Inactivation of Salmonella, Enterococcus faecium and natural microbiota on dry food matrices with microwave-driven plasma-processed air

This study explores the efficacy of plasma-processed air (PPA) as a non-thermal treatment for decontamination of dry food surfaces, such as wheat grains, black peppercorns, and onion flakes. We compared the effect of various inoculum types of Enterococcus faecium, revealing a higher resistance of the powder form compared with liquid and slurry inocula on black peppercorns. Furthermore, Enterococcus faecium NRRL B-2354 appeared to be a suitable control indicator for Salmonella. The PPA efficacy varied across matrices, being higher on medium agar (7.0 +/- 0.5 log(10)), followed by wheat grains (2.1 +/- 0.2 log(10)), black peppercorns (1.5 +/- 0.1 log(10)), and no reduction on onion flakes. Variation was also found across microorganisms, as the natural microbiota was reduced on wheat grains (1.6 +/- 0.6 log(10) for mesophilic count and 0.8 +/- 0.3 log(10) for spores), while on black peppercorns this was only achieved after upscaling (1.3 +/- 0.2 log(10) for mesophilic and 1.5 +/- 0.1 log(10) for spores). The findings of this study give guidance to further optimize the application of PPA treatment by changing the technical parameter settings of the plasma setup to increase the microbial efficacy and/or combining the PPA treatment with other methods to enhance microbial control.