Donkey Milk Meaning and Properties

Donkey Milk Meaning and Properties: Comparison

Please note this is a comparison between Version 3 by Camila Xu and Version 4 by Camila Xu.

Donkey milk (DM) is produced by the female animals of the house donkey (Equus asinus). DM is known for their nutritional and nutraceutical properties for human consumption. DM compared to milk from other dairy animals such cow, buffalo, goat and sheep is most likely to resemble human breast milk. The average daily milk yield of a female donkey over the entire lactation season is 1.57 kg/day and fluctuated between 0.20 and 6.00 kg/day. Furthermore, the average milk concentration of fat, protein, lactose, total solids and ash in DM are 0.63%, 1.71%, 6.34%, 9.11% and 0.39%, respectively. It is noteworthy that DM is rich in whey proteins, polyunsaturated fatty acids such as linoleic acid C18:2 and linolenic acid C18:3 and vitamin C. In addition, the average lysozyme concentration of DM is approximately 1.07 g/L, similar to human milk (0.30-1.10 g/L). The high lysozyme content may be the cause of low bactarial count of DM and also makes this milk useful for preventing intestinal infections in infants. Due to the significantly lower casein content, many people with a cows milk protein allergy can tolerate DM, as they find DM do not trigger an allergy reaction.

donkey milk
milk yield
milk composition
Lactation curve
Fat
Protein
Lactose
Mineral
Vitamin

1. Introduction

Donkey milk (DM) is produced by the female animals of the house donkey (Equus asinus). Compared to cows, donkey mares give low amounts of milk ^[1][2], which initially serves as food for the donkey offspring. The small but valuable milk of the female donkey is particularly noticeable in the prices of the products. However, donkey milk production has recently become more and more an issue ^[3][4][5].

Donkey mares have been known for thousands of years for their valuable milk ^[6][7]. Thus, DM has been known since the time of ancient Egypt ^[2][6]. Hippocrates (460–370 BC) was the first to write about the medicinal properties of DM ^[8]. Furthermore, Cleopatra, the former queen of Egypt, bathed in the milk of female donkeys to preserve the beauty and youth of her skin ^[9]. In ancient Rome, DM was also used for cosmetic purposes ^[10]. In the 19th century, the French Dr. Parrot breast-fed orphaned babies directly on female donkeys, as DM is most similar to human breast milk ^[11]. DM was then sold well into the late 20th century to feed and heal orphaned infants and cure weak and sick children, as well as the elderly ^[7]. Much research has been carried out on DM in recent years, particularly with regard to its properties and chemical composition ^{[12][13][14][15][16][17][18]}. However, there is a lack of information on the morphological properties of donkey udder anatomy and the use of suitable milking machines to increase milk yield and to meet donkey milk requirements.

2. Milk Yield of Female Donkey

The average daily milk yield over the entire lactation season was 1.57 ± 1.12 kg/day and fluctuated between 0.20 and 6.00 kg/day (Table 1). However, the daily milk yield in donkeys is influenced by many factors, such as breed, management of dams and foals, feeding, stage of lactation, lactation number, foaling season, milking procedure, number of milkings per day, milkability degree of donkey mares and production country ^{[1][17][19][20][21][22]}. During lactation season, the milk yield of the Italian Ragusana breed steadily decreased from 1.98 to 1.28 kg/day ^[21], whereas Salimei et al. ^[20] reported that the milk yield per milking did not vary significantly during lactation season. Muhatai et al. ^[23] reported that during lactation season the daily milk yield decreased from 3.30 kg/day at the start of the lactation to 2.20 kg/day by the end of the lactation season. Based on the observations of Doreau et al. ^[19] it is assumed that the diet plays a relevant role in the milk yield and quality of female donkeys, while Salimei et al. ^[24] found no significant differences in milk yield and ingredients in two feed rations. It is noteworthy that the daily milk yield of donkey mares, the length of lactation season and the milk characteristics varied depending on the foaling season ^[21]. However, donkeys foaling in winter and in summer produced more milk than those foaling in spring and autumn ^[21]. Moreover, studies have shown that the average amount of milk yield for morning milking was statistically lower than that for afternoon milking (549.2 mL vs. 949.3 mL) ^[20]. In addition, it was found that parity affects the daily milk yield and parity 3 donkeys produced 22% more milk than parity 4 donkeys (3.30 vs. 2.70 kg/day) ^[23].

Table 1. Average daily milk yield of the donkey mare during the lactation season.

Location	Breed or Type	Daily Milk Yield (kg/day)	Range of Milk Yield (kg/day)	Reference

Table 2. Effects of daily milking number and interval between milkings on milk yield per milking in donkey mares ^[31].

Group	Milking		Milk Yield per Milking (mL)
Group	Ash		(%)	pH	Reference	Number/day	Interval (h)
Daily Milk Yield *	(kg/day)	Fat (%)	Protein (%)	Lactose (%)	SCC ** (log 10 n × 10³/mL)
Mean	Range
Africa	Local type	1.50	*	^[6]
346.0 ± 7.2	^a	40–1220	China	Jiangyue	3.00 (4 milkings/day)	2.20–3.30	^[23]
2	China	Jiangyue	1.28 ± 0.17
^[
¹⁴
^]
Italy
1	1

^[50][52][53]. On the other hand, vitamin B12 was not detected in DM ^[54]. The lack of vitamin B12 in equid milk compared to ruminant milk could be explained by the different digestive systems among these two species, and vitamin B12 is synthesized by the microorganisms of the digestive tract. In addition, the vitamin C content in DM is very similar to that in breast milk but higher than that in cow’s milk (Table 5) ^{[2][49][55][56]}.

Table 5. Vitamin content in donkey milk ^{[2][49][56][57]}.

Water-Soluble Vitamins		Fat-Soluble Vitamins
Vitamin	Concentration µM/L	Vitamin	Concentration µg/100 mL
B1 (thiamine)	0.66	3
3	3	512.2 ± 7.6	^b	50–1500
1.16–1.54	^[	¹	^]
Croatia	Littoral Dinaric	0.17 (1 milking/day)	*	^[25]
Croatia
Croatia	0.16	1.40	*	*	8.74	*	7.03–7.06	^[34]
Spring	1.85 ^a***	0.39 ^ab	1.93 ^b	6.65 ^a	5.35 ^a	Vitamin A	58.00 µg/100 mL
0.28 ± 0.05	1.59 ± 0.02	0.72	6.73 ± 0.06	8.91 ± 0.03	0.32 ± 0.02	7.01 ± 0.01
Summer	1.44 ^b	0.23 ^b
B2 (riboflavin)	0.17	1.81	^ab	6.05 ^c	24.05 ^b	^[	^36]
Vitamin D	2.23 µg/100 mL	1.16	1.57
Autumn	1.41	0.82	6.33	9.53	0.40	7.18	^b	0.38 ^ab^[1]
1.78	^a	6.34	^bc	9.210	^a
B3 (niacin)	18.75	Vitamin E	5.20 µg/100 mL	0.44 ± 0.10	1.9 ± 0.02	0.88	6.40 ± 0.10	*	*
Winter	1.47 ^b	0.50	*	^a	1.92 ^b
B6 (pyridoxine)	5.38	6.50	^ab	^[	²¹	^]	Vitamin K	*	Istrian	0.75 ± 0.30 (1 milking/day)
24.60	^b	*	^[	^26]
0.33	1.55	*
B9 (folic acid)	6.28	0.83		8.80	*	*	^[25]		Littoral Dinaric	0.32 ± 0.15 (1 milking/day)	*	^[
0.49 ± 0.02	2.03 ± 0.15	²⁶	*^]
5.70 ± 0.14	10.71 ± 0.64	0.42 ± 0.02	7.16 ± 0.02	^[	³⁷	^]
B12 (cyanocobalamin)	India	Local type	1.00	*	^[7]
Amiata	0.70 ± 0.10	0.52–0.76	^[	^28]
Italy
3	3	2	325.5 ± 10.9 ^a	40–1640
4	6	2	282.3 ± 7.7 ^c	Italy
1.20	2.14	0.88	5.90	7.50	*	*	^[38]	Local breeds	1.36 ± 0.51	*	^[27]
0.16	1.34	0.56	6.07	Italy	Ragusana	1.64 ± 0.79 (2 milkings/day)	0.38–3.30	^[17]
Italy	Ragusana	1.70 ± 0.10 (2 milkings/day)	1.28–1.98	^[21]
Italy	Ragusana	1.71 ± 0.66	1.25–2.35	Martina Franca	1.50 ± 0.08	1.27–1.47	^[24]
Italy	Martina Franca and Ragusana	1.48 ± 0.06	1.30–1.80	^[20]
Italy	Ragusana	4.87 ± 0.12 (8 milkings/day)	*	^[29]
Italy	Different races	2.68 ± 1.96	0.72–6.00	^[22]
Jordan	Local type	1.00	*	^[30]
Mean ± SD		1.57 ± 1.12

SD: standard deviation; * not specified.

D’Alessandro and Martemucci ^[31] reported that the daily milking management influences milk yield and udder health condition. Thus, the observed differences in milk yield can be attributed to the number of milkings per day and milking interval. However, milking three times a day with a 3 h interval showed a higher milk yield per milking compared to three times a day with a 2 h interval (p < 0.01). In addition, the milk yield per milking increased with an increase from one to three milkings per day, but the values for this variable fluctuated strongly from 40 to 1.640 mL/milking (Table 2

40–1330

Mean values with different letters are significant (p < 0.01).

Furthermore, no differences in milk yield were found between the right and left udder halves of the female donkey, while the effects of lactation day and the individual were significant (p < 0.01) ^[31].

3. Lactation Curve in the Female Donkey

Following the daily milkings shows a typical development of milk production from birth to the end of lactation. The course of lactation can be represented graphically as a curve for a donkey mare ^[31] (Figure 1).

Figure 1. Lactation curve from milk yield data (morning milk) in donkey mares (n = 12) with regression ^[31], by application of Wood’s model (Wood, 1967): Yt = a × t^b × e^−ct (r² = 0.11), where Y is milk yield on day t of lactation, a is the constant level of initial yield, b is the rate of increase to peak, e is the mathematical constant, c is the rate of decline after peak and peak in days (t) is given by b/c.

According to the results shown in Figure 1, the lactation peak was reached after 48 days. However, most female donkeys (42%) showed their maximum milk yield between 64 and 73 days of lactation. Likewise, milk yield decreased progressively and significantly (p < 0.01) until the end of lactation, as shown in the figure (Figure 1). According to Malacarne et al. ^[17], the daily milk yield in donkey mare was around 2 kg/day for up to 160 days and then gradually decreased to 0.13 kg/day at the end of lactation (250–279 days). The effect of the mechanical milking management was also examined, and it was found that the milking management had no significant influence on the total milk yield in the entire lactation ^[32].

4. Physiochemical Properties of Donkey Milk

DM composition has been studied in different parts of the world ^{[1][17][18][33][34][35]}. However, data from the literature indicated ranges of variation in DM composition. The mean values of the DM composition are given in Table 3.

Table 3. Physicochemical composition parameters of donkey milk.

Fat (%)	Protein (%)	Casein (%)	Lactose (%)	Dry Matter (%)
*


Vitamin C	57.00 **
8.19	0.36	*	^[	¹⁷	^]
1.21	1.74	*	6.23	9.60	0.43	*	^[39]
0.30–1.80	1.50–1.80	0.78	5.80–7.00	8.80–11.70	*	7.00–7.20	^[7]
0.38	1.72	0.87	6.88	8.84	0.39	*	^[20]
0.67 ± 0.55	1.88	0.79	6.84 ± 0.15	*	0.41	7.20	^[35]
0.63 ± 0.41	1.71 ± 0.24	0.79 ± 0.11	6.34 ± 0.37	9.11 ± 0.95	0.39 ± 0.04	7.12 ± 0.08	Mean ± SD

SD: standard deviation; * not specified.

DM is characterized by little fat, protein and casein and high levels of lactose and whey proteins and is rich in lysozyme ^{[1][20][36][40]}. DM composition was found to be less than other species such as bovine milk ^[20]. However, variations observed in DM composition could be attributed to several factors such as breeds or type, stage of lactation, age of animals, lactation number, season and country ^[41]. Breed and seasonal variation were found to be the most effective factors in DM composition (Table 4). It is noteworthy that three hours was described as an ideal milking interval for obtaining the best organoleptic characteristics of DM, while an interval of 5 to 8 h between milking times resulted in a decrease in milk fat and lactose content ^[42].

Table 4. Effect of the production season on the main qualitative variables ^[21].

Season

* Daily milk yield (kg/day): the daily milk yield was achieved by mechanical milking twice a day with an inter-milking interval of 5 h. ** SCC: somatic cell count. *** Means in the same column and same parameter with different superscripts a, b, c are significantly different at p < 0.05.

4.1. Fat Content

The mean content of fat in DM during the lactation season was 0.63 ± 0.41% (Table 3), and fat content ranged between 0.16 and 1.21% ^{[1][17][20][37][43]}, with an increasing, non-linear, trend from partum to the end of lactation ^[1][20]. However, the fat content of DM depends on many factors such as stage of lactation, lactation season, feed, country, breed or type, milking methods ^[34][44]. Malacarne et al. ^[17] reported a very low fat content (0.16 ± 0.06 g/100 g milk) in Italian Ragusana DM. Ivankovic et al. ^[25] also observed that the fat content of DM was low (0.33%). Giosue et al. ^[21] indicated that the season had an influence on the milk fat content in DM, as the female donkeys foaled in spring had 0.28% of fat in DM, while the milk of those foaled in autumn had 0.52% of fat.

4.2. Protein Content

The mean value of protein content in DM was 1.71 ± 0.24% (Table 3), and protein content ranged between 1.34 and 2.20% ^{[20][21][25][33]}. This protein concentration in DM was similar to that of breast milk and much lower than that of cow, goat or sheep milk ^[1]. Compared to cow milk, DM is characterized by low casein (42–52% of total protein) and high whey protein content (48–58% of total protein) ^{[1][17][36][38]}. However, the casein to whey protein ratio of 52:48 was between the lower value of human milk and the higher value of cow milk ^[1]. A specific protein profile with the following DM whey proteins was also found in DM: 4.48% lactoferrin, 6.18% serum albumin, 29.85% β-lactoglobulin, 21.03% lysozyme and 22.56% α-lactalbumin ^[20]. It has also been found that the protein content of DM decreases significantly during lactation ^[20][34]. This may be due to the differential expression of milk protein synthesis genes during the lactation period ^[1][34]. On the other hand, the protein content in the milk produced by female donkeys foaling in winter and summer was greater than the ones foaling in autumn and in spring ^[21]. Furthermore, Guo et al. ^[1] reported that DM had higher levels of amino acid such as serine (6.2%), glutamic acid (22.8%), arginine (4.6%) and valine (6.5%) and a lower level of cystine (0.4%).

4.3. Lactose Content

The mean value of lactose content in DM was 6.34 ± 0.37% (Table 3) which is higher than that of cow milk and very similar that of to breast milk ^[1][33]. However, the average values of lactose were 7.00%, 4.90% and 6.00% in DM, cow milk and human breast milk, respectively ^[16][45]. The lactose content of DM showed a progressive lowering from the beginning of lactation (6.30 g/100 g) to the end of lactation (5.70 g/100 g) ^[17]. On the other hand, the concentration of lactose in DM was nearly constant throughout the lactation season ^[1], since the lactose content of the milk plays an important role in the osmotic pressure of the mammary gland, which is equal to that of blood. However, due to differences in the season and donkey feeding conditions, slight changes in the lactose content of DM were found in different studies ^{[20][21][46][47]}.

4.4. Mineral Content

DM contains an average of 0.39 ± 0.04% minerals (Table 3), which differ depending on the breed and stage of lactation. Malacarne et al. ^[17] showed similar results, and the total ash content was 0.36% and it represented 4.4% of the dry matter. Thus, most of the minerals in DM vary significantly during lactation ^[17][48]. However, Ca, P, Mg, Na, Fe and Zn contents were affected by the days in milk, whereas no significant variations were reported for K, Cl and Cu ^[17]. It is noteworthy that the concentrations of minerals in DM are similar to those in breast milk ^[48]. However, the concentrations of K, Na, Fe, Zn and Mg in DM appear to be similar to those in breast milk ^[2]. Generally, DM is a rich source of various minerals such as Ca, P, Na, K, Mg, Fe, Zn and Cu ^[2][17][20].

4.5. Vitamin Content

In DM, the content of B-complex vitamins such as B1, B2, B3, B6 and B9 is higher than that in breast milk ^[2][49]. However, the B1 and B6 concentrations in DM were very high compared to those in breast milk, and the values were 0.66 and 5.38 µM/L vs. 0.12 and 0.48 µM/L, respectively ^[50][51] (Table 5). Likewise, B2 (0.17 µM/L), B3 (18.75 µM/L) and B9 (0.83 µM/L) concentrations in DM were also higher than the values reported in breast milk

* Not detected, ** mg/L.

The vitamin A content in DM (58 µg/100 mL) is slightly lower compared to that in breast milk (60 µg/100 mL) ^[57], while the vitamin D content in DM is higher than the values found in the milk of many other mammals and in breast milk ^[45]. Compared to breast milk, DM has a very low vitamin E content, and the values were 237 and 5.2 µg/100 mL, respectively ^[57] (Table 5). It is noteworthy that vitamin K has never been detected in DM.