Effect Of Lipoxygenase On Aggregating And Gelling Properties Of Soybean Protein Isolate

Effects of lipoxygenase (LOX) on gelling properties of soybean protein isolate (SPI) were discussed in this paper. It's the first time that connected these two aspects together in the abroad and domestic researches. In order to attain high LOX activity in soybean flour, hexane was used to remove the oil from flour, and all the procedure was under room temperature. Different LOX activity DSF was obtained by dry heating at 100℃. SPI was extracted by alkali extraction, and the gelling properties such as texture, rheology, and permeability were researched. Obviously difference in gelling properties was found with the LOX inactivation extent. In order to disclosure the reasons of these changes, SPI structures and aggregates were investigated and preliminary explanations of the mechanism were made. GC-MS showed inactivation could abate the off-flavor caused by LOX; Fluorescence of chloroform-methanol extract of SPI confirmed the peroxidized lipids existing; Turbidity showed the insoluble protein aggregates induced by LOX was the main reason for the gel deterioration. Molecular weight distribution showed soluble protein aggregates induced by dry heating did some benefit to the gelling properties. Surface hydrophobicity, SDS-PAGE and light scattering showed aggregates caused by LOX were associated by covalent bonds with huge size and hard to dissociate; but the thermal aggregates were associated by non-covalent bonds with small size and easy to dissociate during gelling procedure. Amino acid analysis showed some slight changes caused by LOX. The main effect of LOX on SPI was at the supermolecular structure or aggregate level. Results indicate disadvantages of high LOX in DSF to the gelling properties of SPI, and dry heating pretreatment was an effective way to improve it. Texture and rheology measurement showed the strength of gel was increasing, gelling concentration declining, and gelation time shortening with LOX inactivation extent deepening. Permeability and microscopy illuminated the pore radius of gels reducing and consistence increasing. And the fractal dimensions calculated from SEM images reducing indicated the gel type turned from weak to strong. Three kinds of heating methods such as dry heating, saturated water vapor and microwave were compared to inactivate the LOX activity in defatted soybean flour (DSF), and NSI was assayed as the protein denaturing reference. The main influencing factors of dry heating effect were heating temperature, time and water content in DSF in turn, and the synergism was not notable. To vapor inactivation, temperature and time were important, and to microwave was time. All these three methods could inactivate LOX with the low extent of protein denaturing.