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eFOODLab_International_03_2014

Untreated 15min direct plasma 30 min indirect plasma Figure 4. Scanning electron micrographs of untreated and plasma treated B. subtilis spores on the surface of black pepper. HMC EUROPE Sterilization Technology Autoclaves for microbiology Best quality Maximum comfort Affordable 3/14 eFOOD-Lab international 23 a protection and hence, reduce the inactivation effect by the emitted UV-light. The seconded, slower inactivation phase is mainly attributed to two other inactivation mechanisms: the intrinsic photodesorption and etching (Boudam et al., 2006). The reactive species, present during the direct plasma application, are absorbed onto the surface of the spores and bacterial cells. This results in a decomposition of the cell structures, caused by the formation of volatile components, which are eliminated from the cell surface (Figure 4). Indirect plasma treatment of whole black pepper with a PLEXc- plasma system For the indirect plasma treatment air was used as process gas in a microwave discharge plasma system. Due to the high energy transfer into this thermal-equilibrium plasma in the PLEXc system and the consequently high gas temperature inside the discharge (> 3000 K), all molecules in the feed-gas are dissociated. While the gas cools down chemical reactions produce different nitrogen species (RNS), like NO, NO2, HNO2 and HNO3 as well as reactive oxygen species (ROS) like H2O2 and O 3. These species are well known for their antimicrobial effect and might further react to nitric acid on a wet surface, which would enhance the inactivation of bacterial spores. This plasma processed air was cooled down to ambient temperature and the pepper seeds were exposed to it. Similar to the direct plasma treatments, after a longer exposure to the plasma processed air, a higher inactivation was achieved. Furthermore the assumed formation of nitric acid on the surface of moistened black pepper accelerated the inactivation. In this case, the addition of 5 % water to the dry pepper significantly increased the inactivation rate, but had nearly no impact on the final achieved maximum inactivation of naturally spoiled and inoculated samples. Again a kind of biphasic inactivation was observed, which levelled off for treatment times above 60 min. In this regard, the total aerobic mesophilic bacteria (TPC) on the pepper surface showed a higher sensitivity towards the indirect plasma treatment compared to the natural spore flora. Further, the inactivation data for the inoculated samples showed a higher resistance of B. subtilis compared to B. atrophaeus and the natural spore flora. This finding, points towards a different inactivation mechanism compared to the direct plasma treatment with the plasma jet, which is also visible in (Figure 4). In the case for the indirect plasma treatment a rapid initial inactivation caused by UV-photon could be excluded. Further the amount of radicals in the plasma gas is much lower compared to a direct plasma exposure. Hence, also the described effects of photodesorption and etching could have only a minor impact on the inactivation. However, Setlow et al. (2007), described that a pH drop can permeabilize the inner membrane of a bacterial spore. Similar findings for much higher pH values are also described for vegetative cells (Gianotti et al., 2009; Yuk and Marshall, 2004). Consequently this pH drop in combination with antimicrobial effect of the RNS and ROS species in the plasma processed air are presumably responsible for the observed inactivation. To further enhance the inactivation efficiency of the indirect plasma treatment a continuous mixing of the product, during plasma processed air exposure, was tested. This treatment achieved the often by the food industry recommended 4 log10 reduction of TPC within 60 min. Product quality of plasma treated pepper To evaluate the impact of both plasma sources on the product quality: the colour, the product weight and the impact on the main aroma compound, piperine, as well as the amount of essential oils was determined after the plasma treatment. All tested treatment conditions had nearly no impact on these quality parameters (Table 2). The only product quality parameter which was affected by the direct plasma application was the product weight. This was reduced by approx. 5 %, whereas a 5 % addition of water to the samples prior plasma exposure reduced the weight loss to 1.5 %. The reason for the weight loss is most probably caused be the heat-up of the pepper seeds during long plasma exposure (up to 70 °C) and the dry feed-gas STERILIZING SIMPLY AND STERIL WELL www.hmc-europe.com HMC-Europe GmbH Sterilization Technology Kellerstr. 1 84577 Tuessling - Germany Telefon: +49 8633 505 20 -0 Fax +49 8633 505 20 -99 Chamber volumes from 16 - 150 litres


eFOODLab_International_03_2014
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