Only slight variations in fatty acid compositions of lipids extracted
at different pressure levels were observed (Fig. 2b). The
same dominant fatty acid molecules were identified in all lipid extracts,
with C18:1 and C16:0 being the most abundant. The slight
changes in FAME compositions allude to the differential selectivity
that SCCO2 is known to display towards specific lipid fractions at
different pressures (Cheung, 1999).
The dielectric constant or the solvent polarity of a supercritical
fluid measures the degree of interaction between the solvent and
the organic molecules (Zhang et al., 2005). Table 2 shows the exponential
dependence of lipid mass transfer coefficient (k) during our
SCCO2 extractions on fluid dielectric constant along an isotherm.
This behaviour was anticipated since an increasing solvent polarity
would enhance the disruption of membrane-lipid bilayer and facilitate
the extraction of membrane-bound lipids.