Fatty Acid Composition and Antioxidant Activity of Milk from the Bulgarian Local Donkey Breed

Donkey milk has been increasingly studied in recent years and has been proposed to be a functional food. However, its components undergo changes during lactation, including its lipid profile and redox-related properties. This study analyzed the fatty acid composition, antioxidant parameters, and redox-modulating properties of donkey milk from the Bulgarian local donkey breed at three lactation stages (0-30, 31-60, and 61-90 days postpartum). Milk samples from 40 clinically healthy donkeys were grouped by days postpartum. A cross-sectional design with three lactation stage groups was used; one-way ANOVA tested group differences with Tukey's post hoc test, and associations with days postpartum were evaluated using regression models. Fatty acid methyl esters were analyzed by GC-FID, and the atherogenic (AI) and thrombogenic (TI) indices were calculated. Antioxidant enzymes (SOD, CAT, and GPx-1), GSH, MDA, TAC, and EPR-based redox markers (DPPH, Asc•, ROS, NO•, TEMPOL, and 5-MSL) were analyzed. During lactation, monounsaturated fatty acids decreased (approximately 32% in the first month to ~30% by the third month), while AI increased from ~1.9 to ~2.2, and TI increased to ~2.5. SOD and GPx-1 activities increased with advancing lactation, while total antioxidant capacity decreased (213.4 to 199.7 µmol). DPPH radical scavenging activity remained stable during lactation. EPR-detected ROS and NO• values increased with advancing lactation stage, while thiol-bound 5-MSL decreased, suggesting a shift in the balance between oxidative challenge and antioxidant defense during lactation. Regression modeling confirmed a significant effect of lactation period on multiple compositional and redox-related parameters. Therefore, the stage of lactation should be taken into account when interpreting the biological value, redox stability, and potential functional properties of milk, as well as when developing milk management and yield strategies.