Recent Developments in Inactivation Technologies for Low-moisture Foods

Cold atmospheric pressure plasma(CAPP) has been applied in diverse fields of research to achieve several purposes, e. g., to produce specific functional groups at the surface, increase surface energy andhydrophobicity, introduce surface cross-linking and remove weak boundary layers or contaminants. The application of CAPP is an alternative process for the inactivation of microorganisms on dry and also heat sensitive surfaces. The inactivation of microorganisms on the surface of dry products, like herbs, spices, and/or almonds is difficult, because of the higher resistance especially of sporulated microorganisms when compared to a medium with a lower water activity. The main challenges in plasma processing of food materials are: i)proper selection of the plasma source, ii)characterization of product-process interactions including quality and safety attributes, and iii)optimized process design and up-scaling for industrial application.

In this presentation different plasma sources were discussed regarding their inactivation of selected microorganisms, e. g. Bacillus subtilis spores(PS832). A radio-frequency(RF) plasma jet was used for the direct treatment,working with argon as a process gas with the admixture of O₂ and N₂. Furthermore, a DBD(dielectric barrier discharge)system in a static atmosphere was used with different process gases (air,N₂,O₂). For an indirect treatment plasma processed air(PPA) was used, which was generated by a microwave driven plasma torch. The different generated plasmas were characterized using optical emission spectroscopy, gas analysis tubes and the quantification of ozone. Furthermore, the temperature inside the different plasmas was measured. A quantitative PCR assay was used to detect the effect of the plasma treatment on the spore DNA, by monitoring the destruction of dnaK fragments. Additionally, selected isogenic B. subtilis mutant strain spores were plasma treated to evaluate the main inactivation effects of different plasma sources. Photonsemitted by the generated plasma((V)UV) take a key role in the inactivation process, as shown for direct treatment using DBD and plasma jet systems, but for PPA theinactivation process is dominated by diffusion of reactive species.