## 3 Results
#### 3.1.2 Antioxidant Capacity and Compounds
[30], who optimized the microwave and pressurized liquid extraction of carotenoids from _P. tricornutum_, also described a reduction in carotenoid extraction when the temperatura increased, probably due to compound degradation. Regarding the TPC, the most influential parameter in their extraction was the time (\(p=0.0496\)). The efficiency of the extraction increased with the passage of time (up to 16 min) when the TPC value was at its maximum. After 16 min, this value decreased (Figure 5). This is in close agreement with the results reported by [PERSON] et al. [17] for _Nannochloropsis_ spp., who found that the optimal extraction of the total phenolic compounds assisted by ultrasound (W= 400 W) was achieved after 15 min. [PERSON] et al. [31] also found a huge increase in the TPC extraction from _Nannochloropsis gaditana_, _Scenedesmus almeriensis_ and _Spirulina platensis_ after ultrasound treatment for 5 min. The temperature and pH did not significantly influence the polyphenol extraction (\(p=0.5568\) and \(0.2021\), respectively). Other authors, such as [PERSON] et al. [32], also reported that neither the temperature nor the pH had a significant influence on the UAE of TPC from _Spirulina platensis_. Figure 3: Influence of the different extraction conditions (**A**) and main effects chart for these conditions (**B**) on chlorophyll A’s recovery yield (mg/100 g dry matter). The least relevant factor (highest p-value) has been set at its optimal value. Figure 4: Influence of the different extraction conditions (**A**) and main effects chart for these conditions (**B**) on the total carotenoid recovery yield (mg/100 g dry matter). The least relevant factor (highest \(p\)-value) has been set at its optimal value. Figure 5: Influence of the different extraction conditions (**A**) and main effects chart for these conditions (**B**) on the total phenolic compounds (TPC) recovery yield (mg/100 g dry matter). The least relevant factor (highest \(p\)-value) has been set at its optimal value. Finally, regarding the total antioxidant activity, the ABTS values ranged from 563.82 to 760.11 \(\upmu\)M TE, while the ORAC values where higher, ranging from 1416.81 to 2340.01 \(\upmu\)M TE. The optimal conditions for the maximum antioxidant capacity measured by the ABTS method were 28.36 min, 20 \({}^{\circ}\)C and pH = 5.5, obtaining a theoretical value of 758.28 \(\upmu\)M TE. On the other hand, for the ORAC assay, the best conditions were 30 min, 47.65 \({}^{\circ}\)C and pH = 8.5. The theoretical antioxidant activity measured by the ORAC assay applying the optimal conditions was 2338.54 \(\upmu\)M TE, really close to the experimental values obtained with the extraction for 30 min at 50 \({}^{\circ}\)C and pH = 8.5 (2340.01 \(\upmu\)M TE) (Table 4). These values of the antioxidant capacity are in the same range as those described in the literature for _P. tricornutum_[30; 33]. As can be seen in Figure 6, the antioxidant activity measured by the ABTS assay was strongly influenced by the extraction time (\(p\) = 0.0044), but the temperature (\(p\) = 0.1386) and pH (\(p\) = 0.9547) did not show a great impact. The increase of the extraction time led to a clear improved antioxidant capacity, from 698.57 \(\upmu\)M at 0.5 min to 760.11 \(\upmu\)M at 30 min (at the optimal conditions for the temperature and pH, 20 \({}^{\circ}\)C and 5.5, respectively). This can be explained by the increase in the antioxidant compounds' extraction with the passage of time. [PERSON] et al. [34] also found a significant influence of the extraction time on the antioxidant activity, measured by DPPH, when they optimized the bioactive compound extraction from _P. tricornutum_. However, [PERSON] et al. [15] found a minimal decrease in antioxidant activity, measured by ABTS in _P. tricornutum_ aqueous extracts, comparing 4 h and 24 h of extraction. The differences with our study can be explained regarding the huge time interval.