Küf dekontaminasyonu için akışkan yatak atmosferik basınç plazma reaktörü tasarımı
Özet
In this thesis study, an atmospheric pressure fluidized bed plasma system was designed and its decontamination effect on aflatoxigenic fungi (Aspergillus flavus, Aspergillus parasiticus) on the surfaces of hazelnut and maize samples, which were selected as model food samples, was investigated. Samples were artificially contaminated with A. flavus and A. parasiticus in a controlled manner and subjected to plasma decontamination process within 1-5 min in two fluidized bed reactors with different diameters, at varying plasma parameters, with using dry air and nitrogen as plasma forming gases. Parameters were optimized by tracking the decrease in fungal load. Significant reductions of 4.50 log (cfu/g) and 4.19 log (cfu/g) at D1: 49 mm reactor; 3.82 log (cfu/g) and 3.75 log (cfu/g) at D2: 65 mm reactor in A. flavus and A. parasiticus spores on hazelnut samples were achieved after 5 min treatment of "air" plasma applied at optimum plasma parameter [655 W (100 % V -25 kHz)]. In the case of maize samples, 5.48 log (cfu/g) and 5.20 log (cfu/g) reductions at D1: 49 mm reactor; 5.08 log (cfu/g) and 4.99 log (cfu/g) reductions at D2: 65 mm reactor were achieved in A. flavus and A. parasiticus spores by "air" plasma applied at the same parameters. It was observed that plasma inactivation efiiciency on Aspergillus spp. was higher on maize samples than hazelnuts due to their surface topography. When "nitrogen" was used as the plasma forming gas, reductions of 4.17 log (cfu/g) and 4.09 log (cfu/g) at D1: 49 mm reactor; 3.70 log (cfu/g) and 3.57 log (cfu/g) at D2: 65 mm reactor were achieved in A. flavus and A. parasiticus spores on hazelnut samples after 5 min treatment of plasma process applied at optimum plasma parameter (655 W). In the case of maize samples treated with "nitrogen" plasma at the same parameters, 4.62 log (cfu/g) and 4.68 log (cfu/g) reductions at D1: 49 mm reactor, 4.54 log (cfu/g) and 4.63 log (cfu/g) reductions at D2: 65 mm reactor were achieved in A. flavus and A. parasiticus spores. The killing effect of atmospheric pressure fluidized bed plasma system on A. flavus and A. parasiticus spores were increased with the applied reference voltage and the frequency while decreased with the reactor diameter. In addition to this, it was observed that dry air used as plasma forming gas was more effective than nitrogen. Also, a 3.45 log (cfu/g) reduction in natural mold-yeast flora and a 3.27 log (cfu/g) reduction in natural TAMB flora of hazelnut samples were achieved after 2 min of decontamination treatment applied at optimum plasma parameter. Reductions of 3.84 log (cfu/g) and 3.07 log (cfu/g) were achieved in natural mold-yeast flora and TAMB flora of maize samples after 3 min of plasma decontamination treatment applied at the same parameters. No viable cells were determined on both sample surfaces. D-values obtained at the optimum parameter at which the maximum reduction was obtained (655 W) were 1.11 min and 1.19 min for A. flavus and A. parasiticus on hazelnut samples; 0.91 min and 0.96 min for A. flavus and A. parasiticus on maize samples when dry air was used as process gas at D1: 49 mm reactor. The viable spore load of plasma treated model food samples did not increase after storage (30 days at 25 °C), contrarily 5% and 10% decreases were observed in A. flavus and A. parasiticus counts on hazelnut samples, while 10% and 33% decreases were observed in A. flavus and A. parasiticus counts on maize samples. Nonetheless, fungal spores on control sample surfaces continued to grow at the same storage conditions. While increases of 115% and 76% were observed in A. flavus and A. parasiticus counts on control hazelnut samples, 193% and 116% increases were observed on control maize samples after storage. The Aspergillus spp. spores that remained on the food samples surfaces after plasma process were proved to be also damaged cells that they could not continue growing on sample surfaces during storage by following their growing ability on media modified with potassium sorbate used as an inhibitor agent. The temperature change on the surface of hazelnuts while passing too close to the plasma jet in the course of the plasma decontamination process (300 s) was monitored with a thermal camera at all plasma parameters for the same conditions and it was observed that the temperature varied between 35 and 90 °C on average during the whole plasma process. However, the temperature inside the fluidized bed reactor in which the decontamination process took place during the atmospheric plasma process stayed below 45 °C. In addition to this, it was demonstrated that the temperature increase taking place during plasma treatment did not have a lethal effect on A. flavus and A. parasiticus spores. The alterations on morphological structures of A. flavus and A. parasiticus spores taken place after the decontamination process applied in atmospheric pressure fluidized bed plasma system was observed by Scanning Electron Microscopy (SEM).
