## 3 Results and Discussions  
### Polyphenols  
#### 3.1.1 Experimental Design  
Ultrasonic power and temperature are the variables involved in the CCD that could potentially affect polyphenol extraction efficiency in order to evaluate and optimize ultrasound extraction of polyphenols (TPC) and almond skin separation time (SST). According to CCD experiments provided by STATGRAPHICS, experiments were performed at maximum UI 9.47 W.cm\({}^{-2}\) to fit the limits of the ultrasonic device, and at temperatures between 18 \({}^{\circ}\)C and 61 \({}^{\circ}\)C. In fact, relatively low temperatures were a necessity to avoid the destruction of organic compounds (such as proteins) and to allow future applications in the pilot scale. Finally, at the lowest temperature and intensity, skin separation starts at \(20\pm 3\) min of reaction. In doing so, ultrasonication duration of experiments was fixed at 20 min.  
In order to explore the influence of the operating parameters required during the extraction, a CCD was used to analyze the total polyphenol content (TPC) and the skin separation time (SST).  
Fully coded experiments and responses obtained for each run of the CCD are represented in Table 1. Responses vary according to the system variables (from 0.024 to 0.063 of mg naringenin equivalent for a gram of almond, and from five to twenty minutes of skin separation time). Similar results for experiments 2, 4, 7 to 9 and 13 (central points) are indicators of data quality and statistical reliability of key variables.  
Significance and suitability of the design were studied using an analysis of variance (ANOVA). The statistical significance of each effect (including interaction terms, linear, and quadratic effects) was determined using the probability \(p\) (\(p\)-value) given in Table S1.  
CCD responses were optimized to get optimum conditions for experimentation. Higher TPC and optimum SST (calculated by STATGRAPHICS--multi-response optimization) were achieved with 9.47 W.cm\({}^{-2}\) at 18.8 \({}^{\circ}\)C. Temperature was rounded to 20 \({}^{\circ}\)C.  
Pareto graphics are represented below (Figure 2) to visualize the significant relationship linking levels of variables and responses obtained (TPC and SST). Results displayed in Figure 2A have shown that the temperature of the system had a positive and highly significant effect on polyphenol extraction, which is far more effective than the UI. The lack of significance of the cross-product terms (BB: UI\({}^{2}\)) suggests the absence of interactions between variables. As for Figure 2B, temperature indicated a significant yet negative effect on SST. UI has no significant effect in this case, in addition to its cross-product, implying the absence of interaction between them.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**No.** & **UI** & **Temperature** & **TPC (eq mg Naringenin/g Almonds)** & **SST (min)** \\ \hline
1 & \(-1\) & \(+1\) & 0.061 & 5 \\
2 & 0 & 0 & 0.039 & 10 \\
3 & 0 & \(-\alpha\) & 0.030 & 10 \\
4 & 0 & 0 & 0.041 & 11 \\
5 & \(+1\) & \(-1\) & 0.035 & 17 \\
6 & \(+\alpha\) & 0 & 0.063 & 6 \\
7 & 0 & 0 & 0.041 & 10 \\
8 & 0 & 0 & 0.041 & 11 \\
9 & 0 & 0 & 0.042 & 10 \\
10 & \(+\alpha\) & 0.046 & 10 \\
11 & \(-\alpha\) & 0 & 0.031 & 20 \\
12 & \(+1\) & \(+1\) & 0.057 & 7 \\
13 & 0 & 0 & 0.040 & 10 \\
14 & \(-1\) & \(-1\) & 0.024 & 20 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Fully coded Central Composite Design (CCD) and responses obtained for Total Polyphenol Content (TPC; eq mg naringenin/g almonds) and Skin Separation Time (SST; min).  
A coefficient of determination (\(R^{2}\)) of 95.88% for TPC and 97.87% for SST implied this polynomial model was fully competent to predict the response.