Camels are the most climate resilient dairy species, which plays a great role in livelihood improvement of the pastoral and agro-pastoral communities’ (Faye et al., 2010; Yirda et al., 2020). The world’s camel population is approximately 35 million with Camelus dromedarius (one-humped) representing around 95% and Bactrian camels (Camelus bactrianus) constituting the rest (Mbye et al., 2022). Majority (86%) of the dromedary camels are found in Africa and Somalia, Sudan, Ethiopia and Kenya account the largest share (Eyassu, 2022). According to Central Statistical Agency (2021), Ethiopian has 8.1 million camels and 70.54% were females. Concerning the purpose of camel production 36.68% and 17.95% was mainly kept for milk and transportation respectively.
Camel milk is incredibly nutrient-dense food consumed in arid and semi-arid areas (Kaskous, 2019; Tesfemariam et al., 2017). It has numerous unique characteristics such as functional proteins and a predominance of medium-chain fatty acids (Alhaj et al., 2022; Konuspayeva and Faye, 2021). It has smaller fat globule size and higher bioactive property as compared to bovine milk (Dugassa, 2021; Mbye et al., 2022). Due to these special qualities, camel milk is becoming one of the most promising industrial products in the future (Abdullahi, 2019; Sara et al., 2022).
At world level camel milk production report it shows a considerable annual growth, exceeding 8% in the period 2009-2019 (Konuspayeva and Faye, 2021). Despite of this so far camel milk processing has not received much attention as compared to others (Alhaj et al., 2022; Bakry et al., 2021). Moreover, the scientific evidence shows that the detail information about camel milk chemistry, its physicochemical property and processing characteristics are scarce (Dugassa, 2021). Milk composition, particularly protein and fat contents and composition, will also significantly affect cheese yield and composition. Understanding the specific characteristics of milk like i.e its composition, physical properties before processing will helps to decide the correct method of product processing technology. On the other hand so far dairy processing centers coping similar technologies used as cow milk without considering the physicochemical property of milk, it can have a significant effect on product quality (Konuspayeva and Faye, 2021). Therefore the objective of this review is to highlight the scientific information regarding to physicochemical property and processing characteristics of camel milk as compared to other dairy species.
Physicochemical properties of camel milk as compared to others
Camel milk is frothy and opaque-white in color due to the fine distribution of fat globules throughout the milk (Abbas et al., 2013; Arab et al., 2020; Oselu et al., 2022). Its test changes with stage of lactation; it is sweet in the early lactation but salty in the latter stage of lactation (Dugassa, 2021). Also the type of feed and accessibility to drinking water affect how it tastes (Patel et al., 2016). It is less viscous than bovine milk (Al haj and Al Kanhal, 2010), it has a pH of 6.2-6.7 (Khaliq et al., 2019; Lunda et al., 2013). The specific gravity at 20°C is 1.020°C to 1.022°C (Dugassa, 2021). Additionally, it has a freezing point of -0.576°C to 0.61°C, however for cows; it varies from -0.51 to 0.55°C (Vincenzetti et al., 2022).
Characterizing the milk composition of dairy animal is absolutely necessary, because the nutritional composition of all dairy animal species vary considerably (Khaliq et al., 2019). Specifically researchers have been noted that camel milk composition can vary depending on the feeding habitat; season of the year and genetic make-up (Alhaj et al., 2022; Zeleke, 2007). Despite its proximity with cow milk in term of gross composition, camel milk shows much specificity (Konuspayeva and Faye, 2021). Concerning the milk protein composition, camel milk has a lower protein level than that of cows, ranging from 2.29 to 4.9 percent (Alhaj et al., 2022).
Casein (CN) is the major protein which constitutes about 50-85% of the total proteins (Khaskheli et al., 2005). Camel milk casein has two forms “αS1- casein” and “αS2 -casein” (El-Agamy, 2006). There are four type of casein in camel milk, i.e. αs1- CN, αs2- CN, α- casein (α-CN) and κ-casein (k-CN) (Abbas et al., 2013; Al-haj and AlKanhal, 2010). The estimated molecular mass of camel β-CN and α-CN are 32 and 35 kDa respectively, which are considerably higher than those reported for bovine β-CN 24 and α-CN 22-27 kDa (Saliha et al., 2013). The majority of camel caseins are β-CN (65%) followed by αs1-CN (22%) from total casein, while, bovine caseins contains has high percentage of β-CN (39%) followed by αs1-CN (38%) from the total caseins (Brezovecki et al., 2015; Yirda et al., 2020). Camel milk has lower concentrations of κ-CN which account 3.3% as compared to bovine milk which is 13% (Mbye et al., 2022). The detail information about the relative distribution of camel milk casein and bovine milk is shown in Table 1.
Table 2 shows the protein profile (whey protein) distribution of camel and bovine milk. Amino acid profile of camel milk is similar to that of bovine milk; only few differences in the primary structure of casein were observed as compared to bovine caseins (
EI-Elagamy, 2009). The whey protein content in camel’s milk varies between 20% and 25%, which is slightly more than in cow’s milk (
Dugassa, 2021;
Vincenzetti et al., 2022).
According to
Rafiq et al., (2016) camel milk has a higher amount of whey proteins (0.80%) than buffalo (0.68%), sheep (0.66%), goat (0.53%) and bovine milk (0.47%). This is primarily due to the higher content of albumin and lactoferrin. Camel milk whey protein lacks β-lactoglobulin (β-lg) but it contains larger amount of α-lactalbumin (α-La) (27%) and Serum albumin (SA) (26%) as compared with bovine α-la (20.1%) and SA (6.2%). β-lg is the main protein in bovine whey protein which accounts 53.6% of total whey proteins
(Mbye et al., 2022). Whey protein includes: SA, α-La, lactoferrin (LF), Immunoglobulins and peptidoglycan recognition protein (
Hinz et al., 2012). Lactoferrin content of camel milk is significantly higher than sheep, goat, cow and buffalo milk
(Abbas et al., 2013).
Table 3 shows the number of amino acid residue, its molecular weight and isoelectric point of cow and camel milk casein. The number of amino acid residues of αs1-CN of camel milk is 217 while its concentration in cow milk is 199. The κ-CN contains higher amount of arginine and lysine than cow milk (
Salmen et al., 2012). It has also 162 amino acids, 22.4 molecular weight (kDa) and its isoelectric point (IP) is 4.11, while its concentration in cow milk is 19.038 in molecular weight (kDa) with isoelectric point of 5.97
(Khaliq et al., 2019). The number of amino acids residues of camel milk lactoferrin is slightly lower from bovine (
EI-Elagamy, 2009). Moreover the concentration of antioxidant amino acids residues (cysteine, tryptophan, and methionine) is higher than others (
Salami et al., 2009).
The lactose content of camel milk rages 3.12-4.5%
(Alhaj et al., 2022; Dugassa, 2021;
Konuspayeva et al., 2009). Buffalo milk also contains lower lactose compared to camel milk as well as cow milk
(Jaydeep et al., 2015). The variation in the concentration of lactose in camel milk is associated with water intake and type of feed consumed (
Al-haj and AlKanhal, 2010;
Kula and Dechasa, 2016). In cases of dehydration of the camel the lactose content will decreases, thus the taste of milk become less sweet
(Al-Juboori et al., 2013). According to
Bekele et al., (2011) study milk lactose concentration was higher 4.3±0.2% for camel deprived of water for 4 days as compared with 4.1±0.2% for 16 day.
Fat is the other important component of camel milk, including a complex mixture of natural fats (
i.e., triglycerides, phospholipids, cholesterol, and other elements), representing one of the main sources of energy
(Bakry et al., 2021). In dromedary camels, milk fat varies from 1.2% to 4.5% depending on the nutritional status, lactation stage, breed type and the season of the year. The fat contents decrease from 4.3 to 1.1% in thirsty camel
(Konuspayeva et al., 2009). Triacylglycerol accounted for 96% of the total lipids in camel milk (
Dugassa, 2021;
Nikkhah, 2011). Milk fat has a wide range of physical characteristics since it includes over 400 different fatty acids (FAs). Nevertheless, it is mostly composed of 16 major FAs that are responsible for its physical properties,
i.e., melting and solidification temperatures, solid-phase content, firmness/hardness, and spread ability of the resulting butter
(Bakry et al., 2021).
Fatty acids are divided based on the linkage of the carbon atoms into saturated and unsaturated fatty acids. In saturated fatty acids, the carbon atoms are linked in a chain by single bonds, in unsaturated fatty acids by one or more double bonds. Camel milk fat contains lower amounts of short chain fatty acids and a higher proportion of long chain fatty acids (C
14-C
18) as compared to buffalo and bovine milk
(Abbas et al., 2013; Sara et al., 2022) as shown in Table 4 below. Its fat globule membrane is thicker and smaller in size than cow milk
(Sunita et al., 2014). The milk fat globules size (MFGs) range from 1.1 to 2.1 µm, which is lower than those of buffalo (3.9-7.7 µm), cow (1.6-4.9 µm), and goat milk (1.1-3.9 µm)
(Bakry et al., 2021; Richard, 2017). Unsaturated FA in camel milk is 65.02 g/100 g FA which is highest proportion followed by the cow milk (40.76 g/100 g), goat milk (40.23 g/100 g) and 58.17 g/100 g for human milk
(Wang et al., 2011; Oselu et al., 2022). The amount of saturated fatty acids found in camel milk is lower (67.7%) than cow milk (69.9 %) (Konuspayeva et
al., 2008); it exhibit 50-65% of saturated and 35-50% unsaturated fatty acids. Dominant fatty acids found in camel milk are Palmitic and oleic acid
(Attila et al., 2000).
Reviewed literature shown that water content of camel milk ranges 88.7-89.4%, while cow milk ranges 87.7-89.2%
(Dahlborn et al., 1997; Haddadin et al., 2008; Sulieman et al., 2014). Feed availability and water consumption frequency were found to be the main factors affecting the water content of camel milk (Brezovecki
et al., 2015). According to research finding, when the camel has free access to water the water content of the milk is decrease 86%, while when water is scarce; the water content of milk has increased up to 91%
(Yadav et al., 2015). Similarly
Kanca (2017) reported that camels produce diluted milk when water is scarce.
On the other hand the other research finding indicates that the milk osmolality increased in parallel with plasma osmolality during dehydration. This finding is not consistent with previous study result which indicates milk becoming diluted during dehydration. According to
Bekele et al., (2011) study do so far to investigate the effect of water provision or deprivation for (1, 4, 8, 12 and 16 week) on camel milk composition. Accordingly milk volume was decreased when the number of deprivation day increases but milk osmolality was increased from 315±3 on 1
st day to 333±3 mosm/kg in 4
th day of 4
th week. Moreover during 16 week it increased from 318±3 to 336 ±3 mosm/kg during the first 4 day of water deprivation. The result of this study indicates that, camels did not dilute their milk when they are dehydrated. Instead milk osmolality will increase in parallel to blood osmolality.
Table 5 shows the mineral content of different dairy species. The total ash content of camel milk range from 0.6-0.9%
(Konuspayeva et al., 2009), which is relatively higher than buffalo and cow (
Dugassa, 2021; Yoganandi
et al., 2014). It is rich in iron, zinc and copper content than cow milk
(Oselu et al., 2022; Raghvendar et al., 2017). Calcium and Phosphorus content as well as its proportion found in the milk are the most important determining factor that affects the final dairy product quality. The proportion of Calcium to Phosphorus ratio in camel milk is 1:5 as compared to 2:1 and 1:29 for human and cow milk, respectively. The fat soluble vitamin (A, D and E) and water soluble vitamins (C and B) are found in higher quantities in camel milk
(Haddadin et al., 2008; Konuspayeva et al., 2011; Stahl et al., 2006). The content of vitamin C is of specific interest as its levels are three times higher than cow milk and one-and-a-half that of human milk
(Oselu et al., 2022; Stahl et al., 2006).
Processing character of camel milk
The physical characteristics of the milk have its own effect on the final product quality during processing. Camel milk has a high alcohol stability number which is 32.3 versus 20.8, 10.3, 12.5, and 14.1 for cow, buffalo, goat and sheep milk respectively (
Eyassu, 2022). Camel milk has a higher melting point than cow milk due to high content of unsaturated long chain FA and very high levels of higher molecular weight of triacylglycerols (TAGs) C48-C52 (
Eyassu, 2022). The shelf life of camel milk is higher than others milk due to its higher concentration of lactoferrin which have anti-microbial qualities
(Dukwal et al., 2007; Khaliq et al., 2019; Sara et al., 2022).
Pasteurization, fermentation, cream separation, butter and cheese making were the processing techniques applied (
Konuspayeva and Faye, 2021). The processing techniques applied for camel milk was copied from technologies used for cow milk (
Konuspayeva and Faye, 2021). Manufacturing milk by using the same technology as other dairy species milk can lead to processing difficulties or result to poor quality products. Pasteurization of milk is a commonly applied technique in dairy product processing industry. However the pasteurization procedure and indicators method used for camel milk vary from cow milk (
Konuspayeva and Faye, 2021). The presence or absence of Gamma glutamyl transferase (GGT) is an indicator used to assess the effectiveness of pasteurization in camel milk, while lactoperoxidase is used for cow milk.
In cheese making, milk coagulation properties are considered as a good indicator of processing effectiveness as it is correlates with cheese yield
(Mbye et al., 2020). Camel milk cheese is characterized by a weak crusting and a continuous loss of moisture, due to serum release, leading often to a very dry curd which hinders correct ripening. Furthermore, milk coagulation process done by using calf chymosin in cheese making from camel milk was not effective, the site for hydrolysis of k-casein is different in camel milk it is Phe
97-Ile
98 as compared with bovine (Phe
105-Met
106) (
Hailu et al., 2016;
Kappeler et al., 1998). The other difficulty in camel milk product processing features is associated with the low amount of k-casein content and the large casein micelle structure
(Vincenzetti et al., 2022). An average diameter of camel casein is 380 nm as compared to bovine (150 nm), caprine (260 nm) and ovine (180 nm) milk (
Dugassa, 2021;
Khaliq et al., 2019; Oselu et al., 2022). The scholars reported that when the surface area of casein micelles is enhanced it will result in strengthening of gel and firming the texture of curd as compared to poor aggregated gel in case of large size micelle. Additionally casein-to-whey protein ratio affects the texture and protein network formed during the cheese-making (
Roy, 2021). In contrary to cow milk processed cheese where ripening is started after curdling with the rapid appearance of crust formation.
The process of fermentation, including camel milk, is a traditional ancestral method used all over the world (
Konuspayeva and Faye, 2021). It is the transformation of lactose into lactic acid by the action of natural microflora or in some cases produced by yeasts. Fermented camel milk has higher lactic acid bacteria, which have been shown to be effective against pathogens including
Bacillus, Staphylococcus salmonella and
Escherichia (Marwa
et al., 2013). Mesophilic, thermophilic, or their mixture types of starters are used for fermentation of camel milk; it’s led to an acidification rate at 37
oC between 33% and 79% which is lower than for cow milk (
Konuspayeva and Faye, 2021). The proteolysis rate in fermented camel milk has been reported to be greater compared with cow’s milk (
Marwa et al., 2013). Fermented camel milk products have different name at different areas of the country,
i.e. shubat in Kazakhstan and China,
khoormog in Mongolia,
Garris in Sudan,
Suusac in Kenya,
Ititu and
dhanaan in Ethiopia
(Berhe et al., 2019). The difference was mainly based the processing method and the starter culture type used during processing.
Butter processing is heavily depends on the size of fat globule and its outside component or the fat globule interface. Camel milk fat contains less than 0.5% butyric acid as compared to cow milk 5% (
Konuspayeva and Faye, 2021). It has characterized by a small fat globule size and a thicker fat globular membrane (
Eyassu, 2022). These properties make it difficult to produce butter. Moreover the melting range of camel milk butter ranges from 41-42°C and is on average 8°C higher than the corresponding value for cow milk butter. Moreover it has a lower content of carotene as compared to bovine milk. Hence the color of processed camel’s milk butter becomes white
(Swelum et al., 2021).