## 3 Results and Discussion
### Comparative Analysis of the AOA of Aqueous Extracts of Treated and Untreated Coffee Beans
Percentage indicates the change in the average value of AOA after treatment. Bars indicate the upper boundaries of the SD. Statistically significant differences relative to the control are indicated by one (\(p\) < 0.05) and two (\(p\) < 0.01) asterisks. coffee beans decreased significantly (Table 1), their dielectric capacity declined, and they became less sensitive to MWR treatment with specified parameters (800 W, 2.45 GHz, 60 s). After combined SF + MWR treatment, green, medium roasted and dark roasted coffee showed an increase in the AOA of the extracts to 979.8 \(\pm\) 29.5, 565.1 \(\pm\) 15.4 and 505.0 \(\pm\) 32.1 \(\upmu\)mol Fe\({}^{2+}\)-E/g DW (0.01 \(<\) \(p\)\(<\) 0.05), or an increase in the mean AOA value by 15.7%, 15.0% and 23.0%, respectively. After combined FF + MWR treatment, green, medium roast and dark roast coffee showed an increase in the AOA of the extracts to 940.7 \(\pm\) 25.9, 543.8 \(\pm\) 13.7 and 492.8 \(\pm\) 33.5 \(\upmu\)mol Fe\({}^{2+}\)-E/g DW (0.01 \(<\) \(p\)\(<\) 0.05), or an increase in the mean AOA value by 11.1%, 10.6% and 20.0%, respectively. After combined treatments, an increase in the AOA of green coffee extracts was approximately the same as the total effects of individual treatments. In the case of roasted coffee, combined treatments led to a synergistic increase in the AOA of the extracts. Thus, SF + MWR and FF + MWR are the best pre-treatment methods that can be applied to coffee beans to increase the release of extractive antioxidant compounds. The results showed that CAP treatment of green coffee for 5 and 15 min reduced the extract AOA to 814.6 \(\pm\) 18.5 \(\upmu\)mol Fe\({}^{2+}\)-E/g DW (\(p\)\(<\) 0.05) or by 3.8%, and to 765.1 \(\pm\) 19.8 \(\upmu\)mol Fe\({}^{2+}\)-E/g DW (\(p\)\(<\) 0.01) or by 9.7%, respectively. The changes in the AOA of roasted coffee extracts were insignificant. The obtained results are explained by heterophase oxidative processes involving reactive oxygen and nitrogen species, as well as by the presence of a protective oily film on the surface of roasted beans. The DBD is known to produce reactive oxygen and nitrogen species that can work as strong oxidants [56; 57; 58; 59; 60]. Since radical particles have a short lifespan and there is a spatial gap between the DBD and the treated sample (Figure 1a), then the main contribution of heterophase oxidation is caused by the most stable molecules, such as ozone (O\({}_{3}\)), nitrogen oxides (NO\({}_{\rm X}\)) and hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)). The mechanisms of generating O\({}_{3}\), NOx and H\({}_{2}\)O\({}_{2}\) by the DBD were discussed in [56; 57; 58; 59; 60]. The only report on the effect of cold plasma treatment on the AOA of coffee extracts was found in [31]. In that study, samples of ground roasted coffee were placed in a Petri dish and treated with a stream of cold helium plasma for 30 min. The AOA of the extracts was determined by spectrophotometric methods using FCR and 2,2\({}^{\prime}\)-azinobis-3-ethyl benzthiazoline-6-sulfonic acid (ABTS) radical cation. After a 30 min treatment with cold plasma, Folin-Ciocalteu assay showed a 12% reduction in the extract AOA, while the ABTS assay showed that the AOA of the extracts increased by 14%. [PERSON] et al. [31] explained the obtained results by oxidation of phenolic antioxidants with molecular ozone to hydroxylated and quinone compounds (Folin-Ciocalteu assay) and by non-phenolic antioxidant synthesis due to the effect of UV-B radiation during the exposure to cold plasma (ABTS assay). Meanwhile, UV-B enhanced biosynthesis of phenolics in the plant cellular system was also taking place [61]. Apparently, by reason of the complex nature of plasma, the exact mechanisms of degradation and synthesis of bioactive compounds still remain unclear. It is also worth noting that the results for coffee beans and ground coffee may differ due to differences in the active surface area.