4.1 Basic composition of ewes, goats, and Negga’s milk
In terms of milk composition, our results revealed a significant variation between all species. The results showed that sheep species were clearly superior to the population of goats and camels in all chemical contents being studied, with the exception of the casein-protein ratio, which favors goat species. The distinctions are more significant, particularly in terms of dry matter, fat, protein, casein, and ash content. Many more studies have confirmed that sheep milk has the greatest average value of the above constituents than goats (Park et al. 2007; Kondily et al. 2012.) and camel’s milk (Claeys et al. 2014; Ysamin et al. 2020). Likewise, the ancestry of goats relative to camel species in the above chemical constituents has been reported by several authors (Legesse et al. 2017; Yasmin et al. 2020). Nevertheless, many factors might significantly affect the major and minor composition of milk, such as individuals, parity, season, diet, management, environmental conditions, locality, lactation stage, and udder health status (Park et al. 2007), yet the special difference because of genetics always has pronounced contributions (Yasmin et al. 2020).
According to the literature, the milk composition of sheep, goats, and camels varies from medium to large-scale (Park et al. 2007; Rouissi et al. 2008; Chamekh et al.2020; Hilali et al. 2011; Kondily et al. 2012; Claeys et al. 2014; Monteiro et al. 2019). Several studies were conducted in Tunisia to study the chemical composition of milk from native and exotic breeds of sheep, goats, and camels. Milk from local QFO gave in this study total solids, fat, protein, casein, and ash content comparable to those reported by El Gharbi et al. (2015) for Barbarin sheep breed reared in a semiarid climate but was lower than findings of Aloulou et al. (2018) for Sicilo-Sarde ewes and those of Rouissi et al. (2008) for Sicilo-Sarde and Comisane dairy sheep breeds raised in an intensive system in the extreme north of the country. The variation from the reported results could be attributed mainly to the difference in the breed, management conditions, including feeding, and environmental factors, among others. Compared to the results from the exotic D’man breed maintained under an intensive oasis system in southeastern Tunisia (Dhaoui et al. 2019), our findings are higher for all chemical compounds, except for the fat content, which was lower.
Concerning lactose content, QFO milk showed higher levels than those reported for Sicilo-Sarde (Rouissi et al. 2008; Aloulou et al. 2018), Barbarin (El Gharbi et al. 2015), and Comisane (Rouissi et al. 2008). Similar lactose content in the D’man breed was found by Dhaoui et al. (2019) in Tunisia.
The relatively high concentrations of dry matter, fat, and protein, as well as other nutritious components, can assure the growth of a newborn kidlet. This result implies that abundant milk fat and total solids are relevant characteristics of Tunisian sheep milk. It is commonly accepted that milk from breeds with low potential milking yield from the Mediterranean and tropical areas is more concentrated in total solid, fat, and protein than the highly productive breeds from temperate regions (Hernández-Castellano et al. 2019).
The results of goat milk comply with those of Tunisian native goats (Ayeb et al. 2016), native Greek breeds (Capra Prisca) (Kondily et al. 2012), and Algerian Arabia (Hamidi et al. 2020). Higher content levels were noted in the Boer and Arsi-Bale Ethiopian goat breeds (Mestawet et al. 2012) and Murciana-Granadina, Boer, and La Mancha goat breeds (Ferro et al. 2017). The variation in the results from different literature sources could be related to the genetic potential of breeds, management conditions, environmental factors, feeding, and lactation stage at which samples were taken (Currò et al. 2019).
Milk from she-camel gave in this study total solid, fat, protein, casein, lactose, and ash comparable to that reported by Jemmali et al. (2016), Hamed et al. (2012) but was lower than findings by Ayadi et al. (2009) and higher than that founded by Chamekh et al. (2020) except for ash which is higher than in our study. The variation from the reported results could be attributed mainly to the difference in management conditions, including feeding and environmental factors. Out of Tunisia, current results approached those of the Egyptian Maghrebi camel (Abdalla et al. 2015) and native Turkish breed (Karaman et al. 2021). Lower content levels were noted in Ethiopian and Saudi camels (Al Haj and Al Kanhal 2010). The variation in the results from different literature sources could be related to the region, the genetic potential of breeds, management conditions, environmental factors, feeding, and lactation stage at which samples were taken (Chamekh et al.2020).
The casein-protein ratio with the largest value was found in goat milk (0.80), followed by sheep (0.77), and the smallest was found in camel milk (0.74). Similarly, the results for the casein-protein ratio are in line with other investigations in goats (Vacca et al. 2018) and sheep breeds (Rafik et al. 2016). Our finding is higher than that attained with the same Maghrebi dromedary population from the south and the center of Tunisia (Attia et al. 2000; Hamed et al. 2012) and similar to that in other breeds (Farah, 1993). The results inferred that the lowest concentration in whey protein was that of goat milk, and the highest concentration was that of camel milk (Hilali et al. 2011). The increase in whey protein has technological implications, such as a weaker texture of curd and lower cheese yield (Barlowska et al. 2020). Proteins are a determinant factor affecting the quality of dairy products, as the reduction in proteins and casein contents results in poor dairy technology-making properties (Hilali et al. 2011). In another way, a lower casein-to-whey-protein ratio (i.e., a higher proportion of whey proteins) has been shown to be more desirable for faster digestion of the milk proteins in infant formula than a casein-dominant protein composition (Roy et al. 2020), which is the case of camel milk, recently declared very similar to human milk in terms of qualitative whey protein profile (El-Hatmi et al. 2015) and as a most suitable substitute for cow milk when considering the preparation of infant formulas (Mudgil et al. 2022).
4.2 Physical characteristics
On the whole samples, the mean value of all physical characteristics (pH, density, and Dornic acidity) in milk collected from sheep breeds appeared higher than in milk collected from goat species. Compared to the camel populations, sheep species produced milk with a similar pH but a higher density and acidity content. Between camel and goat species, pH and acidity were higher in Negga, whereas the density was identical in both species.
Many earlier findings on the physical parameters of sheep, goats, and camel milk corroborate the results of the current investigation (Park et al. 2007; Ayeb et al. 2016; Al Haj and Al Kanhal, 2010) and differ from other works (Hilali et al. 2011; Kondily et al. 2012; Ismaili et al. 2019). The pH and acidity levels are indicators of the health status of animals and hygienic quality milk. However, milk from healthy animals should have pH values from 6.5 to 6.8 in small ruminants (Park et al. 2007) and from 6.4 to 6.7 in camels (Singh et al. 2017). A lower pH of fresh milk may be because of bacterial actions, and higher pH values indicate udder infection or mastitis (Carloni et al. 2016). The pH of milk is the most critical factor in producing various dairy products. It determines the conformation of proteins, the activity of enzymes, and the dissociation of acids present in milk (Rafik et al. 2016). High acidity indicates high numbers of microorganisms and consequent development of lactic flora, influenced by the combined effect of temperature and storage conditions (Ismaili et al. 2019). Along with pH, titratable acidity provides information about the total solids content of the milk and freshness, making it an important feature when defining quality.
The physical properties of milk are widely reported to be associated with its composition and animal species (Park et al., 2007; Hilali et al. 2011). The fat content associated with total solids in milk has a determining influence on its density, as has already been explained by other authors. (Park et al. 2007).
4.3 Mineral concentration
The variations in mineral concentrations among animal species are indicated in Table 3. The current results revealed considerable differences in the mineral concentrations of milk from different species. The levels of Ca and P are higher in sheep than in goat and camel milk. Compared to small ruminants, milk from the camel is the richest in Na and K. Additionally, more Ca is present in milk from camels than goats. Goat milk, the poorest type of milk in Ca and Na, contains on average more P than camel milk and more K than sheep's milk.
When comparing sheep breed versus goat milk for macroelements, the current study found lower levels of K and higher concentrations of Ca, P, and Na. Other research studies reported higher concentrations of K in sheep milk (AL-Wabel, 2008). The ascendance of sheep in Ca, P and Na and lower K levels compared to goat species was mentioned by Hilali et al. (2011). In the same way, Monteiro et al. (2019) declared that sheep milk is the type of milk that has the highest amount of calcium and phosphorus, while a high potassium content distinguished goat milk. Our results lie within the ranges reported in the literature for sheep (Hilali et al. 2011) and goat breeds (Monteiro et al. 2019).
The phosphorus content in camel milk from Tunisian oasis areas appeared at similar levels to those of the literature (Konuspayeva et al. 2010; Singh et al. 2017) and at lower concentrations than others (Faye et al. 2008). The results from the current study revealed that a high concentration of potassium was detected. These results align with those of several authors (Singh et al. 2017).
Camel milk is a rich source of minerals, especially Ca and K (Benmeziane-Derradji, 2021), because of the forage eaten by camels such as Atriplex and Acacia, which usually have a high salt content and are possibly the reason for the salty taste of milk (Singh et al. 2017). According to Benmeziane-Derradji (2021), camel species have greatly higher concentrations of Na and K than small ruminants, which agrees with the results of our study. Nevertheless, variations in mineral content are closely dependent upon animal species (Clayes et al. 2014), breed differences (Al Haj and Al Kanhal 2010), individual animals, stage of lactation, udder health status (Stocco et al. 2019), production system (Singh et al. 2017), analytical procedures (Attia et al. 2000), water intake (Singh et al. 2017), and nutritional status and diet (Pietrzak-Fiećko and Kamelska-Sadowska 2020).
Low Na concentrations were obtained for all species, which is similar to previous works in camel (Benmeziane-Derradji, 2021), goats (Currò et al. 2019), and sheep breeds (Khan et al. 2006) but in contrast with other studies, which reported higher levels in camel (Singh et al. 2017), goats (Stergiadis et al. 2019), and sheep species (Monteiro et al. 2019). This variation could be attributed to the lactation stage; in fact, the Na concentration in sheep and goat milk is higher at early lactation than during middle and late lactation (Khan et al. 2006).
4.4 Microbiological features
The microbial quality of goat milk was higher than that of ewes and Negga’s milk based on TMAB, TCC, and E. coli counts. Ovine milk is of better quality, referring to LAB, Y/M, and S. aureus values. The poor bacteriological quality was that of camel milk for all microbial counts.
S. aureus and E. coli were prevalent in all milk types with different levels of contamination. Two primary sources cause Staphylococcus in milk: the first is the lack of adequate hygiene measures and inappropriate handling during milking (Fatima et al. 2013), whereas the second is mastitis, which affects animals (Benmeziane-Derradji, 2021). In the current study, the selected animals were healthy and milked respecting hygienic practices; thus, Staphylococcus prevalence in the milk samples may be linked to subclinical mastitis occurrence (Alebie et al. 2021).
No significant differences in the numbers of total coliform populations (TCCs) were noted between the milk of the studied species, except Maghrebi animals, which generate milk with the highest count. Higher TMAB, TCC, LAB, Y/M, S. aureus, and E. coli counts in ewe milk were obtained by Fatima et al. (2013). In goat milk, Tabet et al. (2016) detected higher counts, and Kalhotka et al. (2015) reported higher bacterial results in both species. Lower total bacterial counts have been observed in goats (Abd El Aal and Awad 2008) and sheep's milk (Tonamo et al. 2020).
As stated in the results, camel milk exhibited a high rate of FAMT. These results are nearly similar to those reported by Adugna et al. (2013) and Wasie et al. (2015) and higher than those cited by Karaman et al. (2021) and Abera et al. (2016). Extremely high burdens of FMAT exceeding 8 log10 CFU/mL were found by Ismaili et al. (2019). Concerning total coliform counts, our findings were closer to those advanced in the literature by Wasie et al. (2015) and lower than the values of Benkerroun et al. (2003), Benyagoub and Ayat (2015), and Ismaili et al. (2019).
The average count of LAB was 3.77 ± 0.65 log10 CFU/mL at a low level. The count number was lower than those reported by Benkerroum et al. (2003) and Ismaili et al. (2019). The high levels of lysozyme and ascorbic acid in camel milk may explain the low level of LAB, as mentioned previously by other researchers (Karaman et al. 2021). The yeast and mold count of the camel’s milk samples in this study was 4.22 ± 1.13 log10 CFU/mL. The average value is less than the values found in camel’s milk samples in Sudan (Karaman et al. 2021) and Morocco (Ismaili et al., 2019). The lower yeast and mold counts could be because the natural milk pH favors bacterial growth and lowers yeast and mold content, as detected in the samples of this study (Karaman et al. 2021). In studies achieved in Tunisia on the same camel breed and focused on the enumeration of the mesophilic count, total LAB, and coliforms, lower levels were cited by Fguiri et al. (2018).
High total bacterial counts in raw milk mainly reflect the poor hygienic condition under which the milk was handled, storage temperature and time elapsed since milking, and the poor health of milking animals (Adugna et al. 2013). With the current study, the main source of contamination could be attributed to the contamination of the camel udder by the hands of unhygienic milkers or unhygienic milking procedures. Microorganisms can be transferred from the environment, i.e., feces, bedding, and soil, from contaminated hands, clothing, and mouth of milk handling personnel (Alebie et al. 2021).
The conducted study showed a complete absence of the two dangerous pathogens Salmonella and sulphite-reducing Clostridium in all examined samples of milk, suggesting that both pathogens are uncommon in small ruminant and camel’s milk in the herds sampled. Several studies advanced a similar finding for goat milk (Tabet et al. 2016), sheep milk (Fatima et al. 2013), and camel milk (Benyagoub and Ayat 2015). Various studies have shown that several factors can affect the bacteriological quality of milk, including animal breed (Tonamo et al. 2020), milking practice, stage of lactation (Nagy et al. 2013; Fguiri et al. 2018), farm characteristics and practices (Abera et al. 2016), years and season (Kondily et al. 2012; Ismaili et al. 2019), housing conditions and feeding practices (Fguiri et al. 2018), animal health, flock size and hygiene of premises and milk tank parlors (Carloni et al. 2016).
The results from the current study showed that the levels of microbial contamination of raw small ruminant milk in the oasis regions of Tunisia were satisfactory. Microbiological analysis meets the standard criteria required by Tunisian legislation on the hygiene of milk and dairy products (NT 14.141 (2004)). In contrast, the results showed that the levels of microbial contamination of raw camel milk in the study area were unsatisfactory and could not comply with the standard requirements of Tunisian legislation. Camel milk is commonly produced, conserved, and transported under unhygienic conditions. The bacteriological quality of raw milk should therefore be a major concern for farmers, processors, and the general public because bacteria in milk can degrade milk components, decrease shelf life, and cause illnesses in human beings (Adugna et al. 2013).