## 3 Results and Discussion  
### Valorization Process and Composition of the Red Grape Pomace  
The valorization process consisted of a thermal blanching, crushing, and a hydrostatic high pressure (HHP) treatment. Previous studies were carried out to know the effect of HHP on RGP and demonstrated that HHP treatment does not reduce PPO activity, resulting in a reduction of bioactive compounds in the valorized products during storage [29; 30]. For that reason, the grape pomace was thermally blanched (TB) with the purpose of inactivating the PPO enzyme before HHP treatment. The one-way analysis of the variance (Table 1) showed a complete inactivation of PPO after TB (1 min, 103 \({}^{\circ}\)C), and it remained inactivated until the end of the valorization process. Table 1 shows the total phenolic compound content (PPC) during the valorization process, indicating that TB significantly increased PPC. This initial rise in the PPC of the RGP ingredient aligns with previous research, demonstrating that blanching vegetables increases their phenol content [40]. Some studies have attributed the reported PPC increase to the reduction and inactivation of polyphenol oxidase, along with the release of bound phenolic acids resulting from the breakdown of cellular constituents in the plant cell walls of the leafy vegetable [14]. On the other hand, a significant PPC reduction is observed on the RGP ingredient (TB + HHP) after HHP treatment, which is an unexpected result. Previous studies reported that HHP maintained or even increased the extraction of PCC in grape pomace [41; 42]. This increase is induced by HHP through structural changes in the cell matrices, leading to the extraction of phenolic compounds [41]. In the same vein, a parallel study in our research group reported slight non-significant reductions in PCC in \"Tempranillo\" red grape pomace after treatments at 600 MPa for 5 min [30]. Similarly, the valorization process by HHP of white grape pomace has been recently published [31], and an important increase in PCC was observed after TB. However, the levels were well-preserved after HHP. Compared to those previous studies, in our study, the equipment used to crush the pomace was different, as it allowed the obtaining of a fine powder. The grapes were scadded and then frozen before crushing, resulting in an intense reduction of particle size before HHP treatment. This reduction in particle size was more intense than that obtained with white grape pomace, potentially facilitating the degradation of phenolic compounds by exposition to the air. At this point, an improvement in crushing methodology would be required to avoid the loss of phenolic compounds before HHP treatment. The proximate composition of the percentage wet base (%WB), pH, and aw of the RGP ingredients are shown in Table 2. \begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline  & \multicolumn{2}{c}{**Initial (1)**} & \multicolumn{2}{c}{**TB (2)**} & \multicolumn{2}{c}{**TB + HHP (3)**} & \multicolumn{2}{c}{\(p\)**-Value**} & \multicolumn{2}{c}{\(p\)**-Value**} \\  & & & & & & & & & & & **(1–2)** & **(2–3)** & **(1–2–3)** \\ \hline PPC (mg 100 g\({}^{-1}\)) & 467.6 \(\pm\) & 3.3 & 882.0 \(\pm\) & 41.5 & 379.5 \(\pm\) & 43.7 & \textless{}0.001 & \textless{}0.001 & \textless{}0.001 \\ PPO (\%) & 100.0 \(\pm\) & 4.9 & 0.0 \(\pm\) & 0.0 \(\pm\) & 0.0 & 0.0 \(\pm\) & 0.0 & \textless{}0.001 & 0.341 & \textless{}0.001 \\ \hline \end{tabular} TB: thermal blanching, HHP hydrostatic high pressure; \(p\)-value (1–2) (Student’s \(t\)-test between initial pure and TB pure); \(p\)-value (2–3) (Student’s \(t\)-test between TB purine and HHP pure); \(p\)-value (1–2–3) (Tukey’s HSD test between initial, TB, and HHP). Different letters in the same row indicate significant differences in the Tukey test (\(p<0.05\)).  
\end{table} 
Table 1: Changes in the total phenolic compounds content (PPC, mg 100 g\({}^{-1}\)) and polyphenodoxidase (PPO, % percentage respect to the initial) enzyme during the process of manufacture of the ingredient of red grape pomace.