| Compared with the traditional pasteruzation technology, pulsed electric field(pulsed electric fields, PEF) technology is a promising non-thermal technology because it can work effectively at low or moderate temperature conditions. Meanwhile, PEF processing period is very short and has little effect on food ingredients, which would extend the shelf life. Liquid egg is convenient to use and transport, but the current pasteurization process tends to make heat-sensitive components in egg aggregate and flocculate, which would cause a lower quality of the final products. So it is very promising for those more effective technologies like PEF to replace gradually the current pasteruzation processing. However, PEF processing usually causes protein aggregation, and the mechanism is still unclear. This paper is to discuss the stability of the egg white solution, and the mechanism of aggregation, and the method of inhibiting the formation of aggregates. These will promote the PEF technology in the liquid egg processing.In this paper, colloidal properties, protein oxidation and electrophoresis patterns were used to investigate the changes of egg white solution during Pulsed Electric Field(PEF) processing. Treatment at electric field strength of 25 k V/cm for 400 μs did not change the protein solution colloidal properties, including soluble protein content, z-average size, PDI(polydispersity index) and zeta potential. However, when the processing time exceeded 600 μs, a small peak of large particle size was seen, and a decrease in soluble protein content(7.84%) and an increase in z-average size(36.9%) were also observed. Besides that, there were a slight increase in free sulfhydryl content and no increase in protein carbonyl content was detected during PEF treatments. All this could suggest that protein oxidation did not occur during all treatments but partial protein unfolding or SH ionization was enhanced. Electrophoresis patterns showed that the reason of forming insoluble aggregates after PEF treatments could be covalent and non-covalent bonds. The main components of the insoluble aggregates were lysozyme, ovalbumin and ovotransferrin.In this paper, the aggregates formation of a multi-protein system(containing ovalbumin, ovotransferrin and lysozyme) was studied through several methods, like turibidity, size exclusion chromatography and SDS-PAGE patterns. Results from size exclusion chromatography indicated that there was no soluble aggregates formed during PEF processing, And lysozyme was very important for forming insoluble aggregates in the chosen ovalbumin solutions. The results of SDS-PAGE patterns indicated that lysozyme was prone to precipitate, and it was relatively a higher component in aggregates. We also found that citric acid could be effective in inhibiting lysozyme from interacting with other proteins during PEF processing, whereas blocking the free sulphydryl by N-ethylmaleimide(NEM) can not stop aggregating.The effects of PEF on the structure and the mechanism of aggregation were also studied in the paper. During heat treatment, soluble and insoluble aggregates were observed to form, and we found the main forces were disulfide bonds. But only insoluble aggregates formed during PEF treatment, and the aggregation increased with the increase of field strength, the treatment time and the concentration of protein solution. In contrast to seven kindsof egg white treated by PEF, we also found that the components of egg white solution had an important influence on the aggregation. And contents of ovotransferrin and lysozyme were supposed to be responsible for the increase of turbidity and the decrease of soluble protein content. In addition, the aggregation mechanism of egg white protein by PEF treatment was proposed.The microscopic structure of protein molecules in citric acid and sodium chloride solution was used to study the stabilization effect of citric acid on the three main components of egg white protein(ie. ovalbumin, ovotransferrin and lysozyme). The results showed that the p H could affect the interaction of proteins during PEF processing. Besides PH, citric acid can inhibit the interaction between proteins, which could make the solution more stable. With endogenous fluorescence spectroscopy and synchronous fluorescence spectroscopy, we found the tertiary structure of the protein did not change significantly, but the formation of insoluble aggregates can cause endogenous fluorescence intensity decrease. In addition, the secondary structure of proteins after PEF processing were also determined, and the results were that ordered structure decreased and disordered structure increased with the processing time. Less changes happened in the secondary structure of citric acid system, which meaned that the protein molecule structure had been maintained. This study also found that the stabilizing effects of citric acid for egg white proteins did not go with the ovotransferrin denaturation temperature, but related to the zeta potential values.Finally, the effects of addition of citric acid to egg white’ s functional properties and the bactericidal effect during PEF processing were studied. Finally, the functional effects of citric acid and the bactericidal effect had been also researched. The results showed that citric acid could enhance the lethal and sub-lethal effects of Salmonella. When turning the egg white protein solution’s p H to 7 before PEF treatment, colloidal properties of the solution can be well maintained after PEF processing. For example, after treating at 25 k V/cm for 800 μs, soluble protein content declined by only 6.69%, and zeta potential increased to 13.8%, and particle size increased. From all these above, we can conclude that citric acid have a protective effect on protein components during PEF processing, including stabilizing surface tension of protein solution, and improving foamability and emulsibility and maintaining stability. |
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