Coffee phenolics–milk bioactive peptides conjugation: modulation of structural and functional properties and stability of encapsulated lutein

Protein–polyphenol conjugation has emerged as an effective strategy for improving the functional and antioxidant properties of food proteins. However, limited information is available regarding covalent conjugation of coffee-derived phenolics with milk bioactive peptides. Therefore, this research examined the modification of BioZate1 (BZ1) and glycomacropeptide (GMP) through covalent conjugation with chlorogenic acid (CQA)-rich green coffee extract under alkaline and enzymatic conditions. Native BZ1 and GMP were compared with their enzymatically (EBZ1 and EGMP) and alkaline (ABZ1 and AGMP) conjugated peptides. Structural changes were assessed using fluorescence spectroscopy, while functional properties were evaluated through solubility, antioxidant activity by TEAC and DPPH assays, and encapsulated lutein stability. In BZ1, conjugation markedly reduced total reactive groups, decreasing from 1197.15 to 825.55 and 830.00 nmol/mg for BZ1, ABZ1, and EBZ1, respectively. GMP showed increased exposure of reactive sites. BZ1 surface hydrophobicity decreased substantially, accompanied by improved solubility, emulsion stability, and foam stability. Antioxidant activity increased significantly in all conjugates. ABZ1 showed the highest TEAC value (312.49 μM TE/g protein), 42% higher than native BZ1. Conjugation also improved lutein ester photostability. These findings support the use of coffee polyphenols to enhance milk peptide functionality for foods and antioxidant delivery systems.