4.1 Milk chemical composition
A moderate to large variability in the composition of sheep, goats, and camel milk has been found in the literature (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). Despite the fact that many factors, such as individuals, parity, season, diet, management, environmental factors, locality, lactation stage, and udder health status, may have a significant impact on the major and minor components of milk (Park et al. 2007), the special variation engendered by genetics often significantly contributes (Yasmin et al. 2020). Our results showed significant heterogeneity in milk content across all species. With the exception of the casein-protein ratio, which favors goat species, the analysis indicates that sheep species were clearly superior to the populations of goats and camels in all chemical contents investigated, particularly at the levels of dry matter, fat, protein, casein, and ash contents. According to numerous researches, sheep’s milk has the highest average value of the above ingredients than goats (Park et al. 2007; Kondily et al. 2012) and camel's milk (Claeys et al. 2014; Ysamin et al. 2020). Similarly, various authors have reported on the ancestry of goats in relation to camel species in the above chemical contents (Legesse et al. 2017; Yasmin et al. 2020).
Total solids, fat, protein, casein, and ash content in milk from native Gharbi were similar to those described by El Gharbi et al. (2015) for Tunisian Barbarin sheep breeds raised in a semiarid climate, but lower than those observed for Sicilo-Sarde and Comisane dairy sheep breeds (Rouissi et al. 2008; Aloulou et al. 2018) reared under an intensive system in the north of Tunisia. Our results are higher for all chemical constituents except fat content, which was lower when compared to the results of the oases D'man breed kept in an intensive system in south-eastern Tunisia (Dhaoui et al. 2019). The variation in the reported findings could be linked mainly to differences in the genetic potential of the breeds and management factors, including feeding and environmental factors.
The major carbohydrate lactose of ovine milk in this study was higher than most of the reported results for Tunisian Barabrin, Sicilo-Sarde, and Comisane sheep breeds (Rouissi et al. 2008; El Gharbi et al. 2015; Aloulou et al. 2018). However, lactose content was in line with the finding of Dhaoui et al. (2019) for the D’man breed.
The current result implies that the relatively high concentrations of dry matter, fat, protein, and other nutritive components are relevant characteristics of Tunisian sheep's milk. It is generally accepted that milk from breeds with low potential milking yields from the Mediterranean and tropical areas has a higher concentration of total solids, fat, and protein than the high-productive breeds from temperate regions (Hernndez-Castellano et al. 2019).
Goat milk results are consistent with local Tunisian goats (Ayeb et al. 2016), native Greek breeds (Kondily et al. 2012), and Algeria-Arabia (Hamidi et al. 2020). Higher levels were found in the Ethiopian goat breeds Buren and Arsi-Bale (Mestawet et al. 2012) and in the goat breeds Murciana-Granadina, Buren, and La Mancha (Ferro et al. 2017). The genotype potential, production system, environmental factors, and the stage of lactation at which samples were gathered could all explain the variation of current results from diverse literature sources (Currò et al. 2019).
The chemical components of camel milk studied were lower than those of Ayadi et al. (2009); however, they were nearly identical to those cited by Hamed et al. (2012) and Jemmali et al. (2016) and higher than Chamekh et al. (2020) with the exception of ash, which is higher than that in our study. The difference in results might be mostly ascribed to the variations in the feeding system and environmental conditions. Current Tunisian results were in accordance with those of the Egyptian Maghrebi camel (Abdalla et al. 2015) and the native Turkish breed (Karaman et al. 2021). Al Haj and Al Kanhal (2010) found that Ethiopian and Saudi camels had lower levels of content. The discrepancies in results from different existing literature could be attributed to the breeds, region, lactation stage at which samples were collected, and management conditions, including environmental factors and diet (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). The current results approached those cited in other camel breeds (Farah, 1993), whereas the same Maghrebi breed in a different region (Tunisia's south and center) provided lower casein to protein ratio (Attia et al. 2000; Hamed et al. 2012). The casein assay implied that goat milk had the lowest whey protein concentration and camel milk had the highest (Hilali et al. 2011). Increases in whey protein have technological impacts, including weaker curd texture and lower cheese yield (Barlowska et al. 2020). Protein content and casein content are critical elements in dairy product quality, as lower protein and casein content leads to reduced technical manufacturing properties of dairy products (Hilali et al. 2011). Otherwise, a reduced casein-to-whey-protein ratio (i.e., a higher proportion of whey proteins) has been proven to be better beneficial for rapid digestion process of milk proteins in infant formula than a casein-dominant protein composition (Roy et al. 2020), which is the fact of camel milk, which has just been recognized to have a whey protein composition that is extremely comparable to human milk (El-Hatmi et al. 2015) and it is indeed a perfect substitute for cattle milk in infant formulas preparation (Mudgil et al. 2022).
4.2 Physical parameters
Milk from sheep breeds had a higher average of all physical parameters (pH, density, and dornic acidity) than milk from goat species. Sheep population produced milk with a similar pH to camel population, but with a higher density and acidity content. The pH and acidity were higher in neggas than in goat species, while density was equivalent in both.
Several previous reports on the physical characteristics of sheep, goat, and camel milk were consistent with the findings of the present study (Park et al. 2007; Ayeb et al. 2016; Al Haj and Al Kanhal, 2010) and were distinct from other studies (Hilali et al. 2011; Kondily et al. 2012; Ismaili et al. 2019). Both pH and acidity levels of milk are indicative of animal health and milk hygienic quality. However, the pH value of milk in healthy animals should be 6.5 - 6.8 (Park et al. 2007) for small ruminants and 6.4 - 6.7 for camels (Singh et al. 2017). Bacterial activity may generate a lower pH in fresh milk, while higher pH readings suggest udder infection or mastitis (Carloni et al. 2016). The pH of milk is one of the most critical factors in the production of a variety of dairy products. It regulates the conformation of proteins, the activity of enzymes, and the dissociation of acids found in milk (Rafik et al. 2016).
High acidity suggests a high bacterial load and, as a result, the expansion of lactic microbiota, which is directly, impacted by the joint effect of temperature and storage conditions (Ismaili et al. 2019). Titratable acidity, like pH, provides information on the total dryness and freshness of the milk, making it an important factor when establishing the quality.
The composition and animal species of milk are commonly known to influence its physical qualities (Park et al., 2007; Hilali et al. 2011). As previously stated by other authors, the fat content associated with total solids in milk has a significant impact on its density (Park et al. 2007). Parmar et al. (2020) stated that the composition of milk influences physical properties such as density and, as a result, the basis for weight–volume calculations in the milk industry. Milk density changes are inextricably linked to its fat-free solids and fat content. The milk's high fat content corresponds to a lower density, and inversely. Variations in density can be related to a number of factors, involving lactation stage, climatic conditions, feeding behavior during the study period, housing conditions, genetic group, and analytical method, among others (Parmer et al. 2020).
4.3 Mineral content
The current results revealed significant differences in the mineral levels of milk from the three species. The concentrations 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 in 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.
Our results are consistent with those previously published for ovine (Hilali et al. 2011) and caprine (Monteiro et al. 2019) species. The present study revealed that sheep's milk had higher concentrations of Ca, P, and Na and lower K levels than goat's milk. K levels in sheep milk were shown to be greater in other experiments (AL-Wabel, 2008). Hilali et al. (2011) pointed out that sheep have higher Ca, P, and Na levels and lower K levels than goats. Similarly, Monteiro et al. (2019) stated that sheep's milk contains the most calcium and phosphorus, whereas goat milk contains the greatest potassium concentration.
The phosphorus concentration of Maghrebi camel milk raised in Tunisian oasis was found to be lower than that found by Faye et al. (2008) but comparable to that cited by Konuspayeva et al. (2010) and Singh et al. (2017). Referring to the potassium result, a high concentration was observed, which is in agreement with other findings (Singh et al. 2017).
The salty flavor Camel milk contains a high concentration of minerals, particularly Ca and K (Benmeziane-Derradji, 2021), as a result of the salt-rich pastoral plants consumed. 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. Nonetheless, mineral content varies greatly depending on 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 camels (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 camels (Singh et al. 2017), goats (Stergiadis et al. 2019), and sheep species (Monteiro et al. 2019). This fluctuation could be explained by the lactation stage. In fact, the Na level in small ruminant milk is higher in the early stages of lactation than in the mid and late stages (Khan et al. 2006).
4.4 Bacteriological quality
According to TMAB, TCC, and E. coli counts, goat milk had a higher microbiological quality than sheep’s and Negga's milk. In terms of LAB, Y/M, and S. aureus values, ovine milk is superior. Across all microbiological loads, the poor bacteriological quality was that of camel milk.
S. aureus and E. coli were common and found at various levels of contamination in all milk types. Staphylococcus in milk is generated by two main sources: the first is a lack of adequate hygienic practices and improper milking procedures (Fatima et al. 2013), and the second is mastitis, which affects animals (Benmeziane-Derradji, 2021). Because the animals in this study were healthy and milked in a sanitary manner, the incidence of Staphylococcus in the milk samples could be associated with the development of asymptomatic mastitis (Alebie et al. 2021).
Except for Maghrebi animals, which produce milk with the greatest count, there were no statistically significant differences in TCC between the milk of the investigated species. Fatima et al. (2013) stated higher TMAB, TCC, LAB, Y/M, S. aureus, and E. coli levels in sheep milk. Tabet et al. (2016) observed increased loads in goats milk, while Kalhotka et al. (2015) found greater contamination rates for both species. Smaller overall microbial numbers were observed among goats (Abd El Aal and Awad 2008) and ewe's milk (Tonamo et al. 2020).
Camel milk, as stated in the results, contained a high level of FAMT. These findings are identical to those mentioned by Adugna et al. (2013) and they are higher than the results of Abera et al. (2016) and Karaman et al. (2021). Ismaili et al. (2019) discovered FMAT loads exceeding 8 log10 CFU/mL. In terms of TCC, our results approached those cited by Wasie et al. (2015) but lesser than those of Benkerroun et al. (2003), Benyagoub and Ayat (2015), and Ismaili et al. (2019).
At a low level, the mean of the lactic bacteria number was 3.77 ± 0.65 log10 CFU/mL, which is smaller than what Benkerroum et al. (2003) and Ismaili et al. (2019) reported. The elevated concentrations of lysozyme and ascorbic acid in camel milk, as stated earlier by other scientists, could explain the low level of lactic acid bacteria (Karaman et al. 2021). The Y/M count in Negga's milk found in the current study was 4.22 ±1.13 log10 CFU/mL. The mean level is lower than that of Sudanese (Karaman et al. 2021) and Moroccan (Ismaili et al., 2019) camels. The reduced yeast and mold counts could be attributed to the natural milk pH, which promotes bacterial growth and reduces Y/M, as detected in the study's samples (Karaman et al. 2021). Fguiri et al. (2018) reported lower levels in studies conducted in Tunisia on the same camel breed and focused on counting the mesophilic bacteria, LAB, and coliforms.
High total bacterial counts in raw milk are primarily due to the unsanitary conditions, under which the milk was managed, as well as the storage temperature and time since milking, and also the health problems 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. Microbes could be transmitted from the environment, such as excrement, bedding, and ground, by contaminated milk handling personnel's hands, clothing, and mouth (Alebie et al. 2021).
This study showed that all milk samples studied were completely free of two dangerous pathogens: Salmonella and sulfite-reducing Clostridium, which indicates that the two pathogens are rare in the milk of small ruminants and camels in sampled flocks. A comparable conclusion was attained for goat milk (Tabet et al. 2016), sheep milk (Fatima et al. 2013), and camel milk (Benyagoub and Ayat 2015).
A variety of factors, comprising breed (Tonamo et al. 2020), milking practice, lactation stage (Nagy et al. 2013; Fguiri et al. 2018), farm management (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, cleanliness of the premises and milk handling, and storage equipment (Carloni et al. 2016) may influence and all contribute to the bacteriological quality of milk.
According to the findings of the current investigation, the levels of microbiological contamination of raw milk of goats and sheep in the studied area were acceptable. Microbiological analysis meets the requirements of Tunisian legislation on milk and dairy product quality (NT 14.141 (2004)). In contrast, the results revealed that the levels of bacterial loads of raw camel milk in the study area were inadequate and did not meet Tunisian legislation's standard specifications. The camel milk is commonly produced, conserved, and transported under unhygienic conditions. Because bacteria in milk can degrade milk components, reduce shelf life, and cause illnesses in humans, the bacteriological quality of raw milk should therefore be a key concern for farmers, processors, and the general public (Adugna et al. 2013).