Impact of cold plasma on Citrobacter freundii in apple juice: Inactivation kinetics and mechanisms

• Citrobacter freundii bacteria in apple juice can be inactivated by applying cold argon/O2 plasma.
• A 5-log reduction can be achieved by an exposure time of 480 s plus 24 h subsequent storage.
• The inactivation kinetics can be described by Weibull models.
• Plasma-generated compounds in the liquid lead to cell permeabilisation and RNA damage.

Various studies have shown that cold plasma is capable of inactivating microorganisms located on a variety of food surfaces, food packaging materials and process equipment under atmospheric pressure conditions; however, less attention has been paid to the impact of cold plasma on microorganisms in liquid foodstuffs.The present study investigates cold plasma's ability to inactivate Citrobacter freundii in apple juice. Optical emission spectroscopy (OES) and temperature measurements were performed to characterise the plasma source. The plasma-related impact on microbial loads was evaluated by traditional plate count methods, while morphological changes were determined using scanning electron microscopy (SEM). Physiological property changes were obtained through flow cytometric measurements (membrane integrity, esterase activity and membrane potential). In addition, mathematical modelling was performed in order to achieve a reliable prediction of microbial inactivation and to establish the basis for possible industrial implementation.C. freundii loads in apple juice were reduced by about 5 log cycles after a plasma exposure of 480 s using argon and 0.1% oxygen plus a subsequent storage time of 24 h. The results indicate that a direct contact between bacterial cells and plasma is not necessary for achieving successful inactivation. The plasma-generated compounds in the liquid, such as H2O2 and most likely hydroperoxy radicals, are particularly responsible for microbial inactivation.