Milk normally contains some level of somatic cells: neutrophils (PMN cells), lymphocytes (LYM) and macrophages. Macrophages (MAC) comprise the major cell type in milk from healthy udders (Dosogne et al. 2003; Lindmark-Månsson et al. 2006). When there is bacterial infection, tissue damage, or other inflammation processes affecting the mammary tissue, the SCC in milk dramatically increases (Sharma et al. 2011; Katherine et al. 2013). This increase in SCC results from the transfer of white blood cells from the blood to the mammary gland (Kelly et al. 2000; Sládek et al. 2006). In addition, the relative proportions of cell types present in milk change significantly, with an increased in PMN level (up to 90%) to protect the udder from bacterial challenge (Alhussien et al. 2016; Kehrli and Shuster, 1994; Zecconi and Smith, 2000). The increase in SCC milk causes the change in the components of cow's milk. The variation of the many components of cow milk with SCC was observed by many authors (Aysan et al. 2011; Brandt et al. 2010; Kelly et al. 2000; Somers et al. 2003; Bansal et al. 2005; Lindmark-Månsson et al. 2006) have reported changes in the composition of milk obtained from cows with infection, but little is known about such changes in camel milk. The relationship between SCC and lipolysis was investigated. It has been suggested that milk cells contribute to the lipolysis of milk fat to provide flavor defects (Azzara and Dimick, 1985; Ma et al. 2000; Santos et al. 2003; Gargouri et al. 2008). On the contrary, other studies indicated no relationship between milk SCC and lipolysis level (Lee et al. 1980; Cartier and Chilliard, 1990). Milk composition varies according to factors such as breed, age, mammary gland health, lactation stage, nutritional management and season (Dobranié et al. 2008). Similar, the variation in the constituents of camel milk may be attributed to factors such as breed, age, the number of calving, nutrition, management, the stage of lactation, and the sampling technique used (Alshaikh and Salah, 1994). The purposes of this study were, firstly, to investigate the relationship among SCC, lipolysis and chemical composition and, secondly, to evaluate the influence of lactation stage on these variables in lactating camels and cows.
MATERIALS AND METHODS
The study was carried out using individual milk samples from 36 dromedary animals (Camelus dromedarius) of Maghrabi breed from the south and the center of Tunisia. A total of 52 lactating dairy cows housed either in a free stall barn were used. Samples were obtained from each cow at days < 100 (n=15), between 100 and 240 days (n=25) and > 240 (n=12) after parturition. Of the 36 dromedaries, 11 were at early lactation (100 days in lactation), 18 at mid lactation (between 100 and 240 days lactation) and 7 at late lactation (between 100 and 240 days lactation). The camels were fed exclusively on natural browse. For cows, their nutrition is based on forage and concentrates. The milk was collected during the routine morning milking. Bovine samples were obtained by automated milking systems, but dromedary samples were obtained by manual milking. All the animals were free from clinical mastitis during the sampling period. Milk samples were taken to the laboratory immediately after collection and 250 mL were kept at 4 ˚C until the SCC. The rest was stored at -18 ˚C up to the rest of analysis.
Somatic cell counts
Somatic cells were counted using a Fossomatic 5000 (FossElectric, Hillerod, Denmark) according to International Dairy Federation Standard (IDF, 1995).
Milk was analyzed for pH, titratable acidity (AOAC, 1995), total solids by drying at 102 ˚C (IDF, 1987), milk fat by gerber method (IDF, 1981). The extent of lipolysis in milk was measured using the bureau of dairy industries (BDI) method (IDF, 1991) and was expressed as acid degree value in meq FFA/100 g of fat. The mineral content was estimated using an Automate Synchron CX9 (Beckman coulter®). All analyses were performed in duplicate
Statistical evaluations were performed using SPSS software (SPSS, 2011). The effect of lactation stage and lactation number on the different data was analyzed by one-way analysis of variance (ANOVA) and group means were compared by the Tukey’s least significant difference test. Secondly, pearson’s correlation coefficients (r) were also established to determine the relationships between the various parameters studied. The results were considered significant if the associated P-value was < 0.05.
RESULTS AND DISCUSSION
The overall results of physicalchemical parameters of dromedary and cow milk are resumed in Table 1. The Pearson correlation coefficients between total and differential SCC and physicalchemical parameters of dromedary and cow milks are presented in Tables 2 and 3. The pearson correlation coefficients between stage and number of lactation and physicalchemical parameters of dromedary and cow milk are presented in Table 4.
The data obtained showed a wide range of variation in some parameters studied between different individual camel and cow milk samples. There were no significant difference between pH values, titratable acidity and ash content of fresh camel and cow milk (Table 1).
Table 1 Composition of the camel’s and cow’s milk (Mean±SE)
SE: standard error.
The obtained result of titrable acidity of camel milk was lower in comparison to the acidity of camel milk reported by Khaskheli et al. (2005). These results were similar to those reported by Aljumaah et al. (2012) and Hammadi et al. (2010). The value of titrable acidity found in camel milk is similar to that of cow. The ash content (0.63%) of camel milk found in this study was similar to that reported by Mehaia et al. (1995). The average of total solids content in camel milk was significantly higher (P<0.01) in comparison to cow’s milk. The total solids content of camel milk was higher to that reported by others authors (Farag and Kebary, 1992; Ahmed, 1990; FAO, 1982). According to Mehaia et al. (1995), the total solids content ranged from 10.0 to 14.4% in camel milk. The average value of fat in camel milk was 3.7%, which is similar to the content of fat in cow’s milk (Table 1). The findings of this study agrees with Konuspayeva et al. (2009), who as a result of a meta-analysis of the literature data reported 3.82% an average fat content of camel milk. From the results shown in Table 1, it appears that lipolysis level in camel milk was not significantly different from lipolysis level found in cow’s milk. The results of lipolysis of cows’ milk were comparable to of the results reported by Andrews (1983). It was estimated from previous research (Bodyfelt et al. 1988) that the sensory threshold for detection of off-flavor would be about 1.0 meq/100 g of fat.
Table 2 Correlation coefficients (r) between total and differential SCC and physicochemical parameters of camel’s milk. (values noted in bold are significant at P<0.05)
SCC: somatic cells count; MAC: macrophage; LYM: lymphocyte; PMN: polymorphonuclear leukocytes; D˚: acidity (˚Doronic) and TS: total solids.
Results from this study are in general higher than this threshold and the average lipolysis neared 2 meq/100 g of fat. The higher lipolysis level has been described as the most important factor that contributed to the lower sensory quality and shorter shelf life of milk (Azzara and Dimick, 1985; Ma et al. 2000). The levels of K, Cl, Na and Ca were significantly higher (P<0.05) in dromedary (Table 1), which is in agreement with others studies (Mehaia et al. 1995; Sawaya et al. 1984). An average Ca concentration in camel milk was 10.32 mmol/L, a little lower than that reported by Faye et al. (2008). The K, Na and Cl contents of camel milk were higher than the value reported by Kamoun (1990). Magnesium content of camel milk mean value was higher than the value reported by Ahmed (1990). High variability was observed in some studies regarding the mineral content of camel milk (Dukwal et al. 2007; Haddadin et al. 2008; Ayadi et al. 2009) and it could be attributed to the breed difference, intervals between milking, feeding, analytic procedures and water intake (Haddadin et al. 2008; Mehaia et al. 1995). Mehaia et al. (1995) considered that genetic factors could significantly affect the milk composition, especially under non controlled environmental conditions, as is mostly the case locally. The calculated Na:K ratio in camel milk was higher than that reported by Aljumaah et al. (2012). The variation in concentration of minerals and the increments in Na:K ratio were studied in dairy goats (Boutinaud et al. 2003) and dairy cows (Stelwagen et al. 1999; Delamaire and Guinard-Flament, 2006). Alterations in the Na:K ration could interfere with a number of intracellular processes. Increased Na:K ratio reduce mammary protein system in dairy goats (Stelwagen et al. 1999). In dairy camels, the regulatory mechanism seems not to operate (Ayadi et al. 2009). Instead, this difference might be related to the adaptation of the camels to the desert conditions.
Effect of SCC
The results show that the SCC and differential cell count did nothave any significant correlation with pH, titratable acidity, ash and total solid values in both species. Table 3 shows that there was no significant relationship between lipolysis and total and differential SCC count in cow milk, which in agreement with Chazal and chillard (1986); Lee et al. (1980) and Cartier and Chilliard (1990). However, a positive correlation (P<0.05) between lipolysis and MAC was found in dromedary milk (Table 2). This suggests that the macrophages secreted lipolytic enzymes into the gradient while fractions containing polymorphonuclear leukocytes and lymphocytes did not possess lipolytic activity. These results confirm those of Russell et al. (1977) and Azzara and Dimick (1985), who found that lipolytic enzymes produced by monocytes and macrophages are believed to play a role in the degradation of cow milk fat ingested by those cells. A positive correlation (P<0.05) between fat, SCC and PMN count was found in cow milk but not in camel milk, which is in accordance with others studies (Aysan et al. 2011; Paura et al. 2002; Sawa and Piwczynski. 2002).
Table 3 Correlation coefficients (r) between total and differential SCC and physicochemical parameters of cow’s milk. (values noted in bold are significant at P<0.05)
SCC: somatic cells count; MAC: macrophage; LYM: lymphocyte; PMN: polymorphonuclear leukocytes; D˚: acidity (˚Doronic) and TS: total solids.
This positive correlation reported in this study and others (Barbano et al. 1989; Pereira et al. 1999; Ma et al. 2000) may be ascribed to the strong reduction in milk production consequently to mammary epithelium damages (Akers and Thompson, 1987). Mineral composition was significantly affected by the SCC in camel milk. Table 2 illustrates the positive correlation between SCC, PMN and Mg content. A high positive correlation was also observed between the SCC and the Na content, which is in agreement with Bruchmaier et al. (2004). However, mineral compositions was not significantly correlated with SCC in cow’s milk, except in the case of K content which is in a negative correlation with LYM. Potassium declines because of paracellular passage out of the alveolar lumen between damaged epithelial cells (Harmon, 1994). The ion concentrations in milk may be due to increased blood capillary permeability, the destmetion of tight junctions, and the destruction of the active ion-pumping systems.
Effect of lactation
The pH in camel milk was significantly (P<0.05) affected by the stage of lactation (Table 4), in agreement with Aljumaah et al. (2012).
Table 4 Correlation coefficients (r) between stage of lactation, number of lactation and physicochemical parameters of camel’s and cow’s milk (values noted in bold are significant at P<0.05)
SL: stage of lactation and NL: number of lactation.
Fat content was not affected by the stage of lactation (SL) in both species, which also observed Abeni et al. (2005). The ash content in camel milk was higher in the late stage compared to the initial stage of lactation. These results confirmed those of El-Hatmi et al. (2004) and Raziq et al. (2011), who reported that the ash content increased during lactation. The higher ash contents during late lactation stage suggest that camel milk can provide a satisfactory level of minerals (Mal et al. 2007). There was a negative relationship between total solid and lactation stage in dromedary. This decrease may be due to the increase in the milk water content during the last stage of lactation. These results confirmed those of Zeleke (2007), who demonstrate that total solid of camel milk decreased from 11.7% in the first stage of lactation to 10.1% by the end of lactation. In this study, Ca and Na content in camel milk showed a significant (P<0.05) decrease throughout the lactation, as observed by Aljumaah et al. (2012). The variations in the major mineral contents of camel milk could be due to breed, feeding, stage of lactation, drought conditions, or analytical procedures (Haddadin et al. 2008; Farah, 1993; Mehaia et al. 1995). There was a negative, but not significant, relationship between lipolysis and lactation number (NL) in cow milk (r=-0.308; P>0.05) and camel milk (r=-0.09; P>0.05). This suggests that lipolysis seems to be higher in primaparous cows than in multiparous. These results showed no effect of lactation number on camel’s milk composition.
In view of the observed results on the camel milk, it could be concluded that physicochemical properties was comparable to that of cow’s milk. However, in present study, cow milk was found to contained lower mineral content compared to camel milk. The higher level of lipolysis was observed in camel’s milk that contained a high percentage of MAC. This may also indicate that MAC in milk could play an important part in determining the lipolysis level in camel’s milk. Negative relationship between lactation number and lipolysis level was found in cow’s milk. For this species, the lactation stage not affected the physicalchemical compostion. The present study emphasizes that the variations in the camel’s milk composition could be attributed to SCC and lactation stage.
The authors would like to thank the “Ministère de l’enseignement Supérieur et de la Recherche Scientique, Tunisie’’ for the support of this research work.