Novel rutin-casein composites as functional dry ingredients for the delivery of high concentration of rutin in dairy beverages: in vitro bioaccessibility, cytotoxicity, absorption, and intestinal barrier integrity

<p dir="ltr">Rutin, a flavonoid with antioxidant and anti-inflammatory properties, has poor solubility (highly hydrophobic) and is unstable during gastrointestinal digestion, limiting its use in functional foods. To overcome this challenge, we developed two rutin-caseinate composites (RCC1 and RCC2) as delivery vehicles for incorporation into functional foods/beverages. While both systems deliver rutin at high concentration, they differ in terms of methodology, loading capacity, and applications. The gastrointestinal stability, bioaccessibility, and antioxidant potential of these delivery systems, both alone and incorporated into a functional dairy beverage (flavoured milk), were assessed. We also examined the cytotoxicity, absorption, and intestinal barrier integrity of rutin using an intestinal epithelial cell model. The bioaccessibility of rutin from RCC1 and RCC2 was found to be 63 % and 45 %, respectively, compared to untreated rutin (UR), which was undetectable due to precipitation. Additionally, RCC2 exhibited superior intestinal barrier integrity with a trans-epithelial electrical resistance (TEER) value of 1655 Ω/cm² for 24 h , outperforming both RCC1 (1384 Ω/cm²) and UR (915 Ω/cm²). Intracellular antioxidant activity was significantly higher for both composites in terms of lower relative fluorescent units (RFU); 44 RFU for RCC1 and 42 RFU for RCC2, compared to 63 RFU for UR, demonstrating their enhanced protective effects. Caco-2 cell viability of the composite samples was higher, with no cytotoxicity observed compared to UR, confirming their safety. When incorporated into milk, both systems improved rutin bioaccessibility, with RCC1 showing a stronger antioxidant response (87 RFU) than RCC2 (100 RFU) and untreated rutin (140 RFU) during extended incubation. These findings suggest that both RCC1 and RCC2 are stable, soluble, and safe for physiological systems. Their incorporation into dairy matrices enhances rutin bioaccessibility and antioxidant potential, making them a promising approach for functional foods development.</p>