Study On The Effects Of Ultra-high Pressure On The Gelation Properties Of Peanut Protein Isolates And Its Mechanism

Peanut protein isolate is an ideal protein source, but its gelatin properties yet still fail therequirement of practical production, which severely strangled its application in food industries. In thisstudy, peanut protein isolate has been treated by ultra-high pressure and its heat-induced gelatin,physiochemical and structural properties have been tested, the relationship among those three propertieswas determined, in addition, modification mechanism of ultra-high pressure was discussed. Thosefindings would promote the improvement of gelatin properties of peanut protein isolate, and thedevelopment of further process of peanut protein, and lay a theoretical foundation for its application.1. the optimal ultra-high pressure modification conditions were determined for peanut protein isolategelatin properties. We have studied the effects of different pressure, treatment time, PPI concentrationand pH on heat-induced gelatin properties, including hardness, springiness and cohesiveness of peanutprotein isolate. Orthogonally rotational combination des ign was adopted to optimize parameters ofultra-high pressure process conditions with gelatin hardness as the investigation indication. The optimalconditions were determined as PPI concentration3.11%under pressure115MPa treated for5min,resulted in hardness174.37, springiness0.78and cohesiveness0.32.2. Impacts of ultra-high pressure on physiochemical properties of peanut protein isolate have beeninvestigated. After ultra-high pressure treatment, water-holding capacity (WHC), oil-binding capacity(OBC) and gelatin hardness of PPI were significantly improved compared with that of untreated PPI （p＜0.05）. PPI treated under the optimal conditions of ultra-high pressure modification, its water-holdingcapacity increased by8.9%, oil-binding capacity by50.0%and gelatin hardness by50.6%, while gelatinspringiness hardly changed. Compared with commercial SPI, both PPI‘s oil-binding capacity andgelatin hardness shown significant increase（p＜0.01）, but decrease in water-holding capacity (exceptsoluble SPI).SDS-PAGE study shown the amount of conarachin Ⅱ was significantly affected by ultra-highpressure treatment, under condition of200MPa, amount of conarachin Ⅱ decreased form16.0%tojust2.6%and the band almost disappeared. DSC scanning analys is of natural peanut protein isolateshown tow heat-absorption peaks, one at93.53℃for conarachin and another at107.25℃for peanutprotein. Ultra-high pressure could change heat stability of PPI, when pressure ranged from50to200MPa, conarachin was completely denaturalized when pressure surpassed150MPa, with heat-absorptionpeaks vanished. Total△H of PPI was gradually decreased with increasing pressure. Took results ofgelatin hardness into consideration, we found when PPI was rapidly denaturalized, its gelatin hardnessalso rapidly decreased.Particle size of peanut protein isolate was apparently decreased after ultra-high pressure treatment,and it reached the smallest size6.77um, decreased by69.76%from untreated protein, while the gelatinhardness reached the peak value under the same condition100MPa. Findings shown particle size of PPIdecreased to certain level may contribute to the improvement of PPI gelatin hardness.Distribution of PPI molecular weight was investigated by laser scattering method. Results indicated there were two major peaks in untreated PPI, one for3.648e5Da (group I) with concentration of70.87%,and another for2.327e5Da (group II) with29.13%. After ultra-high pressure treatment, PPI of group Igradually degenerated, when under200MPa, it remained only36.08%, while group II increased to62.95%and appeared some small groups of larger molecules. Took results of gelatin hardness intoconsideration, we found that when the contents of group I and II reach equal may lead to better gelatinhardness, and huge polymer might not favor the improvement of gelatin harness.Amino acid composition of PPI was tested by acidic hydrolysis methods and results indicatedultra-high pressure treatment hasn‘t changed it. Both total and individual amount of amino acids (exceptcystine) were slightly increased, the most three amino acids were glutamic acid, aspartic acid andarginine, whose combined content accounted for about55%of total amino acid in samples. Furtherinvestigation shown, neither composition nor content of amino acids leaded to significant changes ingelatin hardness.3. Effects of ultra-high pressure on secondary structure of PPI were determined. Results shownsecondary structure of PPI solution and PPI powder contained ɑ helix9.60%and13.35%, β strand32.10%and45.71%, β sheet10.60%and28.10%, coil47.70%and12.84%, respectively. Secondarystructure of PPI solution and powder were separately studied by circular dichroism spectra and infraredspectrometer and shown significant changes. When compared with untreated, PPI solution and powdertreated with100Mpa leaded to an increase in ɑ helix by31.8%and14.58%, while β strand decreased by28.9%and12.87%, respectively, and the gelatin hardness reached the peak value. Regardless of thedifferences in samples treatment and analysis methods, all the results indicated there were significantincreases in ɑ helix and decreases in β strand after ultra-high pressure treatment. Took results of gelatinhardness into consideration, change of secondary structure of PPI might lead to change of gelatinhardness, and increases in ɑ helix and decreases in β strand reached certain level might contribute to theimprovement of gelatin hardness.4. Effects of ultra-high pressure on tertiary structure of PPI were determined. Content of sulfhydrylgroup (SH) increased from4.10μmol/g pro to4.78μmol/g pro with pressure in the early phase, butdecreased to1.92μmol/g pro in the following, while disulfide bond (S-S) steadily increased from44.22μmol/g pro to66.18μmol/g pro. However, content of sulfhydryl group (SH) and disulfide bond(S-S) increased with concentration and treatment time in the early phase but decreased in the following.Ultra-high pressure help exposure of sulfhydryl group (SH) in PPI, and part of SH turned into disulfidebond (S-S) through oxidation. When sulfhydryl group (SH) and disulfide bond (S-S) reached4.29μmol/g pro and59.55μmol/g pro respectively, gelatin hardness reached the peak value. Florescenceanalys is shown surface hydrophobicity (H0) of PPI increased with pressure, concentration and time inthe early phase but decreased in the following, and it reached peak value6.09under condition of100MPa,5min and5%. All the results indicated after ultra-high pressure treatment the structure of PPI gotlooser.5. Mass spectrometry analysis indicated there were580amino acids in conarachin Ⅱ andultra-high pressure hasn‘t changed PPI‘s primary sequence. Further investigated its steric conformation by imitation, we found most β sheet and β strand transformed into disordered coil conformation andresulted in a more looser structure for conarachin Ⅱ.