Donkey Milk Study: Raw, Stored, Pasteurized, Powder (Zhang 2026)

Along the processing chain of a farm-scale dairy, the protein content was 13.5 g/kg in raw milk and 16.5 g/kg in reconstituted powder. A total of 1505 metabolites were identified; 354 differed between the four milk forms.

The study in brief

• Original title: Changes in milk composition of raw, frozen, pasteurized, and freeze-dried jenny milk from a farm-scale dairy
• Authors: Zhang L.-T., Zhang K., Degen A. et al.
• Journal: Food Chemistry: X, 38, 104240 (2026)
• Country: China · Study type: Laboratory study

How it was investigated

In a commercial farm-scale dairy in Kuqa (Xinjiang, China) with around 200 lactating jenny, a production batch was followed from milking to powder. Fresh raw milk from seven individual animals in mid-lactation (90 ± 10 days; RM, n = 7), pooled raw milk stored for 3–4 days at −15 ± 3 °C in stainless steel tanks (DM, n = 8), pasteurized milk made from this (85 °C, 15 s; PM, n = 6), and subsequently freeze-dried powder reconstituted with water at a ratio of 1:11 (weight/volume) (FM, n = 6) were investigated. Basic components and pH value, a non-targeted metabolome by liquid chromatography-mass spectrometry, 15 trace elements by ICP-MS, and free short- and medium-chain fatty acids and total medium- and long-chain fatty acids by gas chromatography-mass spectrometry (trace elements and fatty acids n = 3–4 each) were determined.

Important figures

• 4 – Milk forms: raw (individual animals), stored, pasteurized, freeze-dried
• 7 / 8 / 6 / 6 – Samples for basic components and metabolome (RM / DM / PM / FM)
• 13.5 vs. 16.5 g/kg – Protein content raw milk vs. reconstituted powder
• 60.5 g/kg – Lactose in raw milk (stored and pasteurized: 62.9 and 62.8 g/kg respectively)
• 1505 – Identified and tentatively assigned metabolites
• 354 – Candidate metabolites with differences between the four milk forms

Results

The protein and ash content in the raw milk of the individual animals was lower than in the three processed forms; protein increased from 13.5 g/kg (raw) to 15.1 g/kg each (stored, pasteurized) and to 16.5 g/kg in the reconstituted powder. Lactose was 60.5 g/kg in raw milk and 62.9 and 62.8 g/kg in stored and pasteurized milk respectively. Fat content (4.3–8.8 g/kg) and pH value (6.80–7.02) did not differ significantly.

Of the 1505 identified metabolites, 354 were classified as candidates with differences. Compared to raw milk, 326, 556, and 425 metabolites differed in stored, pasteurized, and freeze-dried milk respectively, predominantly from protein, amino acid, nucleotide, and carbohydrate metabolism. Between stored milk and the two processed forms, the differences mainly concerned lipid metabolism pathways.

Of the 15 trace elements, cobalt (9.32 µg/L), manganese (24.8 µg/L), zinc (2661 µg/L), nickel, and chromium were each higher in the reconstituted powder than in individual other milk forms, iodine in pasteurized milk (52.2 µg/L). Copper, iron, selenium, and molybdenum did not differ significantly. Molybdenum (11.8–15.1 µg/L) and tin were described for the first time in donkey milk according to the authors.

Limitations of the study

• Raw milk not from the same batch: The raw milk came from seven individually sampled jenny, while the remaining samples came from pooled batch milk from many animals. Differences between "raw" and "processed" can therefore also be due to the animal selection, not just storage or processing.
• Small and unequal samples: Trace elements and fatty acids were determined for only three to four samples per milk form, from one batch from one farm on one day. Large fluctuations, for example with iron (223 to 2478 µg/L without a significant difference), show the limited conclusiveness.
• Contradiction to basic components: The conclusion states that the basic components are not changed by freezing and heating, but Table 1 shows significant differences in protein, lactose, ash, and density. In addition, the values of the powder depend on the reconstitution ratio of 1:11.
• Text and table are not consistent: According to the abstract, monounsaturated, n-6, and n-9 fatty acids were lower in stored milk than in pasteurized milk and powder; according to the letters in Table 4, this only applies compared to the powder. For lauric acid, the text states p = 0.020, the table p = 0.040.

Level of evidence: Basic research (laboratory) – describes the properties of the milk, no statement about effects in humans.

Our assessment

The study analytically describes how the composition of donkey milk from a Chinese farm-scale dairy changes through storage, pasteurization, and freeze-drying. Unlike the older study by Polidori et al. on fresh and freeze-dried donkey milk, it considers different animals, a complete processing chain, and additional measurements such as the metabolome and trace elements. Statements about effects or individual commercial products cannot be derived from this.

Source: Zhang L.-T., Zhang K., Degen A., Xu Q., Xiong J.-L., Zhou X.-L. (2026). Changes in milk composition of raw, frozen, pasteurized, and freeze-dried jenny milk from a farm-scale dairy. Food Chemistry: X. DOI: 10.1016/j.fochx.2026.104240 · Summarized by the DON DONKEY'S editorial team, as of October 2026.

End of the study summary. The study was conducted independently by DON DONKEY'S® and does not refer to our products. Our donkey milk is a food.