Donkey milk knowledge archive · Historical source

Friedheim 1909: how donkey milk curdles with acid and rennet – an early protein analysis

In his 1909 Bern doctoral thesis, the veterinarian Willi Friedheim examined how nitrogen is distributed in five types of milk when they curdle. He placed donkey milk between women’s and cow’s milk in protein and casein content.

Historical account: The information reproduced here reflects the knowledge of its time. It is not scientifically substantiated, is not a recommendation and cannot be applied to today. Donkey milk is a food, not a remedy.
1909Düsseldorf (laboratory) and Bern (faculty)Scientific doctoral thesis
Author
Willi Friedheim, veterinary practitioner in Solingen; experiments under Dr Engel at the Düsseldorf children’s clinic (dir. Prof. Arthur Schlossmann)
Work
Die Stickstoffverteilung in der Kuh-, Büffel-, Ziegen-, Frauen- und Eselsmilch bei Säure- und Labfällung (nitrogen distribution in cow’s, buffalo, goat’s, women’s and donkey milk on acid and rennet precipitation), doctoral thesis, veterinary faculty Bern, Berlin: Julius Springer 1909
Passage
Section “Säurefällung” (pp. 5 and 10–12), section “Eselsmilch” with table IX (pp. 24–25), overall result (pp. 25–26); plate I, fig. 2
Time & place
1909 (approved for printing on 11 June 1909) · Düsseldorf (laboratory) and Bern (faculty)
Type of source
Scientific doctoral thesis

In the original words

What the original says

Auch die Frauenmilch und die Eselsmilch, von denen man früher glaubte, daß sie eine Ausnahmestellung einnähmen, lassen sich ausgezeichnet durch Säure zur Gerinnung bringen, wenn man nur die richtigen Mengen anwendet.

Women’s milk and donkey milk, too, which were formerly thought to occupy an exceptional position, can be curdled very well with acid, provided the right amounts are used.

German (1909)Friedheim 1909, Abschnitt „Säurefällung“, S. 5

Das Material stammte von einer Eselin, welche etwa 4 Wochen vor den Versuchen geworfen hatte.

The material came from a jenny that had foaled about four weeks before the experiments.

German (1909)Friedheim 1909, Abschnitt „Eselsmilch“, S. 24

Die Eselsmilch charakterisiert sich also als eine Milchart, welche bezüglich des Gesamteiweißgehaltes und des Caseingehaltes in der Mitte steht, zwischen der Frauenmilch und der Kuhmilch.

Donkey milk is thus characterised as a type of milk that lies midway between women’s milk and cow’s milk in its total protein and casein content.

German (1909)Friedheim 1909, Abschnitt „Eselsmilch“, S. 25

English rendering: our own translation by the DON DONKEY’S editorial team, following the sense of the text.

Background

Who, when, what kind of text

Around 1900 interest in donkey milk shifted from materia medica to chemistry. Children’s clinics wanted to understand why types of milk behave differently in the stomach. At the Düsseldorf children’s clinic under Arthur Schlossmann, the group around Dr Engel therefore investigated how milk curdles when acidified and when rennet, the enzyme from the calf’s stomach, is added.

Willi Friedheim, a veterinarian in Solingen, submitted the results in 1909 as a doctoral thesis to the veterinary faculty of the University of Bern. The thesis is an offprint from a Springer biochemistry journal (vol. 19). It compared cow’s, buffalo, goat’s, women’s and donkey milk with the same method and documents the experiments in tables and a plate.

Content

What the source reports about donkey milk

Friedheim counts women’s and donkey milk among the “albumin milks”, which caused the greatest difficulties in precipitation. He shows that, contrary to earlier assumptions, both curdle completely with acid. Donkey milk, he finds, needs about twice as much acid as women’s milk: 50 to 60 instead of 25 to 30 ccm hydrochloric acid per 100 of milk, 150 to 180 instead of 60 to 80 ccm acetic acid. It curdles at room temperature and in larger flakes.

The milk came from a single jenny about four weeks after foaling. Fat content was mostly 1.2 per cent, nitrogen 385 to 397 mg per 100 parts of milk, above the average of 243 mg that Schlossmann had reported for pooled milk from many animals. On acid precipitation about 61.9 per cent of the nitrogen belonged to casein and 38.2 per cent to dissolved proteins.

With rennet, only about 5.5 per cent of total nitrogen additionally passed into the whey, roughly half the value for cow’s, goat’s and women’s milk. From this Friedheim concludes that donkey milk lies between women’s and cow’s milk in total protein and casein. Whether the laboratory conditions match the processes in the stomach he explicitly leaves open.

At a glance

Figures & facts from the source

1909Bern thesis approved for printing on 11 June
1 jennysupplied the milk studied, about four weeks after foaling
385–397 mgnitrogen per 100 parts of donkey milk (table IX)
61.9 %share of nitrogen in casein on acid precipitation
5.5 %additional nitrogen in rennet whey, about half the value of cow’s milk
5milk types studied: cow, buffalo, goat, human, donkey

Critical assessment

How reliable is this source?

Source value

A printed, examined doctoral thesis with disclosed methods and measurements, fully available as a scan. Page numbers are partly cropped in the scan; we derived them from the printer’s sheet signatures and also give the scan leaf number.

Limits of the study

All donkey milk values are based on the milk of a single jenny at an early stage of lactation and on four test series. Friedheim himself points out that pooled milk from many animals gives different values and that laboratory precipitation does not necessarily reflect conditions in the stomach.

Today’s assessment

The thesis is an early example of the chemical comparison of protein fractions that modern proteomic studies continue far more precisely. It was carried out at a children’s clinic; donkey milk is not a substitute for breast milk or infant formula; infant feeding belongs in medical hands.

Our assessment

With Friedheim’s thesis, donkey milk is measured in the laboratory rather than praised in pharmacopoeias. We appreciate the honesty of the work: it states where its sample came from, compares it with other data and openly names what it cannot answer.

Study 168: Casein micelles in donkey milkStudy 181: 106 proteins in donkey milk wheyStudy 129: Protein profile of whey proteins

References

Sources to check

More sources and studies in the research archive. Summarised by the DON DONKEY’S editorial team, as of November 2026.