Study On The Effects Of Glycosylation On The Properties Of Peanut Protein Films And Its Mechanism

Peanut protein film is a kind of new and green material. However, protein based films, due to the strong interactions and hydrophilicity of protein molecules, have relatively low strength and elongation and poor water resistance compared to synthetic films and consequently limit the commercial employment of peanut protein film in food applications. To solve the above problems, peanut protein isolate(PPI) was glycosylated with saccharides and the changes of protein structure during glycosylation were analyzed. On the other hand, the modified protein was dried and converted into powder and then made into films and the changes of film properties were also analyzed to investigate the mechanism of glycosylation for film properties improvement. In the end, peanut protein film was applied in the seasoning packages of instant noodle.Conditions of glycosylation of PPI with saccharides were optimized. Peanut protein was glycosylated with xylose, mannose, lactose, galactose, sucrose and glucose respectively, and effects of glycosylation on the properties of protein films were analyzed. The results showed that modification of PPI with xylose glycosylation could markedly improve the mechanical properties and solubility of protein films. The conditions of glycosylation of PPI with xylose were optimized, and the optimum conditions were fixed as followed: p H 9.5, 91.5 °C, 95 min. Under these conditions, tensile strength(TS), elongation at break(E), solubility(total soluble matter, TSM) of modified protein film were 10.37 MPa, 96.47% and 35.94% respectively. Meanwhile, the film showed a compact network structure.Effects of glycosylation on the structure of protein were investigated. SDS-PAGE showed that, after glycosylation, no differences of the bands of protein fractions could be observed. But the content of arachin and conarachin II decreased after glycosylation due to the formation of polymer(>130 k Da) contributing to the crosslinking between PPI and xylose. As xylose concentration, p H, temperature and time increased, degree of glycosylation(DG) of peanut protein increased. In the condition of xylose concentration 10%, p H 9, temperature 90 °C, 90 min, the DG was 10.49%. In Fourier Transform Infrared Spectroscopy(FTIR), new absorption peak at 1117 cm-l was observed, and the intensity of amide I and amide was found to be decreased due to change of protein structure after glycosylation. Sulfydryl content of protein increased form 7.63 μmol/g pro to 9.72 μmol/g pro after glycosylation, and then formed new disulfide linkage in the process of film formation. Surface hydrophobicity of peanut protein increased significantly after glycosylation. Increase of xylose content, p H and temperature made 2.2-2.8 times, 1.5-3.4 times and 1.6-4.2 times rise of surface hydrophobicity of peanut protein. Before glycosylation, hydrogen bond was the main chemical reaction in peanut protein. After modification, ionic bond disappeared, hydrogen bond was destroyed, while the hydrophobic interaction enhanced due to the exposure of internal hydrophobic groups.The film-making process of xylose glycosylated PPI(PPI-X) was study. PPI was glycosylated with xylose and then converted into powder and made into films. The effects of powder dissolution temperature and glycerol concentration on the properties of the PPI-X films were evaluated. And the optimum conditions were fixed as followed: films were produced by dissolving the PPI-X powder at 20 °C and plasticizing with 25.0% glycerol, under these conditions, TS, E, TSM and moisture content of were 10.37 MPa, 90.11% and 35.94% respectively.Effects of glycosylation on the properties of films were investigated. In the aspect of solubility, TSM of protein films decreased significantly with the increasing addition of xylose. TSM of film with 10% xylose was 17.4% lower than film with 1% xylose. In the condition of p H 9, temperature 90 °C, protein film showed lowest TSM(33.64%). In the aspect of swelling property, p H values affected the swelling ratio of protein films significantly. Film modified at p H 11 showed highest swelling ratio as 829.70, which was 7.7 times, 3.5 times and 2.0 times as p H 3, 7, and 9. In the aspect of surface hydrophobicity, contact angle(CA) increased at first and then declined with the increasing xylose concentration. With the increase of p H, temperature and time, CA of protein films increased significantly. In the aspect of microstructure, network structure was formed in protein film with the addition of xylose. However, excessive addition of xylose made the network loosened. In alkaline condition(p H=9), network structure can be seen. But excessive crosslinking due to over high p H(p H=11) made the network compact. At low temperature, protein film with small degree of protein denaturation showed sheet structure. With temperature increased, the protein molecular unfolded, and showed network structure. On the other hand, color of protein film turned dark after protein modification. Glycocylation can also improve the water permeability of peanut protein films. However, no obvious effects on the oxygen permeability and thermal stability could be observed.The mechanism of glycosylation of PPI for improving film properties was studied. The correlation analysis between protein structure and film properties was performed. DG, sulfydryl content and hydrophbocity of protein had a significant positive correlation with TS and E, and had a significant negative correlation with TSM, repectively. The results demonstrated that protein molecular unfolded after glycosylation with xylose, greatly exposing the internal hydrophobic groups and sulfydryl group. And then hydrophobic interaction and disulfide linkage formed during film formation, which can bear high degree of deformation, resulting in improvement of tensile strength and elongation of the films. On the other hand, the exposure of hydrophobic groups weakened the hydrophilicity of protein and enhanced the hydrophobicity, therefore improve the water resistance of the protein film.Peanut protein film was applied in the seasoning package of instant noodle. With peanut protein film packaging, shelf life of plant oil could reach 180 days, compared to 40 days of plant oil with no package or plastic package. After packaging with peanut protein film, the moisture content and bacterial count can still meet the standard after storage of 60 days.This study showed that glycosylation of PPI with xylose could significantly improve the mechanical properties and water resistance of peanut protein films. And the mechanism of glycosylation of PPI for improvement of film properties was revealed. The results of this study will provid the scientific basis for the application of peanut protein film in food packaging.