Milk Proteins as Molecular Gatekeepers: Comparative Modulation of Sulfonamides, Natural Phenolics, and Zinc–Polyphenol Complexes at the Food–Drug Interface

Milk is a complex biochemical mixture in which proteins significantly influence the behaviour of xenobiotics and bioactive compounds. Interactions between milk proteins and substances such as veterinary drugs or natural bioactives can modify molecular stability, binding dynamics, and exposure pathways, affecting food safety and the One Health concept. This research presents a comparative, matrix-based method to investigate how three chemically distinct ligand groups: sulfonamide antibiotics, phenolic compounds from essential oils, and zinc–polyphenol complexes, interact with the main milk proteins, β-lactoglobulin and casein. Fluorescence spectroscopy was employed to study protein–ligand interactions, providing data on quenching mechanisms and binding affinity, while in silico molecular docking offered structural insights into their interaction modes. Different ligand classes exhibited unique interaction behaviours influenced by differences in molecular structure, polarity, and coordination chemistry. Importantly, zinc complexation altered the binding behaviour of polyphenols, indicating that metal coordination can affect ligand–protein interactions within the milk matrix. This work shifts the focus from proving biological efficacy to suggesting that milk proteins function as molecular gatekeepers, selectively influencing synthetic drugs, natural phenolics, and hybrid metal–polyphenol systems. The findings provide a foundation for hypothesising how the milk matrix affects residue behaviour and bioactive interactions, with relevance to veterinary pharmacology, food safety risk assessment, and translational One Health research.