OPTIMIZATION OF LIPID EXTRACTION FROM ROASTED PISTACHIO NUTS BY SUPERCRITICAL CO2 EXTRACTION WITH ETHANOL AS ENTRAINER
Malformed and unsplit pistachios constitute 20-25% of the production in the U.S.A. These have reduced value and are unmarketable. Therefore, recovering the lipids from these nuts is economically important. Mechanical extraction results in low oil recovery yields, and organic solvent extraction has potential health and safety problems. Supercritical carbon dioxide (SC-CO2) extraction gives similar yields to hexane extraction while producing an oil with superior characteristics, and a residue free of solvents which can be used in other ways. Conditions for maximum oil yield from roasted pistachio nuts by SC-CO2 extraction were established. Three levels of pressure (20.7, 27.6, 34.5 MPa), temperature (50, 60, 70°C), and ethanol concentration (0, 5, 10 wt%) were selected in an incomplete factorial design. Maximum oil recovery (66.14%) was obtained at 34.5 MPa and 60°C in the presence of 10 wt% ethanol by extracting about 10 g ground pistachio nuts with 200 L of CO2 at STP at a flow rate of 2.6 g/min. Effects of temperature, pressure, and ethanol concentration on the extraction yield were determined by response surface methodology using the coded variables (-1=low level, 0=medium level, and 1=high level). Temperature did not have a significant effect on the yield. The following equation adequately represented the experimental data (r²=0.94):
% Yield = (52.14) + (9.707)*P + (21.164)*EtOH - (11.049)*EtOH²
Solubility of pistachio oil in SC-CO2 and SC-CO2-ethanol (10 wt%) mixture at 34.5 MPa and 60°C was found to be 0.27 and 1.29 wt%, respectively. The effects of experimental conditions on the fatty acid composition of pistachio oil were analyzed by response surface methodology. The relative concentrations of fatty acids were independent of temperature. Palmitic (16:0), oleic (18:1w9), and linoleic (18:2w6) acid relative concentrations changed with pressure and/or ethanol concentration, whereas no change was observed in palmitoleic (16:1w7) and stearic (18:0) acid relative concentrations with the experimental conditions. Texture and sensory attributes of the original and the pistachio residue treated at optimum conditions (34.5 MPa, 60°C, and 10 wt% ethanol) were compared. Change in texture was detected by puncture and compression tests as well as by sensory evaluation. Sensory evaluation showed a reduction in roasted pistachio flavor intensity with SC-CO2 extraction.