Bidirectional Associations Between Steak Doneness and Wine Co-Digestion: Protein Oxidation, Digestibility, and Phenolic Bioaccessibility During In Vitro Gastrointestinal Digestion

Research on mixed meals using the INFOGEST protocol has often focused on individual foods, leaving co-ingested matrix associations less well characterized. To address this gap, this study investigated bidirectional associations between steaks cooked to three doneness levels (medium, SM; medium-well, SW; well-done, SD) and red wine during simulated gastrointestinal digestion, focusing on protein oxidation, apparent digestibility, secondary structure, and the apparent bioaccessibility of selected phenolic compounds. Results showed that co-digestion with red wine was associated with lower accumulation of protein oxidation markers across all doneness levels. In the gastrointestinal phase, carbonyl content was 44.5–46.8% lower and free sulfhydryl content was 53.4–59.1% higher than in steak digested alone. SW steak co-digested with wine exhibited the highest mean gastric apparent digestibility (37.7%), and FTIR indicated differences in protein secondary-structure distribution. Furthermore, mixed digests showed higher radical-scavenging activity than when wine was digested alone (an average ABTS increase of 61.4% in the gastrointestinal phase), while the steak matrix altered the digestive recovery and gastric apparent bioaccessibility of several target phenolic compounds. Overall, these findings indicate that steak doneness and red-wine co-digestion are associated with distinct matrix-dependent changes in both protein digestion and apparent phenolic bioaccessibility.