Aqueous extraction processing (AEP) of soybeans has the potential to achieve oil extraction yields comparable to hexane extraction without the environmental or safety concerns associated with hexane. The economic viability of this novel process depends upon maximizing the yield of free oil as well the development of a cost effective, high yielding method of recovering protein values in the aqueous by-product.

In order to direct strategies for yield improvement, mechanisms of AEP were studied by microscopic observation of extracted residual solids coupled with yield measurements and mathematical modeling. The nature of the oil-confining matrix varied depending on physical treatment of the soybean. For extruded flakes, oil is sequestered in a matrix of insoluble protein, which is disrupted by proteolytic action. For flour, oil bodies coalesced into large droplets that have a reduced mobility within a matrix of disrupted cells. Proteolysis increased yield through a mechanism that likely involves the disruption of a viscoelastic protein film at the oil-water interface to increase the emulsification of oil. This hypothesis is supported by experiments with low molecular weight surfactants. A model developed on these concepts was able to fit experimental extraction data well. The extraction times of the pool of small oil droplets (i.e. oil bodies) were consistent with diffusion rates.

The oil release mechanism for AEP of extruded sunflower was similar to soy flour for which unextracted oil was contained within disrupted cells; however, unlike the soybean case, proteases did not increase oil extraction yield. Differences between sunflower and soybean oil extraction may result from differences in the nature of the oil-protein interactions, as well as in differing geometries of the disrupted cellular matrix.

Most proteins in an aqueous fraction from a high oil-yield extraction process from extruded soy had molecular weights between 3000 and 10000 Da. Hydrolysis was effective in reducing the trypsin inhibitor activity of the soy protein, while neither the extrusion nor the hydrolysis affected amino acid profile, indicating that the AEP protein nutritional properties would be as good as if not superior to existing soy protein products. Antinutritional oligosaccharides were effectively eliminated through the use of either galactosidases or by ultrafiltration. Ultrafiltration had the added benefit of being the most effective single step purification strategy, but was ineffective in purifying the smallest polypeptides. Isoelectric precipitation also achieved acceptable purity, but with reduced yields because of the presence of emulsified oil in the skim as well as from increased solubility of the hydrolyzed protein. Ion-exchange chromatography using expanded bed adsorption allowed effective separation of proteins from the emulsified oil and oligosaccharides, but was also incapable of capturing the smallest polypeptides.