Gently dried cells of Salmonella spp., Franconibacter pulveris (earlier classified as Enterobacter pulveris, a surrogate of Cronobacter sakazakii) and spores of Bacillus subtilis (surrogate of B. cereus) have been used to contaminate powders of corn-starch, hydroxy-methylcellulose, whey proteins, Spirulina algae, and cricket. These binomials are relevant for the food industry and enable conclusions on inactivation principles of microorganisms.
Spores of B. subtilis are much more resistant to heat compared to cells of Franconibacter pulveris and can be inactivated by 4 log_10 cycles with a very low plasma power of 7-10 mW/cm^2 in just 5 min when exposed on a flat glass carrier. The exposition on flat glass was used as a reference inactivation measurement and for comparisons with inactivation kinetics when the microorganisms are exposed to the plasma in a powder matrix. The inactivation effectiveness is a function of the product of plasma power and treatment time, or the applied surface energy respectively. This is a key finding proved as applicable for the decontamination of powders too and enables process scaling. Contaminated powders protect bacteria to a certain extent from inactivation, but the effect can be adjusted via the applied surface energy. The relations were established for starch powder, hydroxy-methylcellulose films, and algae powder specifically. Inactivation kinetics have been modeled.
For plasma diagnostics, optical emission spectrums have been registered from the UV light to the visible range of the spectrum. In both, air and nitrogen plasma, nitrogen species of the second positive system (SPS, 320 – 400 nm) are dominant. A linear positive relationship was obtained between plasma power intensity applied (mW/cm^2) and the intensity of nitrogen bands. Nevertheless, air plasma alters a Spirulina algae powder faster and stronger than nitrogen plasma. The same trends were observed concerning the inactivation of micro-organisms in the algae powder. It was concluded, that edging (abrasion) is the dominant principle of inactivation of bacterial spores, but oxidation of outer layers of spores accelerates the inactivation. An inactivation of micro-organisms or alteration of food components by UV radiation, cannot be concluded from plasma diagnostics. The distance of the sample to the plasma source and absorption effects by the dense gas layer at ambient pressure are supposed to be the reasons.
The influence of the powder layer thickness and chemical character of the powder on the rate of inactivation of bacterial spores was studied. The distance of plasma penetration into a powder layer is limited and is a factor influencing the dimensioning of a plasma source as well as predictions on the throughput of installations using a plasma for sterilization of powdered food.
The impact on the nutritional value of CAP treated powders was evaluated through the quantification and qualification of (i) antioxidant capacity, (ii) polyphenols, (iii) chromophores, (iv) proteins and (v) lipids for algae, insects and whey protein powders. While whey proteins resist a low plasma power short time CAP treatment, the insect powder was slightly oxidized and the algae powder indicates clear modification, specifically chromophores are sensitive markers.
Long term toxicological tests are still running when writing this report. Lifespan, egg-laying, and mobility of Caernohabditis elegans is studied when feeding the worm with a CAP treated Spirulina powder.
Three papers have been submitted for publication; two are in preparation.
