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Donkey milk is difficult to process into cheese because it contains little protein and, above all, little casein. A team from the University of Bari investigated whether ultrafiltration changes this: A membrane retains the protein, while water and lactose pass through. Only the ultra-concentrated milk formed a curd in the laboratory – the untreated milk did not coagulate.
Food technology laboratory study on cheesemaking – no statement on effects in humans. Donkey milk is a food.
Study at a glance
| Feature | Information |
|---|---|
| Original title | Effect of ultrafiltration on the cheesemaking properties of donkey milk |
| Authors | Natrella G., Maggiolino A., De Palo P., Mefleh M., Faccia M. |
| Journal | International Dairy Journal, 149: 105830 |
| Year | 2024 (online 2023) |
| Country | Italy (Apulia) |
| Study type | Food technology laboratory study (cheesemaking) |
| Sample | Bulk milk from one farm, three batches of 12 liters each; eight formulation variants |
| DOI | 10.1016/j.idairyj.2023.105830 |
What question did the researchers ask?
For milk to coagulate with rennet, it needs enough casein – the protein that forms the curd. Donkey milk contains very little of this. The researchers wanted to know whether ultrafiltration (concentration over a fine-pored membrane) alone or together with pre-acidification or some cow's milk improved coagulation and cheese yield.
How it was investigated
- Milk: Three batches of 12 liters of bulk milk from a farm in Laterza (Apulia), machine-milked.
- Ultrafiltration: 8 liters passed over a membrane with a 10 kDa cut-off until about half the volume remained. 4 liters remained untreated.
- Eight variants: each with and without ultrafiltration – without additives, acidified with lactic acid to pH 6.3, pre-acidified with a slime-forming starter culture to pH 6.3, or with 10% skimmed cow's milk.
- Cheeses: Calcium chloride, 40 °C, calf rennet; viscosity measured after 0, 30 and 60 minutes, curd stored overnight at 4 °C.
- Analyses: Protein, moisture, yield, protein bands in the gel; firmness assessed by touch and cutting.
The results
Ultrafiltration almost doubled the protein content of the milk: from 1.4% to 2.72%. The fat content increased from 0.35% to 0.71%. Without ultrafiltration, no variant coagulated properly: the control and cow's milk variants did not coagulate at all, while the acidified variants formed only occasional, weak curds.
| Variant (ultrafiltered) | Viscosity after 60 min (mPa·s) | Yield (%) | Moisture (%) |
|---|---|---|---|
| UF without additives | 17.07 | 9.60 | 73.53 |
| UF + lactic acid | 55.6 | 8.05 | 70.25 |
| UF + starter culture | 60 | 10.97 | 73.89 |
| UF + 10% cow's milk | 27.1 | 7.7 | 71.58 |
The variant with the starter culture produced the firmest curd and the highest yield based on dry matter (2.86%). However, in terms of overall yield, it did not differ statistically from the pure UF variant. On average, 20 to 25 minutes passed from renneting to filling the molds.
The low protein content was the main obstacle to cheesemaking with donkey milk. Only after ultrafiltration did a curd form, with the firmest curd produced using a pre-acidifying starter culture.
Critical assessment
- Small sample size: Bulk milk from one farm, three batches in early summer. Breed, number of animals and lactation stage are missing.
- No aged cheese: Measurements were taken after one night in the refrigerator. With 70 to 74% moisture, the curd still does not resemble a sliceable cheese – the authors themselves say so.
- Texture not measured: Firmness was assessed by hand, not with a texture analyzer.
- Unclear calculation: The paper does not describe how the yield based on dry matter was calculated.
- Long filtration: Ultrafiltration ran for several hours at room temperature; during this time, the pH dropped slightly (7.08 instead of 7.30).
Funding, conflicts of interest and level of evidence
No funding is mentioned in the paper. The authors declare no competing interests. An economic calculation for implementation on farms is missing, although the authors emphasize its applicability.
Level of evidence: Food technology laboratory study – describes processing properties, no statement on nutrition or effects.
What does this mean for donkey milk as a food?
The study explains why cheesemaking with pure donkey milk is technically challenging: The low casein content is barely sufficient for normal rennet coagulation. Ultrafiltration and pre-acidification are possible methods, but the authors see a need for further research. In the variant with cow's milk, the cheese also contains cow's milk casein. Our donkey cheese was not investigated.
Our research question: How can we clearly explain how much donkey milk is in one kilogram of donkey cheese and what role the casein content plays in this?
Frequently asked questions
What is ultrafiltration?
Milk is passed under pressure over a fine-pored membrane. Large molecules such as protein and fat are retained, while water and lactose pass through. This increases the protein content of the retained milk.
Why does donkey milk coagulate so poorly?
Casein is especially necessary for rennet coagulation. Donkey milk contains little protein overall; in this study, it was 1.4%. At this level, no curd formed.
Was the cheese tasted?
No. The study assessed coagulation, composition and yield. Tasting or ripening was not part of the investigation.
You can find all measurements, interactive graphics and the full critical assessment on our research page: Study 75: Cheese from ultrafiltered donkey milk. Further studies in the research archive.
Natrella G., Maggiolino A., De Palo P., Mefleh M., Faccia M. (2024). Effect of ultrafiltration on the cheesemaking properties of donkey milk. International Dairy Journal, 149, 105830. DOI: 10.1016/j.idairyj.2023.105830
Summarized and reviewed by the DON DONKEY'S editorial team based on the original publication · As of: October 2026. This summary does not replace the original work.
End of the study summary. The study was conducted independently of DON DONKEY'S® and does not refer to our products. Our donkey milk is a food.
