Walnut kernel pellicle polyphenols: subcritical water extraction, digestive characteristics, and their effect on gut microbiota and metabolites

Walnut kernel pellicle, a by-product of the walnut industry, is rich in polyphenols. However, research on walnut kernel pellicle polyphenols (WPP) remains limited, and the composition and bioactivity of WPP are influenced by extraction methods, gastrointestinal digestion, and colonic fermentation. Therefore, this study aimed to optimize subcritical water extraction (SWE) and NKA-9 resin purification of WPP. Next, the changes in WPP throughout simulated gastrointestinal digestion and colonic fermentation were evaluated. The results demonstrated that SWE achieved higher polyphenol yield compared to ultrasound-assisted extraction, with ellagic acid and ellagitannins as the main components. Although digestion and fermentation reduced WPP contents and antioxidant activities, they facilitated the biotransformation of ellagic acid and ellagitannins into urolithins. Meanwhile, WPP modulated the gut microbiota by promoting beneficial bacteria. Additionally, WPP enhanced beneficial microbial metabolites, including short-chain fatty acids and indole derivatives. These findings highlighted the potential of WPP as a functional ingredient for gut health.