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Donkey milk research · Study 168
Why donkey milk flocculates when heated: casein micelles under the microscope
Donkey milk showed a sediment after only 10 minutes at 75 °C; its casein micelles were more sensitive to heat than the whey proteins. At 295 nm the micelles were larger, and with a zeta potential of −15.4 mV less strongly charged, than those of cow’s milk – according to the authors mainly because very little κ-casein is present.
At a glance · swipe or use the arrows
The key figures
Study design
How it was studied
Food technology laboratory study by China Agricultural University (Beijing) with Hunan Agricultural University: donkey milk was heated and checked for sediment formation; electrophoresis compared how casein and whey proteins responded to heat. The size, zeta potential, casein composition, hydration and calcium binding of the casein micelles were then determined and compared with cow’s milk. To exclude the influence of whey proteins, the micelles were isolated from skimmed donkey milk by ultracentrifugation, redispersed in their ultrafiltrate and heated again (transmission electron microscopy, particle size analysis). Based on the abstract; full-text details such as sample numbers, origin of the milk and comparison values for cow’s milk are therefore missing.
Results
What the study measured
Donkey milk had poor heat stability: a sediment formed after 10 minutes at 75 °C.
Electrophoresis showed that the casein micelles were more sensitive to heat treatment than the whey proteins.
Donkey casein micelles had a diameter of 295 ± 11 nm and a zeta potential of −15.4 ± 0.5 mV – larger and with a lower absolute charge than those of cow’s milk. The authors attribute this mainly to the very low κ-casein content.
β-casein made up 51 % of the casein micelles, making them less hydrated according to the authors. Less than 40 % of the calcium in donkey milk was associated with the micelles.
Isolated casein micelles redispersed in ultrafiltrate also aggregated extensively after heating. According to the authors, this confirms that the instability originates in the micelles themselves.
Authors’ conclusion: According to the authors, poor colloidal stability of the casein micelles and a calcium-rich environment contribute substantially to the heat instability of donkey milk. The study describes physicochemical behaviour during heating – it says nothing about the nutritional value or tolerance of heated donkey milk.
Critical appraisal
Strengths and limitations
Only the abstract assessedClear limitation
The full text was not available. Sample numbers, origin and breed of the jennies and the comparison values for cow’s milk are unknown.
Few temperature steps knownNote
The abstract only mentions 75 °C for 10 minutes. It does not show how lower temperatures or shorter times affect the milk.
Technology, not nutritionNote
Stability and structure of the proteins were measured. Whether the changes matter for flavour, digestibility or nutritional value was not examined.
Mechanism tested step by stepStrength
By isolating the micelles without whey proteins, the authors separate the contribution of the casein micelles from that of the other milk components.
Editorial note: The paper offers a physicochemical explanation of why donkey milk has poor stability when heated. This is relevant for processing donkey milk into dairy products; no statements on health properties can be derived from it. To the research archive.
Research question for DON DONKEY’S®
Do freezing and thawing change the size of the casein micelles in our donkey milk – and can this be checked with a simple particle size measurement per batch?
Original publication
Thermal instability and characteristics of donkey casein micelles.
Original title: Thermal instability and characteristics of donkey casein micelles.
Luo J., Jian S., Wang P., Ren F., Wang F., Chen S., Guo H.
Food Research International · 2019 · 119, 436–443
Beijing Advanced Innovation Center for Food Nutrition and Human Health and Key Laboratory of Functional Dairy, China Agricultural University, Beijing; Hunan Agricultural University, Changsha, China (according to author details)
DOI: 10.1016/j.foodres.2019.02.023
Transparency: according to the database record, funded by the Chinese Universities Scientific Fund, the National Basic Research Program of China and a Beijing Science and Technology Planning Project. Conflicts of interest could not be checked from the abstract. No product-related statement for DON DONKEY’S®.
Summarised and checked by the DON DONKEY’S editorial team against the original publication · As of: November 2026
The study summarised above was carried out independently of DON DONKEY’S® and does not relate to our products. It serves solely to inform about the state of research.
DON DONKEY’S® donkey milk and donkey milk powder are foods. They are not intended to prevent, treat or cure any disease and do not replace medical advice.
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DON DONKEY’S® donkey milk
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On this page we report the findings of a published study. Independently of that, you will find our donkey milk here: 100 % donkey milk, frozen as raw milk.
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