## 4 Discussion  
The obtained optical emission spectroscopy experimental results show the presence of various active species that can play a significant role in fruit surface sterilization. The initiated processes can lead to bacteria inhibition, death or even their whole destruction by the cell wall opening. First of all, the presence of a nitrogen monoxide radical (NO) can start more bacteria deactivation processes. The UV radiation of this radical belongs to the UVC region, which is well known for its bactericide effects [13; 36]. Simultaneously, the NO radical is very reactive and, in a reaction with water (present also at the fruit surface), forms nitrous acid, which decreases the surface pH. Also, this effect might  
Figure 9: Antioxidant activity for different fruits treated by the plasma torch at MW power of 9 W (**left**) and antioxidant activity for blueberries treated at different power (**right**). The argon flow of 7 L/min and the cooling air flow of 4 L/min were fixed.  
contribute to the deactivation of some kinds of bacteria. The OH radical, confirmed by optical emission spectra, is well known as one of the strongest non-selective oxidative species and is strongly indicative of bacteria present at the surface. The results obtained for the NO and OH intensities in the presence of different fruits and the deactivation of microorganisms from the fruit surface show some correlation, and these radicals can be assumed as playing the main role in the microorganisms' deactivation in our experiments. The presence of atomic oxygen supports the same effect [37], but in our case its intensity decreases when the plasma is in contact with the fruits. All these species also form other active molecules like peroxnitrile, but their role in bacteria deactivation is still not fully understood. Additionally, the presence of atomic oxygen leads to ozone formation, which was successfully tested for food/sterilization bacterial protection [38].  
The results obtained show that a shorter treatment time and lower power give better effects in microorganism deactivation. We assume that the electromagnetic field frequency in the microwave region (2.45 GHz) is one of the main reasons for this. All the other plasma sources operate up to the MHz frequencies.  
An important result is that the antioxidant activity of the fruits is not significantly affected by MW plasma torch treatment. We are supposing that a very long plasma treatment will lead to its decrease, but long plasma treatment is inapplicable in practice, mainly due to the time and energy consumption.  
All the processes described above are running simultaneously and thus many synergetic effects can be expected. It is well known that bacteria have different sensitivity to the separate effects described above. As was shown in the results, the active species as well as the plasma torch conditions are dependent on the position in the plasma torch. Moreover, all properties are dependent on other parameters like the quartz tube dimensions, the surrounding air (mainly its humidity), etc. Thus, the optimization of any device used for food treatment against given bacteria must be completed separately. Also, model fruit samples with selected contamination should be studied after plasma exposure by other methods to verify the mechanism of bacteria deactivation. Some standard procedures currently developed in the field of plasma medicine can be adapted for food treatment.