently available are not considered adequate for this purpose, as their oil extraction efficiency is quite low (<70% oil extraction) (Bargale et al., 1999, Willems et al., 2008). The yield obtained by mechanical pressing as usually lower than those extracted by solvents viz. hexane and pentane. It is only in the last century that solvent extraction has been used in this field. The advantage of solvent extraction is the high yield that can be obtained economically with this method (>99 wt.%), but this is at the expense of a reduced oil quality. This quality reduction is caused by the extensive solvent recovery processes that are necessary and the fact that the solvent co-extracts undesired components from the seeds. Especially for high value added oils this quality reduction is unacceptable, limiting the production process to mechanical expression (Venter et al., 2007, Willems et al., 2008).

The supercritical-fluid extraction technique has been studied extensively as an alternative to conventional methods of oil extraction (King and List, 1993). Supercritical fluids have gas-like diffusivities but liquid-like densities. These properties vary as a function of pressure and temperature. Supercritical carbon dioxide (SC-CO2) has been the most frequently used supercritical fluid for oil extraction, since it is nontoxic, nonflammable, inexpensive, and easily separated from the extract (Naik et al., 1989, Rout et al., 2008). Furthermore, the low critical temperature of CO2 (31 °C) allows extraction of thermolabile compounds without degradation (Rizvi, 1994). A number of other plant oils including sunflower, rapeseed (Stahl et al., 1980), corn (Friedrich and Pryde, 1984), wheat germ (Taniguchi et al., 1985), palm (Kalra et al., 1987), canola (Fattori et al., 1988), cottonseed (Bhattacharjee et al., 2007), safflower seed (Han et al., 2009) were also successfully extracted with supercritical CO2. Reverchon and De Marco (2006) reviewed, the numerous works carried on the application of supercritical CO2 in food processing, pharmaceuticals and nutraceutical. This process has an advantage to isolate unsaturated fatty acids (omega-3 and omega-6) at lower temperature from flaxseed.

Presently, we extracted the flax seed by supercritical CO2 extraction process and compared with soxhlet extraction, mechanical screw pressing and also determined their chemical compositions.

Section snippets

Materials and methods

Flaxseed used in this study was a commercial seed, procured from agricultural farm, Saskatoon, Canada. The sample was cleaned manually to remove all foreign materials such as dust, stones, dirt, immature seed. The clean and graded seeds were kept in an air tight plastic vessel and stored at 5 °C for study. Before starting a test the seeds were allowed to warm up under ambient room conditions (22–25 °C, 30–40% RH) to the equilibrium moisture. Then the samples were taken for experimental purpose. A

Results and discussion

In literature few works on fatty oil extraction using supercritical CO2 were reported (Stahl et al., 1980, Fattori et al., 1988, Han et al., 2009). Stahl et al. (1980) observed that the solubility of seed oil in supercritical CO2 was more pronounced above 25 MPa pressure and 40–50 °C. Fattori et al. (1988) reported that the optimum condition for extraction of canola seed oil in supercritical CO2 at 50–55 °C with 30–35 MPa pressure. Recently, Han et al. (2009) reported the extraction of safflower

Conclusions

The supercritical CO2 process is superior in comparison to the soxhlet and screw press process in terms of yield of the important components viz. omega-6-fatty acid and omega-3-fatty acid. The process selectively extracted the neutral fatty acids in the above mentioned supercritical conditions. The chemical composition of fatty oil obtained by screw expression is comparable with supercritical CO2 process but yield of the fatty oil is 27.8% less in screw expression process. On the other hand, it

Acknowledgments

This work was supported by a grant from Graduate Student Exchange Program (GSEP) Fellowship by the Canadian Bureau for International Education (CBIE) on behalf of the Dept. of Foreign Affairs and International Trade Canada (DFAIT) and Dr. V. Meda’s research grants. The authors are thankful to Dr. R. Welford for providing flaxseed for this study.

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