where c represents the concentration of extracted SLs at any time t,cm the maximum achievable concentration (or extraction capacity)and k is the first-order extraction velocity constant. Eq. (2) indicates
that the SL extraction velocity is proportional to the amount of non-extracted SLs. cm and k were determined for both non-homogenizedwet biomass (control test) as well as for wet biomass homogenized
at 500, 1000 and 1700 bar. Fig. 2 shows that the cm values depend on the homogenization pressure, i.e., the maximum SL concentration or yield attainable is limited by the cell rupture degree attained in the homogenization treatment. Therefore, the cm values were obtained from Fig. 2, using the corresponding lipid concentrations
at times higher than 20 h, which are shown in Table 3. k values were
obtained by fitting the experimental concentration data to Eq. (2)
and Table 3 shows that their values also increase with homogenization pressure. This table shows that both cm and k increase with the
homogenization pressure. Cho et al. (2012) carried out the extraction of lipids from dry biomass of the microalga Scenedesmus sp.using 30 mL of chloroform:methanol (2:1, v/v)/g biomass. Biomass
was homogenized at 1200 psi and then extracted at 35 °C. In this
case the experimental results were fitted to a second-order model,
with most of the cellular lipids (yield of 24.9% on biomass dry
weight) extracted during the initial period, mostly within 30 min.
In this work, lipids were slowly and continuously extracted for
approximately 20 h at 20–22 °C, which may be due to the presence
of water, to the lower temperature used and because chloro form:methanol (2:1 v/v) was used instead of hexane. The presence
of water reduces the extraction efficacy when extraction is carried
out with hexane, the temperature influences the lipid solubility
and diffusion velocity and chloroform:methanol (2:1 v/v) extracts
to both neutral and polar lipids.