Thermal instability and characteristics of donkey casein micelles.

The thermal stability of donkey milk, which largely depends on the stability of casein and whey proteins, plays a critical role in the manufacture of donkey dairy products. However, the thermal stability of donkey casein micelles is poorly understood. Our study first found that donkey milk exhibited poor thermal stability, with sedimentation observed after heating at 75 °C for 10 min. Moreover, electrophoresis results indicated that donkey casein micelles were more sensitive to heat treatment than whey protein. The characteristics of donkey casein micelles were then studied to determine the thermal instability mechanism. Donkey casein micelles had a larger diameter (295 ± 11 nm) and lower absolute zeta potential (-15.4 ± 0.5 mV) than bovine casein micelles, mainly due to their very low κ-casein levels. The higher proportion (51%) of β-casein in donkey casein micelles made them less hydrated. Furthermore, <40% of calcium in donkey milk was associated with donkey casein micelles. To eliminate the influence of whey protein, micellar casein was isolated from skim donkey milk by ultracentrifugation and then redispersed in its ultrafiltrate. Transmission electron microscopy and particle size analysis showed the extensive aggregation of isolated casein micelles after thermal treatment, which confirmed the thermal instability of casein micelles observed in donkey milk. Overall, poor colloidal stability and a high‑calcium environment largely contributed to the thermal instability of donkey casein micelles.