| Effect of dissociation induced by acid treatment on the structure and functional properties of peanut proteins were studied. Peanut protein isolate(PPI) extracted from defatted peanut meal.Structural changes of the peanut protein isolate subunits caused by acid treatment were observed by SDS-PAGE electrophoresis, the Zeta potential, solubility, fluorescence spectra and size distribution. In pH < 3, the subunits unfold and acid-hydrolyze into a new 33 ku and 18 ku subunits. The acid-hydrolysis of peanut protein is affected by time and iron strength. Peanut protein isolate subunits begin to acid-hydrolyze after 10 min in pH 2.5 at room temperature, and the amount of the high molecular subunits decreases as the time increases. It hydrolyzes in 0-0.1 mmol/L,which however it is inhibited when the iron strength is above 0.2 mmol/L due to the electrostatic shielding effect. Moreover, it was found that, at pH 2.0-2.5, the extent of the acid-hydrolysis reaches to a maximum, the solubility is higher than others and the maximum absorption wavelength in the fluorescence spectra increases by 13.47 nm. When the pH value decreases to 1.0, the zeta potential decreases. At the same time and the subunits aggregate into soluble aggregation.Further, we investigated the structural changes of arachin and conarachin, the main components of peanut protein, under acidic conditions by zeta potential, endogenous fluorescence spectroscopy, the average hydrodynamic diameter analyses, sodium dodecyl sulfate- polyacrylamide gel electrophoresis(SDS-PAGE), and solubility. According to the electrophoresis analysis, the 40.5 ku, 37.5 ku, 35.5 ku, 27 ku subunits of arachin were acidhydrolyzed into a new 32.86±0.10 ku band and 18 ku, 15 ku bands. When pH < 2.0, the acidhydrolysis of arachin was totally inhibited by the electrostatic shielding caused by extreme pH. At pH range of 1.0- 3.0, the 61 ku subunits of conarachin was acid-hydrolyzed into 36.95±0.50 ku, 25.14±1.86 ku, 18.98±0.78 ku and 17.37±1.17 ku bands. Further studies indicated that both arachin and conarachin unfolded and the particle size increased. When pH was at the range of 2.0-3.0, the zeta potential and solubility were relatively higher and the maximum emission wavelength in the fluorescence spectra of arachin was red-shifted by 17.37 nm as compared to that under neutral conditions while that of conarachin was only red-shifted by 5.0 nm,which indicated conarachin unfolded to a less extent. It reveals that conarachin is less acid hydrolysis sensitive than arachin.Peanut protein isolate which are in pH 2.5(PPI-2.5), in pH 7.0(PPI-7.0) and in pH 7.0 after acid treatment in p H 2.5, was homogenized in various high pressure. Little changes were observed in solubility and endogenous fluorescence emission wave length of PPI-2.5, PPI-7.0, PPI-2.5-7.0 after homogenization less than 100 MPa. Endogenous fluorescence emission wavelength of PPI-2.5-7.0 was significantly higher than PPI-7.0, which meant that structural changes of peanut protein isolate caused by acid treatment is irreversible. The particle size of PPI-2.5 or PPI-2.5-7.0 was significantly higher than that of PPI-7.0, and with the increase of homogeneous pressure, particle size decreased, which proved that acid treatment can make the peanut protein structure unfold and increase particle size. SDS-PAGE showed that it can not break down the peanut protein peptide bond and produce new subunit bands under 100 MPa homogenization. In order to improve the utilization of peanut protein, concentration of 2.00% was used as the optimal protein concentration for homogenization. It was found that high pressure could make peanut protein form uniform aggregate. The interfacial pressure π of the oil-water interface increases with the increasing of protein concentration.To explore the influence of acid treatment on the emulsifying properties of peanut protein. Peanut protein emulsion was made with PPI-2.5, PPI-7.0, PPI-2.5-7.0. Results showed that the emulsifying ability of PPI-2.5 and PPI-2.5-7.0 were significantly higher than that of PPI-7.0. Protein concentrations controlled the size of emulsion particle size to a great extent. Interfacial adsorption value Γ increased with the increase of protein concentration, which made the gel network structure in the emulsion more compact and prevent fat coalescence. Under acid condition, the oil drop in peanut protein emulsion selectively adsorbed arachin, and conarachin Ⅱ and the majority of conarachin Ⅰ did not participate in stabilizing oil droplets. Acidmodified peanut protein emulsion(PPI-2.5-7.0) adsorbed all of arachin and part of and conarachin Ⅰ and Ⅱ bands. It proved that acid treatment improved the emulsifying ability of peanut protein. |
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