Page 22

eFOODLab_International_03_2014

22 3/14 eFOOD-Lab international Figure 3. Inactivation behaviour of B. subtilis spores during direct plasma exposure with a plasma jet (pure argon as feed-gas). system is a thermal-equilibrium plasma, ignited by a microwave torch. Hence, the plasma and the plasma afterglow can reach temperatures of 3000 K. Therefore the plasma afterglow and the exhaust gas are cooled down in a tubular heat exchanger and the PLEXc-gas is applied indirectly to the treated product (Froehling et al., 2012). The used plasma jet (Brandenburg et al., 2009) is non-equilibrium plasma sources and hence, the temperature of the plasma and the plasma-afterglow are much lower (< 70°C) and a direct application of the plasma on a food surface is possible. An advantage of the direct plasma application is, that also highly reactive species generated in the plasma plume, can react with the surface and surface residues, like microorganisms, of the treated product. Further a significant amount of UVlight and secondary UV-light (emitted by the recombination of charged particles in the plasma), can reach the surface of the treated food. The amount of emitted UV-light can be influenced by the gas composition (Figure 2). The main advantage of an indirect plasma application is that the plasma generation and application are separated. Hence, larger and more efficient plasma sources can be used; the reactive plasma gas can be retained (at least for some minutes) and applied in a larger quantity and a more efficient way. A case study for the decontamination of whole black pepper Black pepper is one of the most common spices used in the European Union and worldwide. The product as a whole pepper seed or pepper in a milled form can contain a high microbial load, which has no effect on the product shelf-life, if the pepper is sufficiently dried and stored under proper conditions. However, if pepper is a part of the formulation of a high moisture content food and the final product is not sterilized, the microorganisms on the surface of the added pepper or also other dry herbs and spices can reduce the product shelf-life and/or cause serious foodborne diseases. Consequently, several pasteurization and sterilization methods are applied to increase the microbial safety of herbs and spices, which include the treatment with wet steam, fumigation with ethylene-oxide or γ-irradiation. However, all of these methods have disadvantages with regard to sensorial properties, consumer safety or acceptance. Consequently, there is a need for low-temperature pasteurization and sterilization processes for dry foods that achieve a reliable inactivation even of complex shaped surfaces. A promising process with the potential to fulfill these requirements is a low temperature atmospheric pressure plasma treatment, which was investigated besides other technologies like infrared and microwave heating in the GreenFooDec project (www.greenfoodec.eu). Within this project a characterization of the microbial flora on the surface of fresh harvested and sundried whole black pepper was done (Table 1). These data showed that especially bacterial endospores, which are present in a high load on the surface of black pepper and the possible presence of Salmonella, are of concern for minimally processed products. Within the GreenFooDec project two different plasma sources were tested, a direct (plasma jet) and an indirect one (PLEXc), to increase the microbial safety of whole black pepper Direct plasma treatment of whole black pepper with a plasma jet The tested plasma jet system uses argon as feed gas and small quantities of oxygen and nitrogen could be added, to influence the type and amount of reactive species in the plasma afterglow (Figure 2). The jet consists a ceramic nozzle and a wolframneedle electrode which is mounted in the centre of the nozzle. A grounded ring electrode is placed at the nozzle outlet and a gas flow of 10 slm of argon was used to generate plasma from the tip of the needle electrode to the inner wall of the nozzle, by an radio frequency (RF) (27.12 MHz and system power 30 W) exciting of the needle-electrode. The treated black pepper was in direct contact with the generated plasma afterglow. To evaluate the process efficiency, the impact on the natural microbial flora and on with bacterial endospores inoculated samples were evaluated. The resistance of bacterial spores was, as expected, higher and differed between the two tested strains compared to the inactivation of E. coli cells (4.1 log10 after 15 min). For Bacillus subtilis a 2.7 log10 inactivation was achieved (Figure 3), whereas the reference spore strain for chemical sterilization and γ-irradiation, Bacillus atrophaeus, showed a much higher resistance. The natural occurring spore flora on the surface of pepper was reduced by 2.2 log10 cycles in 15 min. Interestingly, all inactivation kinetics showed a biphasic inactivation behavior, pointing towards different inactivation mechanisms during the treatment (Figure 3). The rapid initial inactivation during the first 3 min is followed by slower second inactivation phase with a pronounced tailing in most cases. The detailed inactivation mechanisms of different plasmas are not yet fully understood. Depending on the plasma source, process parameters and process gases the reactive species vary within the plasma, which makes inactivation mechanisms difficult to compare. However, the presence of UV emitting species (Figure 2) is presumably responsible for the initial rapid inactivation (Brandenburg et al., 2009; Laroussi, 2005). The maximum achieved 1.0 to 1.7 log10 i nactivation c ould b e a ttributed t o the formation of spore agglomerates and spores which are located in cracks on the pepper surface. The agglomerates and the structured surface of the whole black pepper can act as Innovative Processi ng Tech nolo gies


eFOODLab_International_03_2014
To see the actual publication please follow the link above