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Can donkey milk be pretreated before freeze-drying without heat – and what does that mean for the proteins in the powder? A team from Cyprus and Italy left donkey milk raw, treated it with UV-C light, or pasteurised it, freeze-dried all three variants, and then “digested” the powders in test tubes. The measurements for raw and UV-C-treated milk were close in several areas, while the pasteurised powder differed more significantly.
Laboratory study using milk powder and simulated digestion in test tubes – no conclusions can be drawn about its effects in humans. Donkey milk is a food.
Study at a glance
| Feature | Details |
|---|---|
| Original title | Processing of raw donkey milk by pasteurisation and UV-C to produce freeze-dried milk powders: The effect on protein quality, digestibility and bioactive properties |
| Authors | Tedeschi T., Aspri M., Loffi C., Dellafiora L., Galaverna G., Papademas P. |
| Journal | LWT – Food Science and Technology, 173: 114404 |
| Year | 2023 |
| Country | Cyprus (milk, processing), Italy (analysis) |
| Study type | Laboratory study: processing comparison and simulated digestion (in vitro) |
| Sample | 2 batches of 7 litres of donkey milk each from one farm; 3 powder variants |
| DOI | 10.1016/j.lwt.2022.114404 |
What question did the researchers ask?
Before milk is dried, it is usually pretreated – most often by heating. UV-C light is a heat-free process that is being investigated as an alternative in food technology. The researchers wanted to know how the three methods – “raw”, “UV-C”, and “pasteurised” – affected the proteins in the freeze-dried powder and what of this could be measured after simulated digestion.
How the study was conducted
- Milk: Two batches of 7 litres each from a donkey farm in Larnaca (Cyprus), hand-milked.
- Pretreatment: raw; UV-C in a flow-through device (4000 litres per hour, dose 1000 J/L, maximum 35 °C); pasteurisation at 62.5 °C for 30 minutes.
- Freeze-drying: frozen overnight at −80 °C, then dried under vacuum (maximum 0.3 mbar) for 72 hours.
- Simulated digestion: powder dissolved to 10% solids and “digested” in test tubes according to the INFOGEST protocol (mouth, stomach, small intestine at 37 °C).
- Analyses: protein content, protein bands in gel (SDS-PAGE), whey proteins by mass spectrometry, degree of hydrolysis, peptides, ACE inhibition and antioxidant tests in the laboratory.
The results
The protein content of the three powders did not differ. After simulated digestion, the pasteurised powder was more broken down. The degree of hydrolysis (proportion of broken peptide bonds) indicates how far the proteins were broken down in the test tubes.
| Measurement | Raw | UV-C | Pasteurised |
|---|---|---|---|
| Protein in powder (% of dry matter) | 15.29 | 15.67 | 15.34 |
| Degree of hydrolysis after digestion (%) | 37.02 | 37.19 | 44.31 |
| Peptides found per sample | 16 | 30 | 36 |
| ACE inhibition in test tubes (%) | 66 | 56 | 36 |
In the protein gel, three weak bands appeared at around 75 kDa, which the authors tentatively identify as lactoferrin. These bands were missing in the pasteurised sample. According to mass spectrometry, β-lactoglobulin was lower in the pasteurised powder, while α-lactalbumin was lower in the UV-C powder. ACE is an enzyme; only how strongly the digested samples inhibited it in the laboratory was measured. The antioxidant tests (ABTS, DPPH) showed a similar pattern, but the values are only shown in figures.
Freeze-dried powder from raw and UV-C-treated donkey milk was similar in several laboratory measurements. The pasteurised powder differed – these are measurements in test tubes, not effects.
Critical assessment
- Small sample size: two milk batches, with most measurements done in duplicate. The paper does not state the number of donkeys, the breed or the stage of lactation.
- Inconsistency: A total of 27 different peptides are mentioned, but 16, 30 and 36 per sample. How these numbers are related is not explained.
- Laboratory measurements only: ACE inhibition and antioxidant tests were carried out in test tubes. The authors themselves state that the bioactivity of the peptides was not tested.
- Many values only shown graphically: Whey proteins, amino acids and antioxidant values are shown in figures and supplementary tables, not as numbers in the text.
- No storage study: How stable the powders are over longer periods was not investigated.
Funding, conflicts of interest and level of evidence
The work was funded by the Research and Innovation Foundation Cyprus (DELIVER project). The authors declare no competing interests. The milk came from a donkey farm in Cyprus, where it was also pasteurised.
Level of evidence: Basic research (laboratory, in vitro) – describes processing and measurements, but does not draw any conclusions about effects in humans.
What does this mean for donkey milk as a food?
The study shows that pretreatment before drying leaves measurable traces in the powder – for example in whey proteins and in the breakdown in test tubes. It compares processes, but does not evaluate them. No recommendation for or against a process can be drawn from it. Our donkey milk powder is freeze-dried, but this study investigated milk from Cyprus, not our product.
Our research question: How do proteins and protein bands in freeze-dried donkey milk powder change over months of storage – and how can we document this transparently?
Frequently asked questions
What is UV-C treatment of milk?
The milk flows past UV-C lamps in a thin, turbulent layer. The process uses no heat; in this study, the milk remained at a maximum of 35 °C.
What does “simulated digestion” mean?
The samples are treated in test tubes with enzymes and conditions that mimic the mouth, stomach and small intestine, one after the other. This is a standardised laboratory model, not a study in humans.
Does ACE inhibition say anything about blood pressure?
No. Only how strongly the digested samples inhibited an enzyme in the laboratory was measured. Whether this plays a role in the body was not investigated.
You can find all measurements, interactive graphics and the full critical assessment on our research page: Study 74: Raw, UV-C or pasteurised. Further studies in the research archive.
Tedeschi T., Aspri M., Loffi C., Dellafiora L., Galaverna G., Papademas P. (2023). Processing of raw donkey milk by pasteurisation and UV-C to produce freeze-dried milk powders: The effect on protein quality, digestibility and bioactive properties. LWT – Food Science and Technology, 173, 114404. DOI: 10.1016/j.lwt.2022.114404
Summarised and reviewed by the DON DONKEY'S editorial team using the original publication · As of October 2026. This summary does not replace the original work.
End of study summary. The study was conducted independently of DON DONKEY'S® and does not refer to our products. Our donkey milk is a food.
