## 3 Results and Discussion
### In Vitro Antioxidant Activity  
The in vitro antioxidant activity of sweet potato samples was measured as free radical scavenging activity (DPPH and ABTS methods), as well as the ferric-reducing capacity by the FRAP method (Table 2). The results of the DPPH, ABTS, and FRAP methods are expressed in the same units, that is, umol of Trolox equivalent per 100 g of sweet potato DM. Analysis of the antioxidant capacity of the sweet potato samples revealed that the addition of functional ingredients before drying had a significant (\(p<0.05\)) and positive effect on the ability to reduce free radicals in the tested samples (Table 2). A positive correlation was observed between ABTS and the type of impregnating solution (r = 0.84) and dry matter (r = 0.60). FRAP was positively correlated with impregnating solution (r = 0.60) and index b\({}^{*}\) (r = 0.63), while DPPH was only positively correlated with index b\({}^{*}\) (r = 0.71). For DPPH and FRAP analyses, the highest in vitro antioxidant activity was observed in the samples with onion (mean of 2357.94 and 2049.17 umol Tx/100 g DM, respectively), whereas, in the ABTS method, the highest values were determined in the FD samples with the addition of onion and kale juices (1805.77 umol Tx/100 g DM), respectively. The lowest results for DPPH and ABTS methods were obtained in samples dried without functional addition (mean of 242.87 and 708.76 umol Tx/100 g DM, respectively), whereas, in the FRAP analysis, the lowest value was recorded in the VD sample with the addition of sodium chloride, \(-321.37\) umol Tx/100 g DM. Drying sweet potato samples without the addition of functional ingredients and with the addition of NaCl resulted in a reduction in the antioxidant capacity of the samples. The most significant changes were observed with the FRAP method in samples with the addition of NaCl, regardless of the drying method (over 2.5-fold decrease in the ferric-reducing capacity in both cases). A positive correlation was observed between the antioxidant capacity of the samples and the content of bioactive compounds (Table 3). Polyphenols have the highest ability to reduce free radicals. The correlation coefficient ranged between 0.77 for FRAP and 0.97 for the ABTS method. In the case of carotenoids, the highest correlation coefficient was noted with the ABTS method (R\({}^{2}\) = 0.81), and the presence of chlorophyll was strongest in the ABTS method (R\({}^{2}\) = 0.40). Both flavonoids and carotenoids are known for their antioxidant properties [42; 43]. Flavonoids have strong antioxidant properties because of the presence of many hydroxyl groups, and carotenoids due to the numerous double bonds present in the molecule. Compounds showing antioxidant and antimutagenic activities also include chlorophylls. These compounds, which belong to the chromoplastic pigments together with carotenoids, can prevent the development of degenerative diseases and cancer [44]. The influence of different drying methods on the antioxidant capacity has been frequently described in the literature. Other research clearly shows that the drying technique plays a significant role on a higher and lower antioxidant capacity. The power to scavenge free radicals depends on the type of bioactive compounds, their concentration, and the drying technique used. Our results are in agreement with those of [PERSON] et al. [45], who analyzed the influence of different drying methods on the bioactive compound concentrations and antioxidant capacity of Phylanthus amarus.