Swelling of plant material in supercritical carbon dioxide
Graphical abstract
Introduction
Exposure of herbaceous matrix to supercritical fluid leads to partial dissolving of plant material in compressed gas, which may cause changing of tissue properties and accordingly the plant tissue itself might be subject to swelling. The extent to which changes occur depends on the specific structure of plant material, as well as on the operational conditions (pressure and temperature). The time of exposure to supercritical fluid might also influence the swelling process. Swelling of plant material could lead to increased effective diffusion coefficient that would allow faster diffusion through the porous structure of plant material's particle in a supercritical fluid extraction (SFE) process. Hence, if the continuous SFE process starts from already swollen plant material, some amount of supercritical fluid could be saved. Up to date swelling of polymer materials exposed to supercritical CO2 was the subject of reported investigations [1], [2], [3], [4]. Since the polymers are used as tubing or sealing materials in a number of technical applications in contact with compressed gasses, it is essential to know how the pressurized gas influences the polymer properties.
The aim of this work was to analyse the influence of exposing plant material of several families to supercritical carbon dioxide on plant material properties and SFE process. The influence of process parameters, such as pressure, temperature and time of exposure, on the swelling of plant material was analyzed as well. Lamiaceae family species (mint, wild thyme, hyssop, sage and rosemary), valerian root, ginger rhizome and hop cones and pellets were chosen for the study. In the case of Lamiaceae family species, valerian root and ginger rhizome, benefits of swelling behavior on the SFE process were demonstrated. Lamiaceae family species are characterized by peltate glands (type of glandular trichomes) as seats of essential oil production and storage. Peltate glands are placed at the very surface of the leaves. Previously published investigations [5], [6], [7] indicated peltate glands cracking due to the exposure to supercritical fluid. Therefore swelling effects on the SFE of essential oils were investigated and discussed in this study. Influence of swelling on the SFE from Lamiaceae family species at higher pressures was investigated as well. Sage and rosemary were chosen for this investigation because of the well known antioxidant properties of their fractions obtained at higher pressures [8], [9], [10], [11]. Valerian root and ginger rhizome are characterized by small secretory cells placed within the plant tissue. In the case of valerian secretory cells are determined to be 8 μm in diameter [12]. Diameter of ginger secretory cells is somewhat larger and its average value was determined to be 20 μm [13]. Possibility of the SFE from secretory cells enhancement by the plant tissue swelling before the continuous extraction was tested in this study. The SFE experimental results were subjected to mathematical modeling. The influence of plant material swelling was analyzed through the values of the model parameters, namely those indicating diffusion through the particle of plant material.
As the hop is one of the most important raw materials in the industrial SFE processing, hop cones and pellets were also chosen for the swelling investigation. Sorption of carbon dioxide into the hop pellets used in industrial facilities was measured as well.
Section snippets
Materials
Leaves of hyssop (Hyssopus officinalis), wild thyme (Thymus serpullum), mint (Mentha piperita), rosemary (Rosmarinus officinalis), sage (Salvia officinalis), hop (Humulus lupulus) as well as dry root of valerian (V. officinalis L - cultivar Arterner züctung as well as wild grown valerian) and ginger (Zingiber officinalis) rhizome were used in experimental studies. Hyssop, rosemary and wild thyme were cultivated in the southern Balkan region. Mint and Valerian were cultivated in the northern
Swelling detection
Swelling behavior of the Lamiaceae family species will be demonstrated by the experimental results of swelling tests with mint leaves. Fig. 3 shows mint leaf before (upper image) and during (lower image) exposure to supercritical carbon dioxide at 40 °C and 10 MPa. As can be seen the swelling of the leaf under the influence of supercritical fluid is obvious (the distance between two lines indicates the thickness of the sample before exposure; it is clear that the thickness of the sample in the
Conclusion
Explanation of the background of swelling by CO2 is not easy. Mainly there are two effects. The first is swelling during decompression which is caused by very fast decompression and significant tensions created inside a plant material. The diffusion process is overtaken by rapid expansion of gas which generates overpressure inside the solid matrix. The second effect is swelling during exposure to CO2 at higher pressure when CO2 diffuses inside the plant tissue filling the pores of plant
Acknowledgments
Financial support of the Serbian Ministry of Science (Project ON142073) is gratefully acknowledged. The authors would like to thank Dr. Erwin Schütz for the valuable comments on hop swelling.
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