Bioactive Protein Profile and Compositional Evolution of Donkey Milk Across Lactation Reflecting Its Nutritional and Functional Food Value

Donkey milk is increasingly recognized as a functional food due to its unique nutritional profile and richness in bioactive compounds. This longitudinal observational study investigated changes in both chemical composition (total solids, protein, fat, lactose, and ash) and immune-active proteins (lactoferrin, α-lactalbumin, β-lactoglobulin, and lysozyme) across lactation. A total of 153 donkey milk samples were collected from five farms from very early (1-3 days in milk) to late lactation (30-210 days in milk). Chemical composition was determined using mid-infrared spectroscopy, while the concentrations of the immune-active proteins were determined by ELISA Quantitative Sandwich. Chemical analysis showed high values in colostrum, including total solids (10.13%), protein (3.1%), and ash (0.73%), which declined progressively during lactation to 8.45%, 1.14%, and 0.64%, respectively. Fat varied modestly, between 0.55 and 0.25%, while lactose remained stable at 5.75-6.41%. In parallel, bioactive proteins measured between 31 and 210 days exhibited distinct trajectories. Lactoferrin increased from 0.07 to 0.14 mg/mL, α-lactalbumin peaked mid-lactation at 2.58 mg/mL (compared with 1.91 mg/mL early and 2.25 mg/mL late), β-lactoglobulin declined from 0.84 to 0.55 mg/mL, and lysozyme decreased from 0.95 mg/mL early to 0.64 mg/mL late. Across lactation, we observed dilution of total solids and protein, relatively stable lactose and fat, and distinct trajectories of lactoferrin, α-lactalbumin, β-lactoglobulin, and lysozyme, indicating that donkey milk modulates rather than loses its protective protein profile. These results refine reference values for donkey milk and support its nutraceutical relevance for human nutrition and health.