## 3 Results and Discussion
### Effect of Ultrasound and Microwave Extraction on the Antioxidant Activity  
In the case of DPPH radical analysis (Figure 1), extracts obtained using ultrasound at 45 \({}^{\circ}\)C were characterized by the highest antioxidant activity (83.45-87.42 mg ascorbic acid/g dm).  
Moreover, the extract obtained by ultrasound was characterized by a higher antioxidant activity compared to microwaves. Thus, the exposure of the extract to microwaves was more detrimental to its antioxidant activity. In turn, ABTS radical analysis (Figure 2) showed that the highest ABTS value (101.02-99.34 mg ascorbic acid/g dm) was recorded for extracts obtained using ultrasound at 45 \({}^{\circ}\)C.  
Figure 1: Antioxidant activity (2,2-diphenyl picrylhydrazyl (DPPH)) of açai extracts obtained using ultrasound, microwaves and water baths at different time and temperature. \({}^{\text{a, b, c, d, e, f}}\) means with different small superscript letters differ significantly for samples of the same extract at different temperatures.  
[PERSON] [43], observed that the antioxidant activity (IC\({}_{50}\)) of the extract obtained by ultrasound was higher than that obtained by microwaves. Moreover, the antioxidant activity of the ABTS radical was the same for the extract obtained after applying a temperature of 45 \({}^{\circ}\)C with the use of ultrasound (5 min, 25 min) and a water bath (25 min). The same antioxidant activity was obtained by using microwaves (2 min) at 25 \({}^{\circ}\)C. This probably resulted from the higher content of phenolic compounds and flavonoids in those extracts. Higher antioxidant activity was also observed in the case of ABTS analysis in comparison to DPPH, which may be caused by the fact that the ABTS radical has a stronger tendency to donate electrons than the DPPH radical [44]. Thus, it is possible to obtain a similar antioxidant activity in the extract, in shorter time and with less energy expenditure. The highest antioxidant activity of the FRAP radical (74.14-80.89 mg ascorbic acid/g dm) was observed in extracts obtained using microwaves at 25 \({}^{\circ}\)C and at 45 \({}^{\circ}\)C in comparison to ultrasound (Figure 3).  
Figure 3: Antioxidant activity (ferric-reducing antioxidant activity (FRAP)) of acai extracts obtained using water baths, ultrasound and microwaves at different times and temperatures. \({}^{\mathrm{a}}\), \({}^{\mathrm{b}}\), \({}^{\mathrm{c}}\), \({}^{\mathrm{d}}\), \({}^{\mathrm{e}}\), \({}^{\mathrm{f}}\) g means with different small superscript letters differ significantly for samples of the same extract at different temperatures.  
Figure 2: Antioxidant activity (2,2-anizobis-3-ethylbenzzthiaroline-6-sulphonic acid (ABTS) of acai extracts obtained using water baths, ultrasound and microwaves at different times and temperatures. \({}^{\mathrm{a}}\), \({}^{\mathrm{b}}\), \({}^{\mathrm{c}}\), \({}^{\mathrm{d}}\), \({}^{\mathrm{e}}\), \({}^{\mathrm{f}}\) g means with different small superscript letters differ significantly for samples of the same extract at different temperatures.  
Additionally, the analysis showed that a higher FRAP value was obtained using microwave extraction when the processing time was longer. [PERSON] [40], who extracted _Moringa stenopetala_, noticed a reverse relationship. After 10, 20 and 30 min of extraction, FRAP values (expressed as Trolox) at 40 \({}^{\circ}\)C were 141.0, 133.3, 104.9 mg/g dm and 133.2, 128.6, 98.3 mg/g dm at 50 \({}^{\circ}\)C, respectively. The differences in the antioxidant activity of the acai extract may result from the presence of heterogeneous phenolic compounds in the samples, e.g., mono-, di-, and polyphenols. Moreover, the antioxidant activity of phenolic compounds depends on their structural conformation--for instance, compounds with a second hydroxyl group in the ortho or para positions are more active compared to compounds with this group in the meta position [45].