## 3 Results and Discussion
### Oxidation Induction Time  
Oxidation induction time (OIT) is an important oil feature as it reflects the stability of oil and its resistance to the oxidation process [25]. The PDSC method applied in the following study allows assessing OIT in an accelerated mode [26]. The mass transfer improvement during UAE can also include bioactive compound co-extraction [27]. Also, the inactivation of oxidative enzymes, i.e., peroxidase and lipase, may occur during UAE [28]. These mechanisms can be responsible for longer OIT of oils and thus prolong their shelf life. Ultrasound treatment may, however, lead to formation of free radicals, which may induce lipid oxidation reactions [29]. That is why the UAE process needs to be adjusted in order to balance the influence of both factors.  
The response surface 3D graphs with contour graphs showing the extraction time and amplitude influence on OIT are presented in Figure 2. Based on the results, adequate models described by Equations (12)-(14) were fit, tested in ANOVA analysis, and verified by the coefficient of determination (R\({}^{2}\)) (Table 3). As OIT of BB and CH seed oils was dependent on the amplitude and extraction time and their interactions, they were described by quadratic models (\(p<0.05\)) with non-significant lack of fit values (\(p>0.05\)). The OIT of RB seed oil dependence was fit using a linear mathematical model (\(p<0.05\)).  
The impact of amplitude on OIT is consistent for all studied oils. With the increase in amplitude, the OIT was also increasing. Longer extraction time in the case of CH and RB oils also resulted in higher OIT values. The elevated oxidative stability among oils subjected to longer US treatment was also found by [PERSON] et al. [30]. Except the sample that was treated with US for 135 min, the oxidative stability parameter (which was oxidation onset temperature) was increasing gradually as the time of UAE was prolonged. The study of radish seed oil revealed that oil extracted in optimized UAE conditions was characterized by almost two times longer OIT than oil extracted in the Soxhlet procedure (72.5 min vs. 36.5 min, measured in 130 \({}^{\circ}\)C) [31]. Our findings were also in agreement with previous results obtained in the optimization studies for cranberry seed oil [10] and black and red currant seed oils [11].  
Figure 2: Three-dimensional and contour graphs showing oil oxidation induction time dependence from extraction time and ultrasound amplitude.  
On the contrary, in the study by [PERSON] et al. [9], it was described that ultrasound treatment resulted in decreased oxidation induction time of avocado oil when compared to Soxhlet extraction. Furthermore, [PERSON] et al. [21] described that US application significantly accelerated free radical formation during storage, which was correlated with the decreased oxidation resistance of grape seed oils obtained during UAE.  
\[OIT_{BB}=52.53+0.001\cdot X_{1}+2.47\cdot X_{2}-0.04\cdot X_{1}X_{2}+0.005\cdot X _{1}{}^{2}-0.02\cdot X_{2}{}^{2} \tag{12}\]  
\[OIT_{CH}=11.17+0.87\cdot X_{1}+1.82\cdot X_{2}+0.005\cdot X_{1}X_{2}-0.008\cdot X _{1}{}^{2}-0.08\cdot X_{2}{}^{2} \tag{13}\]  
\[OIT_{RB}=39.14+0.17\cdot X_{1}+0.31\cdot X_{2} \tag{14}\]