Effects Of Pulsed Electric Fields On Physicochemical Properties Of Soybean Protein And Lipoxygenase

Pulsed electric fields (PEF) are a new non-thermal food preservation method, which treats liquid or semi-solid food with high electric field intensities (10-50 kV/cm), very short electric pulse (0-100μs) and very high pulse frequency (0-2000 Hz). PEF can is convenient to continuously sterilize and aseptically can. There are many problems to need solve for the industrial application of PEF, for example, mechanism and modeling of inactivation microorganisms and enzymes; effects of PEF on food constituents and structures; product safety assurance after PEF processing etc. Soybean is a full-nutrient food due to have abounding proteins and rational amino acids. So soybean proteins are commonly used as a functional ingredient and additive for food formulation because of its excellent functional properties (stickiness, elasticity, gelling, and emulsification) and good nutritional values. However, soybean and soy products have a rancid off-flavor and negative aspects to human health due to the presence of some enzymes and antinutritional factors. Thermal preservation results in denaturation of soybean proteins and precipitation of beverage containing soybean proteins, thus affects the quality of products. Disruption of nature proteins structure can affect their physicochemical and functional properties. Low thermal does completely remove the rancid off-flavor and antinutritional factors. The objective of this study is to investigate the effect of PEF on the quality of soymilk, inactivation of soybean lipoxygenase (SLOX), and physicochemical properties and structure of soybean protein isolates (SPI) in order to provide the reference for the application of soybean products, modification of soybean proteins, and food safety.Physicochemical properties, color, flavor, enzymes, and microorganisms of soymilk have detected. The viscosity of soymilk decreased possibly due to identical tendency of electric particles in PEF inducement. PEF resulted in mild browning and L increased a little. Electric conductivity, flavor and pH did not change. The contents decrease of cysteine, hydrophobic amino acid (tyrosine and phenylalanine) and serine suggested that PEF had affected on disulfide bond, hydrophobicity and hydrogen bond. PEF inactivated markedly SLOX, soybean trypsin inhibitors (STI) and urease (p<0.05), which urease is more sensitive, SLOX second, STI resistent to PEF. PEF strength and time had remarkable effects on inactivation of E.coil, salmonella and S. aureus (p<0.05), which E.coil is more sensitive, salmonella second, S. aureus resistent to PEF.Effects of PEF treatment (0 to 547μs and 0 to 40 kV/cm) on physicochemical and functional properties of SPI were studied. The viscosity and pH of SPI PEF-treated decreased slightly. Solubility, emulsibility, foaming capacity increased with the increment of the pulsed electric fields strength and treatment time. When the PEF strength and treatment time were above 30 kV/cm and 288μs, solubility, emulsibility, foaming capacity of SPI decreased due to denaturation and aggregation of SPI by hydrophobic interactions and disulfide bonds. The denaturation temperatures of 7S reduced more than 11S after PEF treatment, which indicated 7S was more sensitive to PEF processing. The decrease of denaturation temperatures of SPI showed that PEF changed the structure of SPI and make protein unfold and relax.Aggregation of SPI after PEF was investigated by Ellman reagent, ANS fluorescence probe, size exclusion chromatography (SEC) and laser light scattering analyses (LLS). Surface free sulfhydryls and hydrophobicity of SPI increased with the increment of the PEF strength and treatment time. The stronger PEF conditions caused surface free sulfhydryls and hydrophobicity to decrease due to dissociation, denaturation and aggregation of SPI detected by SEC and LLS. Over 432μs the content of molecule of weight higher than 1000 kDa and larger than 1000 nm increased. These results showed PEF treatment induced polarization of SPI and dissociation of sub-units and molecular unfolding; changed molecular conformation and caused hydrophobic groups and free sulfhydryls buried inside the molecules to expose. Too strong PEF conditions produced stronger molecular polarization and polarized molecules attracted each other and converged again to form larger aggregations by non-covalent bonds such as hydrophobic interactions, electrostatic interactions, hydrogen bonds and S-S bonds.The structure of SPI after PEF was investigated by SDS-PAGE, CD, Raman and FT-IR spectra. Subunits of 11S and 7S fractions did not change after PEF by SDS-PAGE spectra. CD spectra showed thatα-helix content decreased, andβ-sheet content and random coil content increased with the increment of treatment time. But theβ-sheet content reduced and the random coil content increased when PEF treatment time was 547μs. These results showed PEF may destrory the interactional force (for example, hydrogen bond) maintaining the second structure of protein and change the second structure of protein. This fact was confirmed further by Raman and FT-IR spectra. Changes in S-S and C-C stretching vibration frequency by Raman spectra indicated that PEF have certain effects on sulfhydryl and disulphide bond. PEF influenced hydrophobicity by changes in tyrosine vibration frequency of Raman spectra.Residual activity of SLOX decreased with the increase of PEF treatment time, strength, frequency, and width in square wave pulse of bipolar mode. The first-order fractional conversion model, Weibull distribution function, and Fermi's model described successfully the relationship between residual activity of SLOX and PEF paremeters, which Weibull distribution function was the best model. The model showed greater capability to make predictions than the other tested models and was reflected by a higher shape parameter if the curves of inactivation of enzyme exhibited some lag time at low PEF strength. The first-order fractional conversion model provided the residual enzyme activity after a prolonged time of treatment (stabilization). Fermi's model provided the strength when residual enzyme activity is 50%. The data provide the important reference for the application of PEF in soymilk and soybean products.