Comparison of chemical composition and antibacterial activity of Nigella sativa seed essential oils obtained by different extraction methods.

Nigella sativa L. seed essential oils obtained by hydrodistillation (HD), dry steam distillation (SD), steam distillation of crude oils obtained by solvent extraction (SE-SD), and supercritical fluid extraction (SFE-SD) were tested for their antibacterial activities, using the broth microdilution method and subsequently analyzed by gas chromatography and gas chromatography-mass spectrometry. The results showed that the essential oils tested differed markedly in their chemical compositions and antimicrobial activities. The oils obtained by HD and SD were dominated by p-cymene, whereas the major constituent identified in both volatile fractions obtained by SD of extracted oils was thymoquinone (ranging between 0.36 and 0.38 g/ml, whereas in oils obtained by HD and SD, it constituted only 0.03 and 0.05 g/ml, respectively). Both oils distilled directly from seeds showed lower antimicrobial activity (MICs > or = 256 and 32 microg/ml for HD and SD, respectively) than those obtained by SE-SD and SFE-SD (MICs > or = 4 microg/ml). All oil samples were significantly more active against gram-positive than against gram-negative bacteria. Thymoquinone exhibited potent growth-inhibiting activity against gram-positive bacteria, with MICs ranging from 8 to 64 microg/ml.

A biorefinery from Nannochloropsis sp. microalga--extraction of oils and pigments. Production of biohydrogen from the leftover biomass.

The microalga Nannochloropsis sp. was used in this study, in a biorefinery context, as biomass feedstock for the production of fatty acids for biodiesel, biohydrogen and high added-value compounds. The microalgal biomass, which has a high lipid and pigment content (mainly carotenoids), was submitted to supercritical CO2 extraction. The temperature, pressure and solvent flow-rate were evaluated to check their effect on the extraction yield. The best operational conditions to extract 33 g lipids/100 g dry biomass were found to be at 40 °C, 300 bar and a CO2 flow-rate of 0.62 g/min. The effect of adding a co-solvent (ethanol) was also studied. When supercritical CO2 doped with 20% (w/w) ethanol was used, it was possible to extract 45 g lipids/100 g dry biomass of lipids and recover 70% of the pigments. Furthermore, the remaining biomass after extraction was effectively used as feedstock to produce biohydrogen through dark fermentation by Enterobacter aerogenes resulting in a hydrogen production yield of 60.6 mL/g dry biomass.

Modeling the supercritical fluid extraction of essential oils from plant materials.

Different types of mathematical models were applied in the last decade to simulate kinetics of supercritical fluid extraction (SFE) of essential oils from aromatic plants. Compared to the extraction of fatty oils, modeling of extraction of essential oils is more complicated due to their potential fractionation, co-extraction of less soluble compounds, and stronger effect of flow pattern on extraction yield, which is connected with solute adsorption on plant matrix. Fitting the SFE models to experimental extraction curves alone usually does not enable reliable selection among the models. Major progress was made when detailed models for the extraction from glandular structures of plants were developed. As the type of glands is characteristic for plant families, the choice of models for SFE of essential oils is substantially facilitated. As the extracts from aromatic plants contain also cuticular waxes and other less soluble substances, and essential oils themselves are mixtures of substances of different solubility in supercritical carbon dioxide, modeling of extraction of mixtures and their fractionation in time deserves more attention.