Influence of water content on the inactivation of P. digitatum spores using an air–water plasma jet

In order to investigate whether an air–water plasma jet is beneficial to improve the efficiency of inactivation, a series of experiments were done using a ring-needle plasma jet. The water content in the working gas (air) was accurately measured based on the Karl Fischer method. The effects of water on the production of OH (A²Σ⁺ –X²Π_i) and O (3p⁵P–3s⁵S) were also studied by optical emission spectroscopy. The results show that the water content is in the range of 2.53–9.58 mg l^-1, depending on the gas/water mixture ratio. The production of OH (A²Σ⁺ –X²Π_i) rises with the increase of water content, whereas the O (3p⁵P–3s⁵S) shows a declining tendency with higher water content. The sterilization experiments indicate that this air– water plasma jet inactivates the P. digitatum spores very effectively and its efficiency rises with the increase of the water content. It is possible that OH (A²Σ⁺ –X²Π_i) is a more effective species in inactivation than O (3p⁵P–3s⁵S) and the water content benefit the spore germination inhibition through rising the OH (A²Σ⁺ –X²Π_i) production. The maximum of the inactivation efficacy is up to 93% when the applied voltage is -6.75 kV and the water content is 9.58 mg l^-1.