## 3 Results and Discussion
### Changes of Phenolic Bioaccessibility during the In Vitro Digestion  
The GPP-breads were hydrolyzed through a standardized in vitro static digestion method, aiming to explain the changes in the phenolic profile. The UHPLC-QTOF untargeted profiling provided the identification of 109 polyphenols, with an abundance of flavonoids (59 compounds), phenolic acids (29 compounds) and lower-molecular-weight phenolics (21 compounds). Each annotated compound is reported in Table S1, together with its abundance values and composite mass spectrum. [PERSON] et al. [11], analyzing the same experimental breads, showed that the inclusion of higher levels of GPP caused an increase in the total dietary fiber content. The total dietary fiber content represents a key point when targeting the bioactive profile of a certain cereal-based food product. In particular, both chemical and physical characteristics of the food can affect the bioaccessibility, bioavailability and bioactivities of bioactive compounds, including polyphenols [23]. In particular, the phenolic compound bioavailability and bioaccessibility can be affected by molecular interactions between dietary fiber and the other components characterizing the food matrix (such as lipids and proteins) [24]. Therefore, in this work, the phenolic compounds exhibited different bioaccessibility trends, mainly imposed by the different percentage of dietary fiber in each bread sample, in line with previous indications for fortified wheat-based fresh pasta formulated with an increasing level of a high-fiber/polyphenols ingredient [15]. The bioaccessibility of anthocyanins was found to increase in both GP5 and GP10 experimental breads when moving from the gastric to the small intestine phase (Table 1). An average bioaccessibility value of 24% was recorded for this class of compounds. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Phenolic Subclass**} & \multirow{2}{*}{**Bread Sample**} & **Prior to Digestion** & **Gastric** & **Small Intestine** \\  & & **(mg/100 g DM)** & **(mg/100 g DM)** & **(mg/100 g DM)** \\ \hline Anthocyanins & GP0 & nd & nd & nd \\  & GP5 & 20.98 & 3.18 (15.1\%) & 5.88 (28\%) \\  & GP10 & 35.82 & 3.57 (10\%) & 7.25 (20.2\%) \\ \hline Flavones & GP0 & 47.14 & 1.77 (3.8\%) & 1.25 (2.7\%) \\  & GP5 & 54.68 & 4.23 (7.7\%) & 9.33 (17.1\%) \\  & GP10 & 56.80 & 4.57 (8\%) & 6.74 (11.9\%) \\ \hline Other flavonoids & GP0 & 2.85 & nd & 0.53 (18.7\%) \\  & GP5 & 20.44 & 4.54 (22.2\%) & 5.97 (29.2\%) \\  & GP10 & 21.86 & 4.28 (19.6\%) & 5.13 (23.5\%) \\ \hline Other phenolics & GP0 & 8.05 & 1.75 (21.7\%) & 2.14 (26.6\%) \\  & GP5 & 6.30 & 1.46 (23.2\%) & 3.89 (61.7\%) \\  & GP10 & 7.94 & 1.60 (20.2\%) & 3.08 (38.8\%) \\ \hline Phenolic acids & GP0 & 5.71 & 0.57 (10.1\%) & 0.97 (17\%) \\  & GP5 & 4.57 & 0.39 (8.5\%) & 0.94 (20.6\%) \\  & GP10 & 5.35 & 0.71 (13.2\%) & 1.25 (23.3\%) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Cumulative and average (n = 3) phenolic contents and bioaccessibility values (expressed as % of phenolic equivalents) for the different subclasses during the in vitro static digestion of samples formulated with different substitution levels of grape pomace powder (i.e., 0, 5 and 10 g/100 g for GP0, GP5 and GP10, respectively), considering the cumulative phenolic contents of samples prior to digestion and after the gastric and the small intestine digestion phases. The results are expressed as mg/100 g DM of material prior to digestion. Nd: not detected. The investigation of the anthocyanin stability in the gastrointestinal tract could be useful for exploring their final bioavailability and thus related potential health benefit. In particular, human in vitro digestion models demonstrated that anthocyanins are rather stable in the stomach, but easily degradable in the small intestine due to the high pH value [25]. The stability of anthocyanins in the small intestine is extremely important; indeed, as reviewed by [PERSON] et al.