## 3 Results and Discussion
### Development of the Ultrasound-Assisted Extraction Using Box-Behnken Design
Here, the TPC extracted from AOR ranged from: 5.03 mg/g DW (Obs3; obtained after 30 min extraction in water bath (no US application) using pure water as extraction solvent) to 11.44 mg/g DW (Obs16; obtained after 30 min at an ultrasonic bath running at a US frequency of 22.5 kHz using 50% (\(v\)/\(v\)) aqEtOH as extraction solvent) (Table 2). These results provide a first indication on the interest of using an ultrasound and on the choice of an extraction solvent. We noted a good repeatability of thecentral point (i.e., Obs4, 5, 9, 11, 16, and 18), with a mean TPC of \(11.37\pm 0.05\) mg/g DW corresponding to a relative standard deviation (RSD) of 0.47%, thus highlighting the high reliability of these results. Given the nature of the starting material used in the present study, this range of TPC is in fairly good agreement with the data in the literature obtained with almond and/or almond by-products from California, Portugal, Italia, and Tunisia [3; 5; 18; 42]. A multiple regression analysis was applied to the model of the TPC as a function of the three different extraction variables. Under the described conditions, the TPC (Y\({}_{\rm TTC}\), in mg/g DW) as a function of the three different extraction variables (Table 1) in the form of a polynomial equation was (Table 3):  
\[\begin{array}{c}{\rm Y_{\rm TTC}=1  
with the negative values calculated for their quadratic coefficients (X2\({}^{2}\) and X3\({}^{2}\), respectively), but also of all the coefficient involving aqEOH concentration (i.e., linear coefficient X1, quadratic coefficient X1\({}^{2}\), and the interaction coefficients X1\({}_{\text{X}}\)2 and X1\({}_{\text{X}}\)3), indicate that the TPC extracted from AOR according to these extraction parameters will reach a maximum value before decreasing for high values of these parameters. These considerations were clearly observed on the 3D plots (Figure 2). For each 3D plot, a first tendency was observed with a higher TPC extracted from AOR with increased aqEOH concentration, application of US as well as prolonged extraction time. However, after reaching a maximal value for TPC extracted from AOR, a further increase in the aqEOH concentrations as well as application of higher US frequency and/or prolonged extraction duration resulted in a pronounced drop of the TPC (Figure 2). In various concentrations in mixture with water, aqEOH solutions have been widely used as eco-friendly solvents to extract a wide range of polyphenols from plant matrices [11; 21; 30; 38; 40] including various almond products [13; 18; 44]. However, to obtain optimal results, the concentration of aqEOH must be adapted because it is very dependent on the polyphenolic compound(s) as well as on the plant matrix considered [28; 38; 40]. Alongside, it is clearly established that, during USAE, high US frequency associated with extended extraction duration could reveal destructive through the induction of polyphenols oxidation, in particular in the presence of water [21; 28; 30]. Consequently, if these parameters are not finely controlled (optimized), this can lead to a sharp reduction in the extraction yield, quantitatively but also qualitatively with a drastic decay observed in the biological interest of the sample extract [11; 29; 30]. Using the Box-Behnken matrix for optimizing these parameters and using the resulting adjusted second order polynomial equation, optimal conditions for the extraction of phenolics from our Moroccan AOR were: 53.0% (_v/v_) aqEOH as solvent, 27.0 kHz for the US frequency and an extraction duration of 29.4 min. Using these optimal conditions resulted in a TPC