Ultrasound-Assisted High-Voltage Cold Atmospheric Plasma Treatment on the Inactivation and Structure of Lysozyme: Effect of Treatment Voltage

Emerging nonthermal processing techniques are gaining much attention due to their numerous advantages over traditional methods. Thus, in this study, high-voltage cold atmospheric plasma (HV-CAP) was employed separately and synergetically with ultrasound on lysozyme to investigate its activity and structure. The HV-CAP was operated at 120, 140, and 160 kV for 3 min at a frequency of 120 Hz. A significant difference was observed between the untreated and both treatments in lysozyme activity, with a more pronounced result in the ultrasound-assisted HV-CAP treatment. Both treatments resulted in the modification of the tertiary structure of the lysozyme as lipid oxidation, carbonyl content, UV spectra, intrinsic fluorescence, hydrophobicity, and sulphhydryl groups increased with increased voltage. Particle size distribution of lysozyme shows that noncovalent interactions affected the tertiary structure. The effects of both treatments on the secondary structure of lysozyme were demonstrated by CD and relative molar ellipticity. The results indicated that plasma-generated reactive species and ultrasound-induced cavitation influenced the unfolding of tertiary and secondary structures of the lysozyme. The CD spectra postulated that lysozyme inactivation was closely linked to the α-helix and β-sheet components of the secondary structure. This study presents an original novel knowledge on the modification of activity and structure of lysozyme subjected to ultrasound-assisted HV-CAP treatment that has been missing from the literature.