*3.5. The Influence of Experimental Conditions on Individual Bioactive Compounds*

The individual polyphenols and phytosterols that were identified and quantified by LC-MS and LC-MS/MS in HI extracts are presented in Table 5. For epicatechin (Y1), catechin (Y2), syringic acid (Y3), gallic acid (Y4), protocatechuic acid (Y5), and vanillic acid (Y6) (Figure S1), the highest extraction yield was obtained for 2 min of stirring, at pH of 7, in a mixture of acetone/water (3:1). For *p*-coumaric acid (Y7) and ferulic acid (Y8) (Figure S2), the best recovery was attained for 1 min stirring time, pH 5, solvent mixture of acetone/water (3:1), while for hyperoside (Y9), isoquercitrin (Y10), and quercitrin (Y11) (Figure S3), the highest extraction power was observed for 3 min of stirring, at pH 3, in a mixture of solvents acetone/water in equal proportions (1:1). As far as stigmasterol and beta-sitosterol are concerned, their best recovery was obtained after 2 min of stirring the HI extracts in a mixture of

solvents acetone/water (3:1) at pH 5, while the extraction of campesterol was the highest when stirring the sample for 3 min, at pH 5, in a mixture of acetone/water (3:1) (Table 5) (Figure S4).

From the 11 depicted polyphenols, catechin and protocatechuic acid were found in the highest quantity, whereas from the three revealed phytosterols, beta-sitosterol presented the highest amount. The influence of the working conditions on the evaluated bioactive compound recovery from HI extracts is depicted in Figure 3. Furthermore, for predicting the recovery of bioactive compounds considering the working conditions, the response surfaces were generated and presented in Figure 4. By analyzing the two figures, it can be concluded that the amount of water in the solvent mixture exhibited the highest impact on the recovery of bioactive compounds, followed by the stirring time. The pH displayed a minimum impact on the extraction yield.

**Figure 3.** *Cont*.

**Figure 3.** Influence of working conditions on the bioactive compound recovery from hazelnut involucre extracts, depicted as scaled and centered coefficient plots. X1, stirring time (min); X2, pH; X3, water in solvent (% *v*/*v*).

We consider this information especially valuable because it presents the optimal experimental conditions for obtaining the maximum extraction yield.

**Figure 4.** Response surface for prediction of bioactive compound recovery from hazelnut involucre extracts with respect to: X1, stirring time (min); X2, pH; X3, water in solvent (% *v*/*v*). The regions in red represent the domains of working conditions assuring the maximum extraction yield for the evaluated bioactive compounds.
