*3.2. The Influence of Experimental Conditions on TPC, TFC, CTC, and AA-TEAC*

As it can be noticed from the results for TPC, TFC, CTC, and AA-TEAC (Table 3), the responses were influenced by the factors used in the experimental design.

The stirring time was in the range of 1–3 min, the pH varied from acidic to neutral (3–5–7), and the two solvents used were water and acetone mixed in various proportions (0%–25%–50% water in acetone). The best extraction yields were obtained when using binary-solvent systems, while mono-solvent systems displayed a lower extraction power. These findings confirm the rational choice of the solvent mixture based on the previously reported determination of bioactive compounds in a study that aimed to evaluate the recovery efficiency from walnut septum extracts [15]. For a higher extraction yield, the best solvent mixture is between a polar, protic solvent (water) with a polar, relatively acidic, aprotic solvent (acetone). The best results for TPC, TFC, CTC, and AA-TEAC were obtained for pH 3 and 50% water in acetone, proving a positive relationship between the content of bioactive compounds and the antioxidant activity. The influences of working conditions on TPC, TFC, CTC, and AA-TEAC as scaled and centered coefficient plots are depicted in Figure 1, while the response surfaces for predicting the extraction efficiency for the aforementioned parameters are shown in Figure 2. From these figures, it can be observed that the factor with the highest positive impact on the recovery of bioactive compounds is the amount of water in the mixture solvent, a higher amount of water being associated with an increased extraction yield. In addition, the water amount proved to have a statistically significant influence upon TPC, TFC, CTC, AA-TEAC, and bioactive compound recovery (see Tables S4 and S7). The optimal extraction conditions for TPC, TFC, CTC, and AA-TEAC, generated by the software used, are given in Table 6.

**Figure 1.** Influence of working conditions on total phenolic content (**TPC**), total flavonoid content (**TFC**), condensed tannin content (**CTC**), and antioxidant activity (**AA**) by TEAC assay of hazelnut involucre extracts, presented as scaled and centered coefficient plots. X1, stirring time (min); X2, pH; X3, water in solvent (% *v*/*v*).

**Figure 2.** Response surface for predicting the recovery yield for total phenolic content (**TPC**), total flavonoid content (**TFC**), condensed tannin content (**CTC**,) and antioxidant activity (**AA**) by TEAC assay for hazelnut involucre extracts with regard to: X1, stirring time (min); X3, water in solvent (% *v*/*v*); X2, pH = 5. The regions in red represent the domains of working conditions assuring the maximum extraction yield for the evaluated bioactive compounds.



TPC: Total phenolic content expressed as mg GAE/g dw = gallic acid equivalents per dry weight of hazelnut involucre; TFC: Total flavonoid content expressed as mg QE/g dw = quercetin equivalents per dry weight of hazelnut involucre; CTC: Condensed tannin content expressed as mg CE/g dw = catechin equivalents per dry weight of hazelnut involucre; AA-TEAC: Antioxidant activity by TEAC assay expressed as mg TE/g dw = Trolox equivalents per dry weight of hazelnut involucre.
