The Application Of Pulsed Electric Fields (PEF) In Pasteurilization Of Liquid Egg And The Mechanisms Of PEF On Microbe And Protein

Pulsed electric fields (PEF) is a promising non-thermal food preservation technique that uses short pulses of high intensity electric fields to inactivate most microorganisms and some enzymes at room or moderate temperatures. Compared to thermal treatment, PEF can offer fresh-like'minimally'-processed foods with little loss of color, flavor and nutrients. Compared with the extensive studies on the application of PEF in juices, there are limited reports about the application of PEF in protein-based foods. Presently, the mechanisms involved in inactivation of enzymes by PEF and the action of PEF on proteins are not fully understood. Pasteurization of liquid egg is a prerequisite for most applications in food industry; however, most of the functional properties of liquid egg are lost or modified after the mildest heat treatment because of the susceptibility of egg proteins to coagulation or thermal denaturation. It means alternative technologies must be developed and implemented considering sanitation and preservation of liquid egg.In this study, the synergistic effect of mild heat (55oC) and PEF (35kV/cm, 400μs) was employed to process liquid egg. Compared thermal processing (60oC, 3.5min), it could more effectively inactivate the microoganisms in liquid egg and retain functional properties. The PEF induced damage on Escherichia coli cells was studied by using flow cytometry (FCM) in combination with fluorescent techniques. Double-stained cells sorting by FCM successfully proved the occurrence of sublethally injured microbial cells and provided a further quantitative and real time analysis. The results show that cell membrane is more susceptible to PEF at higher temperature to form collapsed cells instead of sublethally injured cells, which might related to the synergistic effect of moderate heat and PEF on the microbial inactivation.The effects of PEF processing on structures and functions such as foaming and emulsifying properties of egg white proteins and ovalbumin were investigated. Unfolding of protein structure occurred induced by PEF, accompanied by the exposure of hydrophobic and sulfhydry groups, which improve the foaming and emulsifying properties. The protein aggregation was formed by disulfide cross-links and hydrophobic interactions when applied PEF dosage was higher than a critical level. As a result, the foaming and emulsifying properties decreased. The protein aggregation induced by PEF and the dominant binding forces were investigated the present study.Circular dichroism, second-derivative UV spectra, intrinsic fluorescence and Raman spectra were employed to investigate the changes of secondary and tertiary structures of lysozyme under PEF stress. Protective effect of sorbitol on ezymes exposed to PEF and different mechanisms involved in inactivation of enzymes by PEF and heat were firstly found.