Aqueous and enzymatic processes for edible oil extraction

Abstract

Industrial processes for the extraction of edible oil from oilseeds generally involve a solvent extraction step which may or may not be preceded by pressing. Hexane is the preferred solvent; hexane-based processes have been in commercial operation for a long time. For such processes, it is possible to achieve oil yields in excess of 95% with a solvent recovery of over 95%. In the past, the main concern of this process has been the safety implications surrounding the use of hexane. This prompted attempts to develop processes based on the use of aqueous extraction media which were unsuccessful mainly due to low oil yields. The scenario at present appears to be changing. Interest in aqueous extraction processes has been revived by increasing environmental concern. An aqueous process is looked upon as an environmentally cleaner alternative technology for oil extraction. Organic solvents such as hexane, in particular, can contribute to the industrial emissions of volatile organic compounds (VOCs). The production of VOCs in the conventional process is particularly worrisome since these can react in the atmosphere with other pollutants to produce ozone and other photochemical oxidants which can be hazardous to human health and can cause damage to crops. Besides this, the VOCs are themselves “greenhouse gases”; some are carcinogenic and have toxic properties. Other advantages of the aqueous process compared with solvent-based processes include: (1) simultaneous production of edible oil and protein isolate or concentration in the same process, (2) lower protein damage during extraction, and (3) improved process safety due to the lower risk of fire and explosion. It is also reported that aqueous extraction processes may be more cost effective since the solvent recovery step is eliminated. The main limitations of this process appear to be: (1) lower efficiency of oil extraction as evident in earlier studies, (2) demulsification requirements to recover oil when emulsions are formed, and (3) treatment of the resulting aqueous effluent. With the objective of improving the yield of aqueous processes, enzymes have been used to facilitate oil release. Selected enzymes have been tried on different types of oilseeds, resulting in extraction yields much higher than the original aqueous process (in some cases of over 90%). These enzymes mainly hydrolyze the structural polysaccharides which form the cell wall of oilseeds or the proteins which form the cell and lipid body membrane. This article aims to review aqueous and enzyme-based processes and discuss related issues.