## 3 Results and Discussion
### Development of the Ultrasound-Assisted Extraction Using Box-Behnken Design
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Source** & **Sum of Square** & **df** & **Mean of Square** & **\(F\)-Value** & **\(p\)-Value** \\ \hline Model & 106.071 & 9 & 11.786 & 567.558 & \textless{}0.0001 *** \\ Lack of fit & 0.166 & 8 & 0.021 & - & - \\ Residual & 0.166 & 8 & 0.021 & - & - \\ Pure Error & 0.000 & 0 & - & - & - \\ Cor. Total & 106.237 & 17 & - & - & - \\ \hline \(R^{2}\) & 0.997 & & & & \\ \(R^{2}\) adj & 0.998 & & & & \\ CV\% & 0.976 & & & & \\ \hline \hline \end{tabular} df: degree of freedom; Cor. Total: corrected total; \(R^{2}\): determination coefficient; \(R^{2}\) adj: adjusted \(R^{2}\); CV variation coefficient value; *** significant \(p\) \(<\) 0.001.  
\end{table} 
Table 4: ANOVA of the predicted model used for USAE of TPC from almond residues. Figure 2: Predicted surface response plots of the TPC extraction yield (in mg/g DW) as a function of aqEOH concentration and US frequency, aqEOH concentration and extraction duration, as well as US frequency and extraction duration. of \(11.63\pm 0.15\) mg/g DW (Figure 2). The optimal aqEtOH concentration obtained here is in line with results obtained for almond phenolics extraction very recently described [13; 18]), although the starting byproduct material or the extraction method used were different from our study. The present method was then validated in respect with the AOAC standards. According to these standards, the parameter values of this validation procedure were adequate in terms of interday and intraday precision, but also repeatability and stability (Table 5). Indeed, the RSDs of both intraday and interday precisions were of 0.05 and 0.28%, respectively. The RSDs of the repeatability corresponding to five different extraction repeats of five samples from the same batch was of 1.30%. The recovery rates at three different addition levels of chlorogenic acid in the sample before extraction were between 100.26 and 101.13% reflect the accuracy of the present method. The efficiency of the present USAE method was compared with conventional heat reflux extraction (HRE) using the same conditions, in particular an aqEtOH concentration (53.0% (_v/v_) and an extraction time of 29.4 min. The difference between USAE and HRE being the application of an US frequency of 27 kHz for the present optimized UASE extraction procedure, while no US was applied for the HRE protocol operating in a classical water bath. The comparison of these extractions is depicted in Figure 2. A significant gain of 30% in TPC extracted from AOR was observed with the optimized USAE (\(11.63\pm 0.15\) mg/g DW) as compared to conventional HRE (\(8.96\pm 0.21\) mg/g DW) (Figure 3). Increasing the extraction time for the HRE to one hour did not achieve performance levels similar to those obtained with USAE (\(9.24\pm 0.37\) mg/g DW). Consequently, it appears that the USAE method developed in the present study is of real interest according to the principles of green chemistry [45], not only in terms of the use of a renewable green solvent, but also in terms of reducing the energy consumption. We hypothesize that this efficiency could be partly explained by the hot spot hypothesis indicating that the cavitation bubbles, after their collapse, act as a microreactor locally generating, in the surrounding solvent, a high temperature environment and pressure leading to more efficient rupture of the plant matrix subjected to extraction and increased release as well as solubilization of phenolic compounds [28]. Figure 3: TPC extracted from AOR using the optimal USAE (with US) conditions and comparison with conventional heat reflux method (HRE; without US). Means \(\pm\) SD standard deviations of three independent extractions; *** significant at \(p<0.001\). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Precision (\% RSD)** & **Repeatability (\% RSD)** & & **Recovery 1 (\%)** & \\ \hline Intraday & Interday & & 0.5 & 1.0 & 2.0 \\
0.05 & 0.28 & 1.30 & 100.26 \(\pm\) 0.07 & 100.90 \(\pm\) 0.80 & 101.13 \(\pm\) 0.50 \\ \hline \hline \end{tabular}
\end{table}