Submitted to: Journal of the American Oil Chemists' Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 13, 1997
Publication Date: N/A
Interpretive Summary: Water vapor becomes liquid water at the normal pressure when water vapor gets cooled below 100 C. Like water, carbon dioxide (CO2) exists in two states liquid or gas depending upon temperature and pressure. When CO2 is put above a high pressure and a high temperature, called critical pressure and critical temperature, it exists only in a special state called "supercritical." This means that CO2 in the supercritical state behaves much like a liquid, but still has some useful properties of a gas. This CO2 in the supercritical state (denoted SC-CO2) has a gentle solvent power. Recently rice bran oil has received some attention from the public because it contains a group of chemical compounds called sterols which may be effective in lowering cholesterol. Conventionally, most vegetable oils including rice bran are produced via solvent extraction using hexane, a petroleum-derived solvent. Because hexane is a powerful solvent, it extracts not only useful oil but also some undesired chemical components such as free fatty acids. Free fatty acids are detrimental to the quality of an edible oil. In this investigation, SC-CO2 was tested to examine whether it is efficient in extracting useful components of rice bran oil. Results indicate that at special conditions, SC-CO2 extracted the neutral components of rice bran oil, including sterols, as efficiently as hexane, but extracted fewer free fatty acids. It was also found that the color of rice bran oil extracted by SC-CO2 was superior to the hexane-extracted oil.
Technical Abstract: Extraction of rice bran lipids with supercritical carbon dioxide (SC-CO2) was performed. To investigate the pressure effect on extraction yield two isobaric conditions, 7000 psi and 9000 psi, were selected. A sohxlet extraction with hexane (a modified AOCS method Aa 4-38; 4 hrs. at 69 C) was also conducted, and used as the comparison basis. Rice bran with a moisture content of 6%, 90% passable through a sieve with 0.297 mm opening, was used for extraction. A maximum rice bran oil (RBO) yield of 20.5%, which represents a 99+% lipid recovery, was obtained with hexane. RBO yield with SC-CO2 ranged between 20.4 and 19.2%. RBO yield increased with temperature at the isobaric conditions. At the 80 C isotherm, an increase in RBO yield was obtained with an increase in pressure. The pressure effect may be attributed to the increase in SC-CO2 density, which is closely related to the value of Hildebrand solubility parameter. Rice bran oil extracted with SC-CO2 had a far superior color quality, when compared with hexane-extracted RBO. The amount of sterols in SC-C02-ex ed RBO increased with pressure and temperature.