Unraveling the nature of ohmic heating effects in structural aspects of whey proteins: the impact of electrical and electrochemical effects

Electric field (EF) variables and electrode material are fundamental factors in ohmic heating (OH) processing. In this study, stainless steel, titanium and graphite were used as electrodes in conventional and OH (at 50 Hz and 20 kHz) treatments of whey protein isolate solutions at pH 3 and pH 7. The treated solutions were characterized in terms of UV absorbance, changes in intrinsic fluorescence, binding of a conformational probe and secondary structure (evaluated by circular dichroism). Differentiated structural features were induced by the different treatment conditions and were more evident at pH 7 and OH at 50 Hz. The metal leakage (quantified with ICP-MS) from the electrodes revealed to influence the equilibrium of early stages of protein aggregation, with stainless steel demonstrating to be the most electro-active material followed by titanium and finally graphite (that did not release metal). These findings highlight that structural properties of whey proteins may not only be influenced by the EF action but also by the leakage of metals from the electrodes. Industrial relevance OH is a recognized emerging processing technology and has a recognized potential to control and enhance protein functionality due its ability to influence protein structural features. Both electrode stability and protein structural changes are related with the EF operational parameters such as voltage, current or frequency. These parameters are decisive in the occurrence and extension of electrochemical phenomena and in the EF action mechanisms over protein structure. The confirmation of EF non-thermal effects at high and low frequencies (independently of the electrochemical phenomena) dissipated previous doubts regarding its effectiveness to modulate protein functionality. The understanding of the electrode material stability and influence of different electric parameters on the product's functionality, stability and safety are fundamental to design OH devices and to the establishment of operational protocols. The stability and material release from the electrodes is a fundamental factor during OH processing, imposing serious implications in equipment costs, maintenance, as well as in product quality and safety. This study highlights the influence of different electrode materials during OH at different electrical frequencies and its impacts on protein structural aspects.
This study was supported by the Portuguese Foundation for Science and Technology (FCT) under the scope of the strategic funding of UID/ BIO/04469/2019 and UID/QUI/50006/2019 with funding from FCT/ MCTES through national funds, BioTecNorte operation (NORTE-01- 0145-FEDER-000004) funded by the European Regional Development Fund under the scope of Norte2020 - Programa Operacional Regional do Norte. Ricardo N. Pereira acknowledge FCT for its Assistant Research contract obtained under CEEC Individual 2017.
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