High intensity ultrasound-heat pretreatments improving gelation properties of cold-set soy protein isolate induced by glucono-δ-lactone

Abstract: In order to explore the effects of high intensity ultrasound (HIU) on the gelation properties of glucono-δ-lactone (GDL) induced cold-set soy protein isolate (SPI), this study used HIU-heat pre-treated SPI to form GDL induced cold-set SPI gels. Texture analyzer, circular dichroism (CD) spectrum, fluorescence chromatography, scanning electron microscope (SEM), sodium dodecyl sulphate - polyacrylamide gel electrophoresis (SDS-PAGE) and particle sizer were used in this study. Compared with heat pretreatment, HIU-heat pretreatments increased the gel strength and water holding capacity (WHC) of cold SPI gels significantly (P<0.05). HIU-heat pretreatment of SPI could not change the primary structure of SPI. However, the secondary structure, tertiary structure, intermolecular forces, protein solubility and particle size of SPI were changed after HIU-heat pretreatments. For instance, HIU-heat pretreatments increased the α-helix amount but decreased the β-sheet amount as demonstrated by far-CD spectra. Moreover, HIU-heat pretreatments of SPI increased the surface hydrophobicity, free or total sulfhydryl groups of SPI. The near-CD spectra and fluorescence spectra indicated that HIU-heat treatments reduced the tertiary structure and increased the hydrophobic environments of SPI. The protein solubility in different solvents (DW, deionized water at pH value of 8.0; Buffer B, Tris-glycine buffer (0.086 mol/L Tris, 0.09 mol/L glycine, and 4 mmol/L Na2EDTA, pH value of 8.0); Buffer BSU, Buffer B containing 0.5% sodium dodecyl sulphate and 6 mol/L urea) suggested that HIU-heat treatments increased the electrostatic interactions but reduced or did not change non-covalent interactions among SPI molecules. SEM showed that the three-dimensional (3D) structure of HIU-heat pretreated SPI gels became denser and more uniform. However, different HIU-heat pretreatments influenced the gelation properties of SPI gels diversely. The gel strength and WHC of gels prepared by Method 1 (M1, HIU for 2, 4 or 10 min then heating at 95 ℃ for 20 min) increased gradually with the increasing of HIU time (gel strength: from (5.83±0.31) to (46.37±1.15) g, WHC: from 42.03%±1.59% to 81.74%±6.22%), while those of gels prepared by Method 2 (M2, heating at 95 ℃ for 20 min then HIU for 2, 4 or 10 min) increased rapidly within 4 min under HIU (gel strength: from (5.83±0.31) to (37.57±2.57) g, WHC: from 42.03%±1.85% to 79.31% ± 3.00%). This means that M2 can shorten the total producing time and reduce the energy, which might have more potential in soy bean protein industry. The mechanism for the above differences between M1 and M2 may be due to that the heating pretreatment could facilitate the unfolding of SPI, causing the following HIU treatments to modify SPI more easily. Therefore, M2 could change the conformational structures of SPI within a short time of HIU (within 4 min), resulting in the improvement of exposing hydrophobic groups, the increase of hydrophobic environment, the increase of surface hydrophobicity and the increase of protein solubility in DW, as demonstrated by CD, fluorescence and spectrophotometer data. Moreover, the intermolecular electronic interactions also increased as demonstrated by the protein solubility changes in DW, Buffer B and Buffer BSU. Therefore, a more uniform and denser micro-structure was formed to increase the gel strength and WHC as demonstrated by SEM. The results of this study can facilitate the application of HIU-heat technology in soy protein industry.