2.5. Experimental design
A multilevel factorial design was carried out in order to
determine the effect of temperature and pressure (experimental
variables) on the yield of the process when CO2 + 20% of ethanol
is used as solvent system, accordingly ethanol was selected as the
best CO2 modifier. The ranges for the factorial design were 35–55 ◦C
for temperature, and 10–40 MPa for pressure. Then, the extraction
process was analyzed through a factorial experimental design
using 22 + 2 central points where temperature and pressure were
adjusted. On the basis of this design a total of 6 experiments were
carried out in a single block. The response variable was the extraction
yield expressed as g/100 g of dry matter. The experimental
data were analyzed by Statgraphics Plus 5.1® (1994–2001, Statistical
Graphics Corp.). Empirical correlations were developed in order
to predict the influence of extraction conditions on the extraction
yield of the process studied.