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Lysozyme is an enzyme that occurs naturally in donkey milk. A team from the University of Pisa measured for one year how active this enzyme is in the milk of a herd of Amiata donkeys in Tuscany. The lactation stage, the age of the animals, and gentle pasteurization were examined. The activity fluctuated with the month of lactation and age; the team found no significant difference between raw and pasteurized pooled milk.
Laboratory milk analysis – enzyme activity was measured in milk samples; this is not a statement about an effect in humans. Donkey milk is a food.
Study at a glance
| Feature | Information |
|---|---|
| Original title | Lysozyme activity in donkey milk |
| Authors | Martini M., Salari F., Licitra R., La Motta C., Altomonte I. |
| Journal | International Dairy Journal, 96: 98–101 |
| Year | 2019 |
| Country | Italy (Tuscany) |
| Study type | Milk analysis over one year (raw and pasteurized) |
| Sample | Pooled milk from 40 donkeys (monthly, 12 months); individual samples from 12 donkeys in the 3rd, 6th, and 9th month of lactation |
| DOI | 10.1016/j.idairyj.2019.04.009 |
What question did the researchers ask?
The team wanted to know what the lysozyme activity in donkey milk depends on: the lactation stage (i.e., how long the donkey has been giving milk), the age of the animals, and gentle pasteurization at 65 °C for 30 minutes. They also examined whether the activity is associated with other milk components such as minerals or fatty acids.
How it was examined
- Herd: Around 160 Amiata donkeys at a farm in Tuscany, hay as desired plus concentrate feed, machine-milked once daily.
- Pooled milk: For one year, milk from 40 donkeys once a month, two raw and two pasteurized samples each time.
- Individual samples: Raw milk from 12 donkeys (9 to 20 years) in the 3rd, 6th, and 9th month of lactation, refrigerated and analyzed within 24 hours.
- Measurement: A fluorescence test: marked cell wall material glows when lysozyme breaks it down. The glow was converted into units per milliliter (U/mL) using a calibration curve.
The results
In the individual samples, the mean lysozyme activity was 1,670.11 ± 411.454 U/mL. It was highest in the 3rd month of lactation and lowest in the 6th month. Donkeys over 15 years had higher values than donkeys under 9 years. In the pooled milk, raw and pasteurized samples did not differ significantly.
| Measurement | Group | Lysozyme activity (U/mL) |
|---|---|---|
| Month of lactation | 3rd / 6th / 9th month | 1,842.21 / 1,433.84 / 1,733.46 |
| Age of the donkey | under 9 / 9–15 / over 15 years | 1,547.83 / 1,610.57 / 1,824.11 |
| Raw pooled milk | 12 months | 1,340.82 ± 371.078 |
| Pasteurized pooled milk (65 °C, 30 min) | 12 months | 1,402.50 ± 294.722 |
In the 3rd month of lactation, higher activity was associated with more ash (r = 0.684) and calcium (r = 0.613), as well as more saturated fatty acids (r = 0.868). There was a negative correlation with long-chain fatty acids (r = −0.877). Correlations show co-occurrence, not causation.
The measured lysozyme activity fluctuated with the month of lactation and the age of the animals. The examined gentle pasteurization did not change it significantly in this study.
Critical assessment
- Small sample: One herd, one breed, individual samples from only 12 donkeys. The age groups are correspondingly small.
- Barely sufficient statistics: The paper does not state F or P values for the group comparisons; the same standard error (446.058) is given for lactation and age.
- Contradiction: For linoleic acid, the table shows a positive correlation (r = 0.884), while the text lists it among the negatively associated fatty acids.
- Not directly comparable: Individual samples and pooled milk are different types of samples; their means cannot be directly compared.
- One method: Only 65 °C for 30 minutes was examined. The study says nothing about other heating methods, freezing, or drying.
Funding, conflicts of interest, and level of evidence
The work was funded by PRA 2017, a research project of the University of Pisa. The freely available author version does not contain a declaration of conflicts of interest; we evaluated this version.
Level of evidence: Basic research (milk analysis) – describes the enzyme activity in milk samples; no statement about effects.
What does this mean for donkey milk as a food?
The study shows that lysozyme activity in donkey milk is not a fixed value: in this herd, it depends on the month of lactation and the age of the animals. Regarding pasteurization, it neutrally reports that the examined gentle version did not significantly change the activity. No recommendation for or against a method can be derived from this.
Our research question: How does lysozyme activity behave in donkey milk that is shock-frozen at −40 °C and stored frozen – and how can this be documented in a traceable way?
Frequently asked questions
What is lysozyme?
An enzyme that can break down certain building blocks of bacterial cell walls. It occurs in many body fluids and in milk. In this study, only its activity was measured in the laboratory.
What does U/mL mean?
Activity units per milliliter. The value indicates how much enzyme activity the test measured in a sample – not how many grams of lysozyme are contained.
Is pasteurized donkey milk therefore equivalent to raw milk?
This cannot be derived from the study. It only compared lysozyme activity and only one heating method.
You can find all measurement values, interactive graphics, and the complete critical assessment on our research page: Study 79: Lysozyme activity in donkey milk. Further studies in the research archive.
Martini M., Salari F., Licitra R., La Motta C., Altomonte I. (2019). Lysozyme activity in donkey milk. International Dairy Journal, 96, 98–101. DOI: 10.1016/j.idairyj.2019.04.009
Summarized and checked 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 by DON DONKEY'S® and does not refer to our products. Our donkey milk is a food.
