| Livestock Research for Rural Development 30 (7) 2018 | Guide for preparation of papers | LRRD Newsletter | Citation of this paper |
In Botswana, donkeys are used for milk production in a limited scale. However, donkey milk is recently getting popular in the country. To date, no research work has been conducted on the nutritional value and safety of donkey milk produced in the country. The chemical composition and microbiological quality of donkey milk sold in Gaborone, Botswana were investigated. Donkey milk samples were collected from street vendors at Main Mall in Gaborone City and examined for physicochemical properties and microbial quality. The average total solids, solids-not-fat, fat, protein, lactose, pH, specific gravity and ash contents of the milk samples were 10.71±0.64%, 10.22 ± 0.69%, 0.49±0.02%, 2.03±0.15%, 5.70±0.14%, 7.16±0.02, 1.03±0.00 and 0.42±0.64%, respectively. The average total viable and coliform counts of the milk samples were 3.72±0.58 and 2.53±0.01 log 10 cfu/ml, respectively. Presence of coliforms in the donkey milk indicates the potential presence of pathogenic microorganisms in the milk. Thus, there is a need for pasteurization of the milk and strict hygienic measures during production and handling of the milk. Quality control measures should be put in place to ensure that donkey milk sold in the market is safe for human consumption.
Keywords: jenny milk, nutritional value, safety
Donkey (Equus asinus) is an important dairy animal and has been used for milk production since the Roman age (Salimei 2011). Donkey milk has been used not only for its nutritional value but also for its beneficial properties in skin care (Salimei 2011). Recently, donkey’s milk has been revalued as an alternative food for infants with cow’s milk protein allergy (CMPA), the most common food allergy in childhood (Monti et al 2007; Salimei 2011). Moreover, the milk is highly palatable making it suitable for feeding young children (Nayak et al 2017).
Donkey milk is produced and consumed in many countries in the world including Italy, France, Spain, Belgium, Bulgaria, Portugal and Greece (Salimei 2011; Aspri et al 2017). In Asia, donkey milk is produced in China, which has the largest donkey population (8.7 million) in the world (Salimei 2011; Aspri et al 2017). The milk is also consumed in Africa and Latin America (Salimei and Fantuz 2013).
Donkey milk has unique properties which resemble human milk in composition. It is similar to human milk in terms of protein composition, that is, low casein content and high whey protein content (Barłowska et al 2011). This similarity to human milk has made donkey milk an alternative as a substitute by people who are suffering from cow milk protein allergy (Barłowska, et al 2011; Aspri et al 2017). Clinical studies have demonstrated that donkey milk could substitute breast feeding in infants affected by severe immunoglobulin E (Ig-E) mediated milk allergies (Polidori et al 2009).
Donkey milk also contains large amounts of lysozyme (13.13-15.34% of total protein) in contrast to cow, sheep, and goat milk (Barłowska et al 2011). The lysozyme has both bactericidal and bacteriostatic properties. Therefore, the high lysozyme content of donkey milk may be associated with the low bacterial count reported in the literature and also makes this milk suitable for prevention of gastro-intestinal infections in infants (Polidori et al 2009). Donkey milk has been reported to have low fat content (Guo et al 2007) and it is also characterized by a fatty acid profile that is different from milk of other animal species. Additionally, it is very rich in polyunsaturated fatty acids (PUFAs): linoleic (C18:2) and linolenic (C18:3) (Barłowska et al 2011).
Donkey milk is reported to contain anti-ageing, anti-oxidant and regenerating compounds. It may prevent certain skin diseases and as a result used in manufacturing of cosmetics such as facial bathing soaps and body lotion (Chiofalo et al 2006). It is recommended for heart and cholesterolemic patients (Cosentino et al 2012; Cosentino et al 2015); it balances the metabolic activities of cells, corrects blotchy skin and rejuvenates facial expression lines. Face creams made with jenny milk allow a better skin hydration and moisturisation compared to conventional cosmetics (Cosentino et al 2013; Cosentino et al 2015). Reports also indicate that jenny milk prevents skin-aging process by hydrating and restructuring the dermal intercellular substance (Cosentino et al 2013).
Botswana possesses an estimated 310,000 heads of donkeys in 2013 and an increasing trend in donkey population has been observed in the country. In Botswana, donkeys are kept by individual farmers at households and mainly used for draught power such as ploughing, cart pulling to transport firewood, building materials, crop harvests and water (Thutwa and Nsoso 2017). Donkey may be used for joy ride at cattle posts and for milk production in Botswana. The milk is usually consumed in its raw state which could pose health risk to the consumers. Recently, a commercial donkey dairy farm has been established in south-western part of the country and this farm is now selling donkey’s milk to the general public and also make some cosmetics (soap and body lotion) from the milk which they sell as skin care products. There is a claim that donkey’s milk alleviates some disease conditions such as asthma, arthritis and heart conditions in Botswana (Thutwa and Nsoso 2017). This clearly indicates that donkey milk is getting popular in Botswana and has a promising prospect in the future. However, no research has been conducted to date on the composition and safety of donkey milk produced in the country. Therefore, this study was conducted to generate baseline information on the quality of donkey milk sold on the street and make recommendations for possible future improvements.
The major aim of this study was to determine the chemical composition and microbial quality of donkey’s milk sold by street vendors in Gaborone City.
A total of five raw donkey milk samples (250 ml each) were obtained from street vendors at Main Mall in Gaborone City. The samples were transported to the laboratory of Botswana University of Agriculture and Natural Resources in a cooler box immediately after collection. In the laboratory, the milk samples were kept in a refrigerator at 4°C and analyzed in less than 24 h. The samples were analyzed for chemical composition, microbial content and physical properties.
The pH of milk samples was measured using digital pH meter (Richardson 1985). Specific gravity of the milk samples was determined by a Lactometer according to O’Connor (1995). Total solids (TS) content was determined according to AOAC (2000). Fat content was determined by the Gerber method (Richardson 1985). Solids-not-fat (SNF) content was determined by difference as reported by Harding (1995). The total nitrogen (N) content was determined by the Kjeldahl method as described by the International Dairy Federation (IDF 1993). The crude protein content of milk was calculated by multiplying N by 6.38. The ash content was determined according to Bradley et al (1993). Lactose content was estimated by quantitative Benedict method (Harvey and Hill 1967).
Total viable counts were determined by pour plating of tenfold serial dilutions of milk samples in 0.1% peptone on standard plate count agar (Oxoid, Hampshire, England). Plates were prepared in triplicate and incubated at 32°C for 48 hours according to Roberts and Greenwood (2003).
Coliform bacteria were enumerated on Violet Red Bile Agar (VRBA) (Oxoid, Hampshire, England) as follows. Nine ml of sample was aseptically transferred into 90 ml of sterile peptone water and decimal dilutions of up to 10-6 were prepared. Triplicate dilutions from each dilution were plated by pipetting 1 ml of the dilution onto sterile petri dishes and adding molten agar (VRBA) and mixing the content. The agar was allowed to set and an overlay of a thin layer (about 5 ml) of sterile VRBA was added. Plates for enumeration of coliforms were incubated aerobically at 30°C for 48 hours. Pink colonies surrounded by bile precipitation were counted as coliforms (McLandsborough 2004).
Descriptive statistics was used to analyze the data generated using the SPSS software. Quantitative data are presented as mean with standard error.
Average values for the physicochemical parameters of donkey milk are presented in Table 1. The fat content of donkey’s milk observed in this study is in line with the findings of Guo et al (2007) and Medhammar et al (2012) who reported that fat content of donkey milk ranged from 0.3 to 1.8 g/100 g. However, it is lower than the value of 1.82% reported by Oftedal and Jenness (1988) for donkey’s milk fat. Fat is reported to be the most variable component of donkey’s milk ranging from 0.1 g/100 ml to 1.8 g/100 ml and it is affected by stage of lactation and foaling season (Salimei et al 2004). It appears that the fat content of donkey’s milk is much lower than that of cow’s milk (3.5–3.9%) and human milk (3.5–4.0%) (Guo et al 2007). The lower fat content in donkey’s milk implies that it can be used for the production of low-fat dairy products. The wide variation in fat content of donkey’s milk observed between the present study and values reported in the literature might be attributed to differences in stage of lactation, age of the animal, number of calvings, type of forage consumed and the hydration status of the animal. Reports indicate that donkey’s milk contains high concentrations of polyunsaturated fatty acids (PUFA) (ω6 and ω3) (52.2%) and low ω6 to ω3 ratio and used to prevent cardiovascular, autoimmune and inflammatory diseases (Chiofalo et al 2001).
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Table 1. Physiochemical properties of donkey milk sold in Gaborone |
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Variables |
Mean ± SEM |
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Fat (%) |
0.49 ± 0.02 |
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Lactose (%) |
5.70 ± 0.14 |
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Protein (%) |
2.03 ± 0.15 |
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Total solids (%) |
10.71 ± 0.64 |
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Solids-not-fat (%) |
10.22 ± 0.69 |
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Ash (%) |
0.42 ± 0.02 |
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pH |
7.16 ±0.02 |
|
Specific gravity |
1.03 ± 0.01 |
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SEM = standard error of the mean. |
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The average protein content of donkey’s milk observed in the present study (Table 1) is in agreement with the results reported for Italian donkey’s milk. Cosentino et al (2012) reported a protein content of 1.2 g/100 ml whereas Giosue et al (2008) reported an average protein content of 1.9 g/100 ml for Italian donkey milk. The protein content of donkey’s milk is affected by stage of lactation and foaling season. The protein content of donkey’s milk observed in the present study is higher than the values 1.72% and 1.89 % reported by Salimei et al (2004) and it is in line with that reported by Giosuč et al (2008) who indicated a protein content that ranged from 1.25 % to 2.18 %.
The average lactose content of donkey’s milk observed in the present study (Table 1) is lower than the value 6.85% reported for Ragusana ass’s milk (Chiofalo et al 2004). Giosue et al (2008) found lactose content of 6.2% in milk of donkeys that foaled in spring and 6.3% and 6.6% in milk of donkeys that foaled during summer and winter, respectively. Donkey milk has high amounts of lactose (5.8-7.4%) which is similar to human milk (Nayak et al 2017). The high lactose content is responsible for the good palatability of the milk and also facilitates the intestinal absorption of calcium that is essential for infant’s bone mineralization (Nayak et al 2017). The total solids content of donkey milk observed in the current study is in line with the value 9.81% reported by Fantuz et al (2010) for donkey’s milk. It is also in agreement with the findings of Sesh et al (2012) who reported that the total solids content of donkey’s milk ranged from 6.0 to 10.60%. On the other hand, Taha and Kielwein (1989) reported total solids content of 7.0 to 10.5% for donkey milk.
The specific gravity of the analysed donkey’s milk (Table 1) is in line with the density of donkey’s milk (1.032) reported by Colavita et al (2011). However, it is lower than the value 1.035 reported by Mariani et al (2001) for mare’s milk. The density of donkey’s milk ranges from 1.02-1.037 (g/cm3) (Salimei and Fantuz 2013). Tadesse et al (2014) reported a density of 1.026 g/ cm3 for Abyssinian donkey’s milk. The specific gravity of milk is used to determine whether the milk has been adulterated with addition of water or other substances.
The pH of donkey’s milk observed in the present study (Table 1) is in line with values reported in the literature. Mariani et al (2001) reported pH of donkey’s milk ranging from 7.14 to 7.22. Similarly, Guo et al (2007) reported a pH value of 7.21 for donkey’s milk, which is higher than the value for cow milk (6.7) but it is closer to human (7.3) and mare milk (7.18). However, Salimei and Chiofalo (2006) reported higher pH values ranging from 7.00 to 7.35 for donkey’s milk.
The ash content of donkey’s milk observed in the present study is consistent with earlier reports. Guo et al (2007) reported an ash content of 0.3 to 0.5% for donkey’s milk produced in northwest China. On the other hand, Sesh et al (2012) reported ash content of donkey’s milk that varied from 0.53 to 0.95% with an average value of 0.70% which is higher than the value observed in the current study. However, the ash content observed in the present study is in line with the findings of Oftedal and Jennes (1988) who reported 0.40% ash content for donkey’s milk. Fantuz et al (2012) reported an average ash content of 0.3 to 0.9 g/100 g for donkey milk which is similar to human milk but higher than cow milk. They also indicated that the ash content of donkey milk varies with breed and stage of lactation.
The average total viable count (TVC) of donkey’s milk observed in the present study (Table 2) is lower than the value 2.4×105 cfu/ml reported by Cavallarin et al (2015) for donkey’s milk produced in Italy. It is also lower than the findings of Salimei (2011) who reported total bacteria count in donkey milk that varied from 3.7 to 5.9 log10 cfu/ml. However, it is slightly higher than the findings of Ivanković et al (2009) who reported an average total bacterial count of 3.58 log 10 cfu/ml for donkey’s milk. The generally lower TVC of donkey’s milk observed in the present study might be attributed to presence of natural inhibitory substances in the milk as suggested by earlier researchers (Polidori et al 2009). Zhang et al (2008) reported that donkey milk has strong inhibitory potential on microorganisms. The low TVC observed could partly be attributed to the low storage temperature of the donkey milk. After milking, the vendors indicated that they freeze the donkey milk before delivering it to the market. Guo et al (2007) found much lower bacterial count (250 cfu/ml) in donkey’s milk kept at 5°C. Colavita et al (2011) and Colavita et al (2016) also reported lower bacterial count in donkey’s milk during storage at lower temperature (3°C). These researchers additionally reported that the potential antimicrobial activity and good shelf life of raw donkey milk might be attributed to its natural compounds acting in synergy with each other and to the hygienic milking procedures applied at farm level.
The coliform count observed in donkey’s milk in the present study (Table 2) is within the ranges of values reported by Conte et al (2006) who found that coliform count of donkey’s milk produced in Italy ranged from 2.14 to 2.85 log10 cfc/ml. Colavita et al (2010) reported that donkey milk was found to have total coliform counts that varied from <10 cfu/ml to 3.7×106 cfu/ml. The presence of coliforms in the donkey milk samples analyzed in the present study indicates the potential presence of pathogenic microorganisms of fecal origin in the milk which might have resulted from poor hygienic conditions during production and handling of the milk. Observation of the donkey farm where the milk is produced revealed that the donkeys are milked in the open air in a kraal which is full of dust and dung. Moreover, there is a critical shortage of water in the area that makes it difficult for the farm workers to routinely wash and clean milk containers, their hand and teats of the donkeys before milking. This predisposes the milk to contamination by coliforms.
The donkey milk sold in Gaborone is not pasteurized. Donkey milk producers claim that heat treatment of the milk will destroy the antimicrobial compounds and the medicinal value of the milk and therefore the milk should be consumed raw in order to retain the medicinal properties of the milk. However, the present study revealed presence of coliforms in donkey’s milk, which suggests the likelihood of presence of pathogenic microorganisms in the milk. Thus, consumption of raw donkey milk may pose public health risk. Therefore, there is a need for pasteurization of the milk and strict hygienic measures during production and handling of donkey milk. There has to be a quality control measure to ensure that the donkey milk supplied to the market in Gaborone is safe for human consumption.
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Table 2. Mean microbial counts (log10 cfc/ml) of donkey milk sold in Gaborone |
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Microbial count |
Mean ± SEM |
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Total viable |
3.72±0.58 |
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Coliform |
2.53±0.01 |
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SEM = standard error of the mean . |
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The donkey milk analyzed in the present study had lower fat and protein contents and higher lactose content. The low fat content in the donkey milk implies that it can be used for the production of low-fat dairy products. Presence of coliforms in the donkey milk indicates the potential presence of pathogenic microorganisms in the milk. Thus, there has to be a quality control measure to ensure that the donkey milk sold in the market is safe for human consumption. Hence, there is a need for concerted effort by all concerned bodies in this regard.
This study was funded by the Department of Tertiary Education of Botswana.
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Received 8 April 2018; Accepted 3 June 2018; Published 3 July 2018