## 3 Results and Discussion
Table 1 and Figure 1b show the variation in target polyphenols during digestion. Despite a reduction in the phytochemical content throughout digestion, the amount of many phenolics, particularly catechins, remained relatively stable after gastric exposure. These findings align with those of [PERSON] et al., who noted increased polyphenol and flavonoid contents after the gastric digestion of blackberry extracts, likely due to the pH and enzymatic effects [19]. However, intestinal digestion consistently reduced polyphenol bioaccessibility, a trend supported by multiple in vitro studies [20; 41; 42; 43; 44]. Regarding the antimicrobial activity, the extract exhibited strong bactericidal effects. As shown in Figure 2a, its effects were more pronounced against Gram-positive bacteria (_L. monocytogenes_ and _S. aureus_), with the complete eradication of _S. aureus_ at concentrations below 1% (\(v/v\)), and approximately 5% for _L. monocytogenes_. On the other hand, Gram-negative strains (_E. coli_ and _S. enterica_) were less susceptible, requiring concentrations exceeding 10% (\(v/v\)). This aligns with other studies that also reported the greater sensitivity of Gram-positive bacteria to white grape pomace extracts [45]. This differential sensitivity may be linked to the protective lipopolysaccharide layer in Gram-negative bacteria [40]. However, previous work by [PERSON] et al. [17] demonstrated that catechin-rich white grape extracts effectively inhibited _E. coli_ and _S. enterica_ at concentrations below 1%, suggesting polyphenol-bacterium specificity. According to a recent review on the polyphenols' antimicrobial potential, this antimicrobial activity is influenced not only by bacterial cell wall structures, but also by the polyphenols' chemical composition and structure, including hydroxyl radicals, molecular size, and degree of polymerization, which affect their ability to disrupt bacterial membranes and inhibit key enzymes [3]. Furthermore, polyphenols may generate oxidative stress within bacterial cells, interfere with quorum sensing, and disrupt biofilm formation [3; 40]. These multifaceted modes of action explain why certain polyphenol-rich extracts show selective efficacy depending on the bacterial species and even strain. During this research, the bactericidal activity was mostly maintained after in vitro gastrointestinal digestion, especially at certain concentrations; however, slight differences were observed among phases and strains. The antimicrobial activity of the samples taken directly after the digestive phases are collected in Figure 2c-f. Control tests using gastric (GF) and intestinal fluids (IF) revealed increased bacterial growth, particularly for Gram-positive bacteria and _S. enterica_. Concretely, resistance to gastric fluids was observed for _L. monocytogenes_ (Figure 2c) and _S. enterica_ (Figure 2f) during fluidic control assessment. This phenomenon was previously reported by [PERSON] et al., who observed that these foodborne pathogens are very resistant to the gastrointestinal environment, proving that the effects of gastric acidity and bile acids alone could not prevent bacterial survival [46]. Gastric fluids are traditionally considered bactericidal at pH < 3.0 [47], although bacterial overgrowth can occur if the pH exceeds 4.0 [48]. Although digestive fluids alone can inhibit the growth of most pathogenic bacteria, for the particularly resistant strains we tested, the presence of the polyphenic extract both in the stomach and the intestine presents marked antimicrobial effects in vitro. In order to compare the antimicrobial stability of the digested extract with the crude sample, conventional inhibitory values, the IC50 and MBC values, were calculated. The inhibitory values are collected in Table 2. Additionally, Figure 2g-j display the bacterial growth trends under different concentrations of crude and digested samples, the measurements needed for inhibitory value determination. Notably, _S.