# Marine Microalgae for Potential Lutein Production
## 7 Extraction of Lutein from Microalgae and Challenges  
The extraction of intracellular microalgae products is most commonly conducted by conventional means. This involves using dry biomass coupled with maceration and thermal extraction employing either organic or aqueous solvents (Table 1), depending on the polarity of the desired compound to be extracted. Compounds, such as carotenoids, display varying polarities and chemical natures. Hence, an appropriate solvent must be chosen with regards to extracting target carotenoids based on the selectivity, efficiency, and purity required. The extraction of carotenoids is typically conducted using non-polar solvents such as n-hexane, dichloromethane, and dimethyl ether, with other solvents such as acetone and octane also used for extraction [58]. The use of these non-polar solvents is due to the high hydrophobicity of carotenoids. Recently, green solvents such as ethanol and biphasic water solvent mixtures have also been researched for the extraction of carotenoids from microalgae [59]. The conventional extraction process of microalgae products still bears limitations such as extraction efficiency, selectivity, and high solvent consumption. [PERSON] et al. [5] discussed that the energy consumption for microalgal cell disruption ranged from 30 to 500 MJ kg\({}^{-1}\). This energy consumption was reliant on the disruption process, where the energy demand is determined by factors such as cell wall compositions, cell wall thickness, and cell size. This value is 1000 times higher than the crushing energy for marigold flower (800 kJ kg\({}^{-1}\)). To overcome this problem, the development and application of a multistage extraction process, combined with varying physical and chemical methods, may prove useful in selectively targeting the carotenoids of interest [58]. Microalgal biomass consists of a rich cellular composition and much thicker cell wall than marigold flowers. Dry-milled marigold petals are usually processed using solvent extraction to produce oleoresin-containing carotenoids in their ester form. This is followed by a multistep purification process which frees the hydroxylated carotenoids from the accompanying fatty acids and finally, a recrystallisation process occurs which results in pure lutein/zeaxanthin. Lutein is sold in oily extracts ranging from 5% to 60% as its crystalline form poses management and stability problems, where microalgal biomass can be extracted into an oleoresin-like extract with 25% lutein in free form that could be used directly for the commercial products [60]. [PERSON] et al. [61] applied two cell disruption methods, glass bead vortexing and ball mill grinding, on _Chlorella vulgaris_, _Chlorella zofingeniensis_, and _Chlorella protothecoides_ to improve the extraction yield of lutein, where the yield of _C. vulgaris_ (0.51 mg L\({}^{-1}\) d\({}^{-1}\)) and _C. zofingeniensis_ (0.53 mg L\({}^{-1}\) d\({}^{-1}\)) was higher compared to _C. protothecoides_ (0.37 mg L\({}^{-1}\) d\({}^{-1}\)). [PERSON] et al. [62] also applied two different cell disruption methods (bead-beating and high pressure) on _Chlorella sorokiniana_ MB-1 and the lutein was extracted by a reduced pressure extraction method. High-pressure pretreatment extracted with tetrahydrofuran (THF) as the solvent resulted in high lutein recovery efficiencies of 87.0% (at 20 min incubation) and 99.5% (at 40 min incubation) at 850 mbar pressure and at temperature 25 \({}^{\circ}\)C.  
Solvents such as hexane and/or ethanol are the most commonly employed methods for lutein extraction due to the easy removal of the solvent from the extract while also retrieving a high content of lutein. In contrast, direct extraction with vegetable oil was described by [PERSON] [63] and does not allow for solvent removal. This patent involves direct extraction on wet biomass using the addition of vegetable oil, followed by emulsification and a resting period. However, in regard to microalgae with thick cell walls, such as _Murielopsis_ sp.