| In this study, ultra-high pressure homogenization is used on soy protein isolate for modification treatment to investigate the effect on the structure and functional properties of soy protein isolate under different homogeneous pressure. And then soy protein isolate composite film was prepared by adding glycerol to modified soy protein isolate, barrier properties including water resistance and oil resistance were inspected respectively; In addition, the effect of different amounts of beeswax composite to barrier properties was considered and the best p erformance one was selected to use in microwave reheating pre-fried popcorn chicken. To improve the brittleness of popcorn chicken reheated by microwaving after frozen; the moisture and oil contents in the shell of popcorn chicken are measured during storage. Specific results of this study were as follows:First, different homogenization pressure, 60 MPa, 80 MPa, 100 MPa and 120 MPa, were considered to explore the effects of pressure on structural and functional characteristics of soy protein isolate. The results showed that the activated sulfhydryl and total sulfhydryl content of soy protein isolate decreased significantly(p < 0.05), 0.78 μmol/g and 6.70 μmol/g after 120 MPa treatments; polyacrylamide gel electrophoresis(SDS-PAGE) analysis showed that high pressure treatment caused protein aggregation by disulfide crosslinking and the higher the pressure, the greater the degree of aggregation; particle size analysis showed that D4,3 value of isolated soy protein occurred significant increase and increasing with pressure increases; circular dichroism spectra showed that high pressure homogenization changed secondary structures of soy protein isolate molecules: α- helix and β- corner contents increase, β- folded decrease and random coil content also changed. In addition, by fluorescence spectroscopy it can be found that the maximum absorption wavelength of soy protein isolate after ultra-high pressure homogenization shows a blue shift, which is indicating that the tertiary structure of protein molecules has be en changed, tryptophan residues was exposed to a hydrophobic environment.Secondly, functional properties of the soy protein isolate were also influenced by high-pressure treatment. Solubility showed a trend of rise first the fall with increasing pressure, the highest solubility is at 100Mpa(p < 0.05); Combining with emulsifying and ζ- potential results of analysis, emulsibility of soy protein isolate increases with the pressure increases(p < 0.05), which increased 110.8%; on the contrary, emulsion stability of soy protein isolate decreased clearly with homogenization pressure increase(p < 0.05).Surface hydrophobicity also increases as the pressure increases(p < 0.05), after 120 MPa treatment it reached the highest, 306.7% higher c ompared with the control group.Subsequently, soy protein isolates after different pressure treatment are used to make protein film for barrier properties study. The film is made at room temperature with using 90 ℃, 30 min treated glycerol as a plasticizer. The result indicated that barrier performance of the film made by high pressure homogenization processed soy protein isolate is better than the control group(without high pressure treatment). Water resistance an d oil resistance properties are the best at 100Mpa(p < 0.05). Meanwhile, the barrier property of film is studied at different time point, the best barrier effect is seven days. Water and oil resistance does not show any difference variation within 7 days(p >0.05), however the barrier effect shows significant difference(p<0.05), soy protein isolate treated under 100 MPa has the best blocking performance(p<0.05).Additionally, different amounts of beeswax(Beeswax / SPI, g / g, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 1%, 2%) were added to 100 MPa treated soy protein isolate to study the barrier property of soy protein isolate- beeswax complex film. When the addition amount of beeswax is 0.5%, results indicate that oil resistance coefficient decreased 102.64% and wate r resistance reduced 19.18% at the 7th day of the composite film. By analyzing the forces forming the composite film, it show that the main force compose soy protein isolate film is disulfide bonds; in addition, the hydrophobic interaction is also involved in the formation of the composite film.At last, best barrier properties of soy protein isolate- beeswax composite film(protein after 100 MPa high pressure homogenization, 0.5% beeswax) was used in microwave reheating popcorn chicken. By analyzing moisture and oil content in the shell of fried and microwave reheated popcorn chicken, it could be seen that the moisture and oil content decreased 36.87% and 8.09% respectively, comparing to the control group( p <0.05). With increasing of frozen storage time, the shell moisture and oil content of microwave reheated popcorn chicken were increasing, the brittleness also significantly higher(p < 0.05). In storage experiment, with the increase of freezing time, moisture and oil content in the shell of microwave re heated popcorn chicken were both increasing, but the increase of the composite films treated popcorn chicken was lower than the control group(p < 0.05). |
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