6.2.3. Supercritical Fluid-Mediated Extraction

Classical extraction with organic solvents has attained purity degrees sufficient to meet commercial specifications for large-scale production of lutein; however, selective precipitation with supercritical CO2 constitutes a promising alternative. Note that conventional liquid extraction of carotenoids from microalgal matrices is time-consuming—as multiple extraction steps are typically required; and large relative ratios of organic solvents have to be used, which are often expensive and potentially harmful. Supercritical fluid extraction (SFE) using modified CO2 permits more straightforward purification and shorter extraction times [81]. 

In general, SFE is relatively rapid and efficient because of the low viscosities and high diffusivities that characterize supercritical fluids. Furthermore, extraction can be made selective by controlling solvent density; the material extracted will be recovered afterwards by simply depressurizing, thus allowing the supercritical fluid to return to its gaseous form and leaving no (or little) residual solvent in the precipitate thus originated[82]. Supercritical CO2 has so far been the most employed supercritical fluid—because it is non-flammable, non-toxic, inexpensive and relatively inert from a chemical point of view. 

Previous studies demonstrated the feasibility of extracting pigments from plants using supercritical CO2—e.g., carotenoids from carrots [83] and cabbages [84]; Mendes *et al*. [85], Careri *et al*. [86] and Macías-Sánchez *et al*. [87–89] have meanwhile extended such a technique to extraction of carotenoids from *C. vulgaris*, *Spirulina platensis*, *Nannochloropsis gaditana*, *Synechococcus* sp. and *S. almeriensis*, respectively— and satisfactory results were consistently reported, as emphasized in Table 3. 


**Table 3.** SFE yields of total carotenoids (including lutein), and of lutein specifically, by selected microalgae. 


**Table 3.** *Cont.*


However, this mode of extraction tends to recover chlorophylls more efficiently than carotenoids, thus producing extracts with relatively poor specifications [90]. Furthermore, the cost of supercritical fluids and associated equipment make it difficult for SFE to compete with classical solvent extraction—especially because the former requires dry biomass. 

The selective adsorption of lutein might constitute an alternative in terms of separation/purification, especially if specific solid phases can be used [91], coupled with contacting conveyed by expanded beds [92]; this allows raw extracts to be processed, and tolerates the presence of cell debris or other particulate matter that causes major problems in conventional preparative chromatography. Selective precipitation was also described by Miguel *et al*. [93], who proposed use of supercritical CO2 after organic solvent extraction; the first solvent (containing carotenoids) was accordingly mixed with supercritical CO2, and the conditions of pressure and temperature were duly adjusted to promote preferential precipitation of lutein. However, simple standard mixtures—rather than complex microalgal extracts have been considered, so a long way of improvement is still anticipated prior to practical use. 
