There are many different techniques for biomass extraction. The choice of extraction procedure depends on the nature of the natural material and the components to be isolated. The main conventional extraction procedures are liquid-liquid extraction and liquid-solid extraction. Liquid-liquid extraction is using two different solvents, one of which is always water. Some of the disadvantages of this method are: cost, toxicity and flammability. Solid-phase extraction can be used to isolate analytes dissolved or suspended in a liquid mixture are separated from a wide variety of matrices according to their physical and chemical properties. Although these techniques are widely used, have several shortcomings: very often time-consuming and require relatively large quantities of polluting solvents, the influence of temperature which can lead to the degradation of thermo labile metabolites. Supercritical fluid extraction, microwave-assisted extraction and pressurised solvent extraction are fast and efficient unconventional extraction methods developed for extracting analytes from solid matrixes.

Source: Adina-Elena Segneanu, Florentina Cziple, Paulina Vlazan,Paula Sfirloaga, Ioan Grozescu and Vasile Daniel Gherman.Biomass Extraction Methods.Chapter 15. Additional information  available at http://dx.doi.org/10.5772/55338.


Production of biodiesel from microalgae by extraction


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The most commonly used methods for algal oil extraction are mechanical, enzymatic, chemical extraction , ultrasonic and supercritical extraction.
Mechanical extraction
In mechanical extraction method, techniques such as mechanical pressing, bead milling, and homogenization are  used and accounts for large scale of cell disruptions. Mechaning pressing puts high pressure on the cells being extracted, ruptures the cell wall, allowing the intracellular lipids to be extracted and collected. Homogenization  ruptures the cell wall by forcing the cells through a small orifice at high pressures. When the cell reaches the opening there is a sudden drop in the pressure along with a strong liquid shear force cause the cell to break open allowing the lipids to be extracted. At laboratory and industrial scales, bead milling, or bead beating, has been used for size reduction of particles and the disruption of cells. In the presence of beads, this technique works by agitating the algal biomass, which results in pulverization of the algal cells and breaking them apart by mechanical force and providing a means to extract the lipids.
Enzymatic extraction
In enzymatic extraction, water is used as solvent with the cell wall degrading enzymes to fractionate oil, proteins and hulls. The oil is found inside plant cells along with proteins and carbohydrates. The cell is surrounded by thick walls which have to be opened to release oil and protein. By using enzymatic extraction it is possible to fractionate the components to a degree which cannot be reached using the conventional technique like mechanical pressing. But the cost of enzymatic extraction process is much higher than the most popularly used solvent based extraction processes and this high cost is the limitation factor for large scale use of this
Chemical extraction
Solvents are used for oil extraction and Soxhlet method is the most commonly used. In this method oil and fat from the solid algal biomass is extracted by repeated washing with organic solvents under reflux in a special glass apparatus called Soxhlet extractor. Petroleum ether and n-hexane are most commonly used solvents. By using this method large amount of extraction can be done using limited solvent and is cost effective so it can be more economical if used at large scale. Poor extraction of lipids, long time required for extraction and hazards of boiling solvent are its limitations. When hexane was used as a solvent it can recover almost all the oil to leave behind only 0.5% to 0.7% residual oil in the raw material. Addition of solvents such as n-hexane or chloroform to the in situ reaction system could lead to a higher biodiesel yield. The use of an additional solvent such as hexane or chloroform helps the easy extraction of oils within microalgae cells and enhances the contact of its oil with the esterification reagent.
Ultrasonic extraction
In ultrasonic extraction method intense sonication of liquid is done which generates sound waves that propagate into the liquid media which causes alternating high-pressure and low- pressure cycles. Ultrasonic waves support the diffusion of solvents into the cell structure during high pressure cycle. As the cell wall breaks mechanically by the cavitations shear forces, the lipids get transferred from the cell into the solvent. The oil gets dissolved into the solvent; the tissues are then filtered out. The oil is separated from the solvent by distillation. This method improves the extraction of oil from algae as well as helps in conversion into biodiesel. This method is not feasible with large scale applications as it is not cost effective with the amount of oil production.
Supercritical extraction
In this process CO2 is liquefied under pressure and heated to the point such that it has properties of both liquid and gas. This liquefied fluid is then used as solvent for oil extraction. This process is more efficient and can extract almost 100% of oil and provide high purity and product concentration.

Source: (PDF) Microalgal Biomass Production and Oil Extraction for Algae Biodiesel Production – A Review. Available from: https://www.researchgate.net/publication/317042399_Microalgal_Biomass_Production_and_Oil_Extraction_for_Algae_Biodiesel_Production_-_A_Review [accessed Jun 25 2018].