In this method, finely ground biomass is placed in a porous bag or "thimble" made of strong cellulose, which is placed in the extraction chamber of a Soxhlet appa­ratus. The menstruum is heated, and the condensed extractant drips into the thimble containing the biomass, ensuring intimate continuous contact with the biomass. When the level of liquid in the extraction chamber reaches overflow, the liquid contents siphon into the heating chamber. This process is continuous and is carried out until complete extraction is achieved (Morrison and Coventry, 2006). The advantage of this method is that large amounts of lipid can be extracted with a much smaller quantity of solvent.

Countercurrent Extraction

Counter-current extraction is a process whereby wet raw material is pulverized using toothed disc disinte­grators to produce slurry in a semicontinuous stream. As the pulverization of the biomass is in aqueous media, the heat generated during comminution is counterbal­anced by the slurry water, preserving thermolabile compounds. The slurry stream is moved in one direction within the cylindrical extractor where it comes into discreet contact with a suitable menstruum (Vishwakarma, 2010). Complete extraction is possible when the quantities of solvent and material and their respective flow rates are optimized. The quantity of solvent required is generally minimal and as the process is most often conducted at room temperature, the threat to thermodegradation of volatile compounds is negated (Handa, 2008).

Ultrasound Extraction (Sonication)

The use of sonication is an emerging technology that is gaining widespread industrial acceptance due to recent advances in the scalability of the technology (Awad et al., 2012b; Dolatowski et al., 2007). In the context of lipid extraction from biomass, ultrasound technology is used to increase the permeability of biomass cell walls by generating cavitation events. These events are created by the use of high frequencies (20—2000 kHz) to generate a microbubble in solution; the intensity of the waves leads to the eventual collapse of the bubble generating extreme localized pressure and temperature events in close proximity to the biomass. These cavitation events assist in the rupturing of the cell walls to release the intercellular constituents into the surrounding environment. Once the biomolecules of interest are released from the biomass they can be recovered using conventional techniques. One disadvantage of using ultrasonics in the occurrence of sonolysis, i. e. the occasional but deleterious effect that when high power (typically greater than 20 kHz) is applied in aqueous media it can lead to the formation of free radicals and hydrogen peroxide. These are generated at the interfacial double layer established during cavitations, which subse­quently diffuse into solution (O’Donnell et al., 2010).