Effects Of High Pressure Treatment On Lipoxygenase And Nutritional Inhibitors In Soybean And Properties Of Soybean Protein

Over the last twenty years, high pressure technology has developed rapidly and its applications in food processing are gradually stepping into industrialization. But, the industrial breakthrough of high pressure technology in food processing, like that of other novel technologies, can only be forced by the establishment of a scientific basis to assess its impact on food safety and quality aspects. Such quantitative assessment is indispensable to fulfill legislative food safety requirements as well as to respond to the consumers'increasing demand for high quality food. Soybean is generally acknowledged as full-nutrient food because of its abounding proteins and rational amino acids composition. And soybean protein is widely used as functional food ingredient or food additive because of its good nutritional values and excellent functional properties, such as gelatin, emulsification, foamability etc. Besides its favorable features, soybean is known to contain some adverse enzymes and nutritional inhibitors. Traditional heat treatment could effectively inactivate these enzymes and destroy microorganisms. However, it would simultaneously cause some adverse chemical changes which could affect the final quality of products. The purpose of this study is to investigate the effects of high pressure treatment on the quality of soymilk, the lipoxygenase and nutritional inhibitors in soybean, and also the physicochemical properties of soybean protein isolates, thus provide some theoretical references for the application and safety assessment of high pressure treatment in soybean products processing, and possible modification of soybean proteins by high pressure treatment.The high pressure inactivation of lipoxygenase in soy milk and crude soybean extract was studied in the pressure range 200-650 MPa with temperature varying from 5 to 60℃. And the results suggest that, for both systems, the isobaric-isothermal inactivation of lipoxygenase was irreversible and followed a first-order reaction at all pressure-temperature combinations tested. In the entire pressure-temperature area studied, the lipoxygenase inactivation rate constants increased with increasing pressure at constant temperature for both systems, indicating an acceleration of the lipoxygenase inactivation by increasing pressure. At constant elevated pressure, lipoxygenase exhibited the greatest stability around 10-20℃in both systems, indicating the Arrhenius equation not to be valid over the entire temperature range. For both systems, the temperature dependence of the lipoxygenase inactivation rate constants in mild temperature area (20℃≤T≤60℃) decreased with increasing pressure, while the highest sensitivity of the lipoxygenase inactivation rate constants to pressure was observed at about 30℃. The lipoxygenase inactivation rate constants in soy milk system were somewhat smaller than those in crude soybean extract, but on a kinetic basis, neither the reaction order of inactivation nor the pressure and temperature sensitivities of the inactivation rate constants were influenced by the different levels of food complexity between the two systems.Based on thoroughly studies of the kinetics for high pressure inactivation of lipoxygenase, two absolutely different mathematical models were used to describe the combined pressure-temperature dependence of the high pressure inactivation rate constants for lipoxygenase in both systems. Results showed that the pressure-temperature dependence of the high pressure inactivation rate constants for lipoxygenase in both systems could be described by either the thermodynamic kinetic model which built on the basis of the thermodynamic equation proposed by Hawley or the empirical mathematical model which used the Eyring equation as a starting point. By comparison, the former could do more accurately than the latter.The high pressure inactivation of nutritional inhibitors in soy milk was studied and the inactivation conditions were optimized. Results showed that the high pressure inactivation of urease in soy milk could occur at room temperature, while high pressure inactivation of trypsin inhibitors was possible only trough combination with elevated temperature (T≥40℃). The inactivation could be speeded up by increasing any one of the three influencing factors (pressure, temperature and processing time). For both urease and trypsin inhibitors inactivation, the three process parameters were optimized by using central composite rotatable design and response surface methodology. Results showed that pressure is the uppermost influencing factor, while temperature is the most minor factor. The ideal high pressure inactivation conditions of nutritional inhibitors in soy milk were as follows: pressure, 750 MPa; temperature, 60℃and processing time, 5 min.Effects of high pressure treatment on physicochemical and functional properties of soybean protein isolates (SPI) in two different buffers were studied. Results showed that the increase of SPI's solubility in pH3.0 Gly-HCl buffer induced by high pressure treatment was more remarkable than that in pH8.0 Tris-HCl buffer. Analysis of free sulfhydryl group and surface hydrophobicity of SPI showed that high pressure treatment produced a molecular unfolding of the protein with the exposure of the hydrophobic groups to the medium and thus formed new S-S bonds through SH/S-S interchange reactions. Aggregation of SPI after high pressure treatment was investigated by size exclusion chromatography, laser light scattering analysis and SDS-PAGE. High pressure treatment could induce structural reorganization of SPI in pH3.0 Gly-HCl buffer with the formation of soluble high molecular aggregates, whereas in pH8.0 Tris-HCl buffer high pressure might produce disaggregation of the protein aggregates. These phenomena demonstrated that the different existing forms of SPI in different buffers might result in the different structural changes of SPI under high pressure treatment. High pressure treatment could improve the emulsifying and foaming properties of SPI and this improvement was much remarkable in pH3.0 Gly-HCl buffer because of the significant increase of SPI's solubility and surface hydrophobicity. High pressure could induce the dispersions of SPI with a certain concentration to form a gel. Compared with temperature and processing time, pressure is the uppermost influencing factor of high pressure treatment on textural properties of the gel. The hardness of the gel is increasing when elevate the pressure, temperature and prolong the processing time. The hardness of pressure-induced gel of SPI (12% protein concentration), treated under 700MPa and 20℃for 15 minutes, was greater than that of heat-induced gel of SPI with the same protein concentration, treated under ambient pressure and 85℃for 20 minutes.Comparing the physicochemical properties, color, flavor and rheological properties of the untreated, heat treated and high pressure treated soy milk, it was shown that high pressure treatment and heat treatment did not affect the pH and electric conductivity of the soy milk; the apparent viscosity of the soy milk was increased by the heat treatment, which might because of the thermo-aggregation effect of the soybean protein; heat treatment and high pressure treatment had little effect on the color of the soy milk; high pressure treatment had little effect on reducing the existed volatile flavor compounds; however, the beany flavor of the soy milk were much decreased when high pressure treatment were done before the refining process; high pressure treatment and heat treatment had no effect on the protein based amino acid composition of the soy milk; high pressure treatment and heat treatment reduced the flow behavior index of soy milk, and the flow behavior tended to be the pseudo-plastic fluid; the consistency factor of the soy milk was dramatically increased after heat treatment, which indicated that heat treatment could elevate the apparent viscosity of the soy milk. In summary, heat treatment had greater impact on the rheological property of the soy milk than high pressure treatment, which might because of the soybean protein folded, denatured and aggregated to a far greater extent under heat treatment than high pressure treatment.