Donkey Milk Study: Detecting Cow Milk Adulteration Using a Rapid Test (Kourti 2024)

A chip sensor detected cow milk in donkey, sheep, and goat milk from 0.05% (v/v) in 12 minutes. The recovery was 92.0–105%, with measurement variation of 4.0% (same day) and 6.5% (different days).

Study at a glance

• Original title: Photonic Dipstick Immunosensor to Detect Adulteration of Ewe, Goat, and Donkey Milk with Cow Milk through Bovine κ-Casein Detection
• Authors: Kourti D., Angelopoulou M., Makarona E. et al.
• Journal: Sensors, 24(17), 5688 (2024)
• Country: Greece · Study type: Laboratory study

How the study was conducted

Laboratory study on the development of an immunosensor on a silicon chip (22 × 2 mm) with two Mach-Zehnder interferometers: One measurement window was coated with bovine κ-casein, the other with bovine serum albumin as a reference. In a competitive test format, the chip was sequentially dipped into milk diluted 50-fold with an antibody against bovine κ-casein and then into a solution with a secondary antibody for signal amplification; pumps or microfluidics were not required. Surface coating, stirring, antibody dilution, κ-casein concentration, and sample dilution were optimized.

Key figures

• 0.05% v/v – Detection limit for cow milk in donkey, sheep, and goat milk
• 0.1–10% v/v – Linear working range
• 12 min – Stated test duration without sample preparation
• 50-fold – Dilution of the milk sample in the test
• 1.69 rad – Zero calibrator signal in donkey milk (sheep 1.66, goat 1.59 rad)
• 54.8–58.5% – Residual signal at 0.5% cow milk relative to the zero calibrator, all three milk types

Results

Coating the chip surface with 2% aminosilane in ethanol for 20 minutes yielded the highest signals; stirring the solutions increased the signal approximately fourfold. The authors chose a 1:200 antiserum dilution and 50 µg/mL κ-casein for coating as the optimum.

At 10- and 20-fold dilutions, milk components, especially fat, reduced the signal; this effect was no longer clearly evident from 50-fold dilution. Centrifugation of the milk was therefore not necessary.

The zero calibrator signals were similar for donkey milk (1.69 rad), sheep milk (1.66 rad), and goat milk (1.59 rad), and the calibration curves of the three milk types overlapped. The addition of 0.5% cow milk reduced the signal to 54.8–58.5% of the zero value.

Limitations of the study

• Only one commercial sample per milk type: Only one commercially available milk from one manufacturer was used for each animal type and mixed with cow milk in the laboratory. It was not tested whether donkey milk from different breeds, lactation stages, or processing forms (e.g., raw milk, powder) behaves similarly; actual suspected samples were not examined.
• Test duration cannot be verified: The described steps (2 min equilibration, 5 min primary reaction, 2 min washing, 5 min secondary antibody) add up to 14 minutes, but 12 minutes are stated. Chip regeneration is not included in this.
• Contradictions in the literature comparison: For the previous sensor, the introduction states 0.1% in 10 minutes, the discussion 0.04% in 25 minutes, and Table 1 0.04% in 10 minutes. The values for mozzarella and feta are swapped between the introduction and discussion. The statement that the sensor is more sensitive than all previous ones contradicts Table 1, which states 0.01% for a voltammetric method.
• Minor reporting errors: The legend of Figure 3a states a 50-fold antiserum dilution, while the text states 100-fold; the sixth condition is not numbered, and the unit is missing for the aqueous aminosilane solution ("0.5 v/v"). Regeneration is stated as 80 cycles, while Figure S6 shows 85.
• Authors' classifications: Statements about the nutritional value and tolerability of cow milk compared to donkey, sheep, and goat milk are not supported and were not the subject of the study. The stated EU maximum limit of 1% refers to the official reference method for cheese made from sheep, goat, and buffalo milk, not explicitly to donkey milk.

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

Our assessment

The study concerns the authenticity testing of donkey milk, i.e., the detection of cow milk adulteration, and describes a laboratory prototype. It does not contain statements about the ingredients or effects of donkey milk.

Source: Kourti D., Angelopoulou M., Makarona E., Economou A., Petrou P., Misiakos K., Kakabakos S. (2024). Photonic Dipstick Immunosensor to Detect Adulteration of Ewe, Goat, and Donkey Milk with Cow Milk through Bovine κ-Casein Detection. Sensors. DOI: 10.3390/s24175688 · 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 product.