## 3 Results and Discussion  
Dietary polyphenols are a diverse group of compounds found in fruits, vegetables, and beverages [31]. They are widely studied due to their broad range of biological activities, largely attributed to their inherent antioxidant properties [32]. Due to their association with promoting overall health, there has been a heightened demand for polyphenonic nutraceutical compounds [33]. Consequently, there has been growing research into identifying new polyphenol sources, developing innovative extraction methods [18], and improving compound stability [34]. These improvements have enhanced the value of polyphenol-rich extracts derived from agro-industrial byproducts [35; 36; 37]. Despite well-documented bioactivities, the application of polyphenols is often limited by their low bioavailability [10]. To address this challenge, in vitro digestion models are favoured due to their speed, cost-effectiveness, and reproducibility, often showing good correlation with in vivo results [38]. Therefore, the effectiveness of food fortification and the use of polyphenols as antimicrobial agents should be evaluated using in vitro bioaccessibility and bioavailability simulations to determine the optimal formulations and concentrations. This study aimed to assess the antimicrobial activity of an Albarino grape marc extract across different stages of in vitro gastrointestinal digestion, focusing on the relationship between polyphenol bioaccessibility and antimicrobial efficacy. Figure 1a and Table 1 summarize the quantification of key polyphenols in the extract. The undigested extract exhibited a rich polyphenolic profile, particularly high in procyanidins (302 mg/L) and quercetin-3-glucosidase (167 mg/L), followed by quercetin-3-glucuron (54 mg/L) and catechin (50 mg/L). Epicatechin gallate and phenolic acids such as gallic acid (31 mg/L) were also present in lower concentrations. These results are consistent with previous evaluations of white grape marc extracts, which generally contain  
Figure 3: [PERSON]’s correlation between the polyphenolic profile and antimicrobial activity of each extract: (**a**) Ce: crude extract. (**b**) GDE: gastric digestion of the extract. (**c**) IDE: intestinal digestion of the extract. Negative correlations (e.g., higher polyphenol concentration associated with lower IC50 values) are represented in lighter colours, while positive correlations are shown in dark greyish green. The calculated IC50 values for each digestion phase and microbial strain are presented at the bottom of each matrix. more flavan-3-ols (e.g., procyanidins) than flavonoids [16]. Previous work by our group found that the bioaccessibility of polyphenols from white grape marc increased during gastric digestion, but declined during intestinal digestion [39], possibly due to interactions with bile salts, which prevents polyphenol detection. Although white grape marc extracts have previously been evaluated for their antimicrobial properties [16; 40] and bioaccessibility, there are no studies, to our knowledge, that specifically assess the persistence of antimicrobial activity after digestion. To better understand the present study, it is important to take into consideration that the polyphenol quantification of the samples is affected by two factors acting in the opposite way: firstly, the dilution of the samples after gastric (\(\times 2\)) and intestinal (\(\times 4\)) digestion would dilute the concentrations of the molecules; secondly, as has been previously reported [39], the amount of some polyphenols can increase during gastric digestion, due to their liberation from insoluble complexes present in the extract when it is exposed to acidic conditions. Since the goal of this research was to assess the antimicrobial activity at each digestion stage, no dilution corrections were applied and only the soluble fractions, representing the potentially bioavailable polyphenols, were analyzed for their composition and antimicrobial activity (Figure 2b).