Influence of processing parameters and stress adaptation on the inactivation of Listeria monocytogenes by Non-Thermal Atmospheric Plasma (NTAP).

This study evaluated the effectiveness of Non-Thermal Atmospheric Plasma (NTAP) treatments against Listeria. Firstly, the impact of gas composition and flow rate on L. monocytogenes and L. innocua (used as a surrogate) inactivation by NTAP was monitored. Secondly, the influence of stress adaptation (growth under suboptimal conditions, using a wide range of temperatures and media acidified up to pH5.5 with citric, lactic, malic or hydrochloric acid, or short-term exposure to acid, cold or thermal shocks) on L. monocytogenes NTAP resistance was assessed. Survival curves obtained were concave upward. A mathematical model based on the Weibull distribution accurately described the inactivation kinetics. Both L. monocytogenes and L. innocua showed a higher sensitivity to plasma when the treatment was performed using air than when nitrogen was used. In fact, the use of nitrogen as working gas made the plasma treatment almost ineffective. The effect of gas flow rate on the effectiveness of the NTAP treatment depended on the type of gas used to generate plasma. Increases in flow rate from 5 to 10L/min caused an acceleration of bacterial inactivation when air was used, while an additional increase of gas flow from 10 to 15L/min had a minor impact on microbial inactivation. On the other hand, gas flow rate hardly affected NTAP treatment efficiency when nitrogen was used to generate plasma. L. monocytogenes growth under sub-optimal temperature or pH conditions or short-term exposure to acid, heat or cold stress conditions did not significantly modify its NTAP resistance. This suggests that temperature and pH stress adaptation does not induce a cross-protection response against NTAP treatments in L. monocytogenes, what makes NTAP an attractive technology for food decontamination within minimal processing strategies targeting this pathogenic microorganism.