Milk is considered as one of the most important staple foods consumed by many populations worldwide. It is a suitable and complete food for children and particularly important for elderly and patients who cannot consume solid foods. Milk is converted into various dairy products primarily cheeses, yoghurts, butter, cream and ice cream. However, the nutritional value of milk and its suitable conversion into dairy products depend on the quality of milk. Nowadays milk from a number of mammals is sought after but the most common and traditional milk types consumed by humans remain the bovine, ovine and caprine milk. The physicochemical analysis for the bovine, ovine and caprine milk obtained from farms in Malta and Gozo, are illustrated in table 1.
|
Bovine
|
Ovine
|
Caprine
|
|
Malta
|
Gozo
|
Malta
|
Gozo
|
Malta
|
Gozo
|
Temp °C
|
30±0
|
32±0
|
33±0
|
33±1
|
32±1
|
33±1
|
Fats % (w/v)
|
4.53±0.16
|
4.60±0.140
|
6.55±0.23
|
6.59±0.29
|
4.97±0.27
|
5.28±0.58
|
Solids Non Fat % (w/v)
|
7.75±0.03
|
7.70±0.03
|
10.57±0.08
|
10.23±0.15
|
8.15±0.09
|
8.13±0.32
|
Density kg/m3
|
26.9±0.3
|
26.9±0.2
|
32.0±0.3
|
30.9±0.5
|
28.5±0.3
|
27.8±0.7
|
Freezing point °C
|
-0.494±0.002
|
-0.491±0.002
|
-0.745±0.007
|
-0.717±0.012
|
-0.525±0.007
|
-0.529±0.027
|
Protein % (w/v)
|
2.79±0.01
|
2.77±0.02
|
3.83±0.03
|
3.70±0.06
|
2.93±0.03
|
2.93±0.12
|
Lactose % (w/v)
|
4.20±0.02
|
4.18±0.02
|
5.71±0.04
|
5.51±0.08
|
4.42±0.05
|
4.41±0.17
|
Salt % (w/v)
|
0.59±0.00
|
0.60±0.000
|
0.82±0.01
|
0.79±0.01
|
0.62±0.01
|
0.62±0.02
|
Added Water % (v/v)
|
0.3±0.1
|
0.2±0.2
|
0.0±0.0
|
0.0±0.0
|
0.0±0.0
|
0.9±0.7
|
pH
|
6.48±0.02
|
6.32±0.01
|
6.36±0.03
|
6.34±0.03
|
6.32±0.03
|
6.31±0.07
|
Conductivity mS/cm
|
4.6±0.0
|
4.8±0.0
|
4.7±0.0
|
4.8±0.0
|
4.5±0.0
|
4.6±0.0
|
Table 1: Comparison of the proximate characteristics between the milk of the three species in Malta and Gozo.
Fat content
Milk fat content varies according to the type of milk produced by dairy plants including: whole milk, semi-skimmed milk and skimmed milk. Whole milk from bovine is usually marketed at the standard of 3.5% fat content [13,14]. Milk processing plants may modify the fat content to develop products according to consumer demands since milk fat is the most important component of milk in terms of cost, nutrition and sensorial characteristics. No significant difference in the mean percentage of fats in bovine, ovine and caprine milk was observed between the two islands (p>0.41). Interspecies differences were significant with ovine milk fat content (6.56±1.72%) being higher as compared to caprine and bovine (5.08±1.41% and 4.56±1.17%) milk fat content (p<0.05).
The milk fat in bovines in this present study was lower than the approximate 4.92% reported for a study on 134 dairy cows: Swedish red and white, Swedish Holstein and Danish Holstein-Frisian breed [15]. The latter is the main breed present in the Maltese Island showing higher milk fat content than the 3.26% and 3.62% obtained in two studies [16,17]. The milk in the latter study was used for the production of teleme cheese in Greece. The fat contents in this study were comparable to those reported on bulk tank milk deriving predominantly from Holstein-Friesian cows [18]. However for ovine milk in Tunisia, the fat content was higher for pasture, feedlot receiving hay and feedlot receiving silage (7.33%, 7.6% and 8.23%, respectively) [19]. Another study reported high fat content in ewe milk [20]. In a study conducted in Greece, fat content varied with seasonality, with an average mean of 6.79% and 7.6% for milk collected in spring and summer, respectively [21]. No significant difference between breeds was reported. With regards tocaprine milk, the same study reported an approximate value of 4.3% and 4.21% for milk in spring and summer, respectively which is lower than the average % for caprine milk obtained in this present study. Different milk fat percentages in ewes and goats may vary according to different stages of lactation [22]. In ewes, mean milk fat was 5.64%, 6.33% and 5.99% for early, middle and final stages of lactation respectively, which were all lower than the mean (6.56%), obtained in this present study. With regards tocaprine milk, the authors reported mean milk fat 3.63%, 4.11% and 4.18% for early middle and final stage of lactation respectively, all of which were lower than the 5.08% obtained in this present study. Lower mean values for milk fat content for ovine and caprine milk (5.75% and 3.74%, respectively) were also reported in a study carried out in Austria [23]. Our results are somewhat comparable to those obtained in Portugal, showing that lambing affects milk composition with the mean fat increased from 5.97% to 7.05% [24]. The forage to concentrate ratio may also affect milk composition, with reported values of 6.77%, 6.23% fat in ovine milk [25], which is similar to what is reported in this present study. A high fat value is ideal for the production of cheeses. A study reported values of 7.60% and 5.10% for ovine and caprine milk, respectively [17], the latter being comparable with values obtained in this study. In another study carried out in France [26], different diets affected the fat content of caprine milk (3.0 - 3.31%), and which are much lower than the mean fat of 5.08% obtained in this study.
Solids non-fat content
The SnF portion in milk consists of protein (mainly casein and lactalbumin), carbohydrates (mainly lactose), and other minerals. Generally, the higher the fat content in the milk, the higher the SnF portion of the milk hence sustains ovine milk having a higher SnF content as compared to bovine and caprine milk. The mean concentration of SnF in bovine, ovine and caprine milk did not vary significantly between the two islands (p>0.07). Interspecies differences were significant with ovine milk SnF (10.50±0.69%) being relatively higher, followed by caprine (8.10±0.65%) then bovine (7.70±0.23%) milk SnF content (p<0.05).
In this study, SnF for ovine milk are similar to those (10.54%) obtained by Mayer and Fiechter [24]. Studies show that breed differences may not reflect significant differences in SnF content [21]. However, the authors observed interspecies differences in SnF (11.28% and 8.64%) for ovine and caprine milk, respectively. These were slightly higher than those obtained in this present study. Other studies reported higher (11.32% for ovine milk) [20] and lower (10.03% and 7.96%, ovine and caprine milk [23]. SnF values in the present study were comparable to those for Greek milk derived from ovine, caprine and bovine (11.65%, 9.05% and 8.89%, respectively) [17].
Protein content
The percentages of proteins in milk are approximately 3.2%, 3.4% and 6.2% for bovines, caprines and ovines, respectively [27]. However, this depends on several factors including breed and locality. In bovine and ovine, these proteins are mainly essential amino acids out of which approximately 80% is casein (αs1, αs2-, β- and κ-casein) [14,27]. The other milk proteins consist of whey or serum proteins such as α-lactalbumin, β-lactoglobulin, serum albumin, immunoglobulins, enzymes and enzyme inhibitors, metal (lactoferrin) and vitamin binding proteins, several growth factors, low molecular weight peptides (proteose-peptone) and bioactive peptides and all of these have important physiological properties [13,27]. The essential amino acids of caseins render milk as an important staple in the human diet primarily for the development and growth of the young while the other proteins play a vital role in nutrient transportation, disease resistance such as antibodies and other growth factors.
No significant differences were observed for protein content in bovine and caprine milk between Malta and Gozo (p>0.28); however a significant difference was observed between protein concentrations in ovine milk for the two localities (p<0.05) Interspecies differences were significant with ovine milk protein (3.79±0.26%) being relatively higher as compared to caprine and bovine (2.79±0.24% and 2.93±0.09%) milk protein content respectively (p<0.05).
In this study, results of protein content in bovine milk were lower than those reported on 134 dairy cows: Swedish red and white, Swedish Holstein and Danish Holstein-Frisian breed (4.78%) [15] and two other studies, 3.53% and 3.30% in the Netherlands and Italy [19,16]. The latter reported higher protein percentage values for both ovine and caprine milk in three different stages of lactation: 5.16%, 5.39% and 5.26% for ovine milk in early, middle and end stage of lactation respectively and 3.29%, 3.62% and 4.03% for caprine milk in early, middle and end stage of lactation, respectively. Higher protein values were demonstrated for ovine and caprine milk (5.21% and 3.15%, respectively) in Austria [23]. Diet may also affect protein content in ovine milk. Values of 4.88%, 5.14% and 5.35% for proteins in ovine milk were obtained on three different diets in Tunisia [19], which values relate to those obtained by other studies with values of 4.98% and 5.47% [22,24] and for ewes fed different diets (5.56% and 5.62 [25]. Higher mean values for protein were obtained in Greece [21] for ovine milk (6.3%) in both spring and summer and for caprine milk (3.71%and 3.44%) in spring and summer respectively. Conversely, in France [26] values for caprine milk (3.22-3.23%) were lower than the above mentioned studies but are similar to values obtained in this present study. A hypothetical reason for lower values of protein in milk of the Maltese Islands could be the forage to concentrate ratio in livestock diet. The cost of concentrates and importation costs may force farmers to feed less concentrates to livestock. Also compared to other countries, Maltese farms hold a lower number of heads leading farmers to feeding more roughage to limit expenses. Protein results in this study highlight the urge for local studies to be carried out on the physico-chemical characterization of local forage to establish forage quality and nutritional aspects.
Lactose content
The major carbohydrate in caprine, ovine and bovine milk is the milk sugar lactose [8,28]. It constitutes an average of 4.1%, 4.9% and 4.7% of caprine, ovine and bovine milk, respectively. The mean concentration of lactose in bovines, ovines and caprines did not vary significantly between Malta and Gozo (p>0.07). Interspecies differences were significant with ovine milk consisting of a higher concentration of lactose (5.56±0.37%) as compared to caprine and bovine (4.42±0.35% and 4.2±0.13%) milk lactose content (p<0.05).
The approximate 4.2% of lactose in bovine milk obtained in this study is lower than values on 134 dairy cows: Swedish red and white, Swedish Holstein and Danish Holstein-Frisian breed [15], and two other studies in Italy and the Netherlands [16,18] (4.78%, 4.53% and 4.51%, respectively). Conversely, in another study in Italy [22], results show that different lactation stages affect lactose of ovine milk (4.70 - 4.72%). Another study in Austria reported values of 4.64% and 4.32% for ovine and caprine milk [23]. Values between 4.44% and 5.06% for ovine milk were obtained in various studies [20,21,24,25]. These were lower than the approximate mean lactose obtained in this present study. On the other hand, the mean lactose content in caprine milk obtained in this study seem to be comparable with results obtained in France (4.4% and 4.33% [26]) and Greece (4.53% and 4.26% [21]), but higher than those obtained in Italy (3.97%-4.31% [22]).
Salt content
Salts in dairy contribute to a substantial amount of nutrients such as Ca, P, K and Mg and even though they are present in small amount in milk (approximately <1%) they play an important role in the milk’s technological properties as some salts influence stability and physical state of protein particularly caseinate [18,29]. No significant difference in the mean percentage of salts for bovine, ovine and caprine milk between Malta and Gozo was observed (p>0.06). Interspecies differences were significant with ovine milk salt concentration (0.80±0.07%) being higher as compared to bovine and caprine (0.60±0.02% and 0.60±0.05%) milk salt concentration (p<0.05).
Several studies [7;18,21;25;29,30-37] report the analysis of the mineral content that contribute to the salt content.
Freezing point
The freezing point of milk is an important physical property as it can be used as a parameter to determine milk adulteration hence to evaluate the milk quality [38]. However, freezing point may be affected by other factors such as heat treatment and seasonality [39]. In general, the average freezing point value in raw ovine milk decreases during the summer months [39]. High temperatures and dehydrated livestock may be potential causes for this depression.
There was no significant difference in the mean freezing point for bovine, ovine and caprine milk between the two localities (p>0.15). However interspecies differences were observed with ovine milk (-0.727±0.09°C) being significantly different from the freezing point bovine and caprine milk (-0.493±0.02°C and -0.527±0.05°C) respectively (p<0.05).
These results correlate to results (-0.519°C) obtained from milk deriving from Holstein-Friesian cows [18]. In another study the freezing point for caprine and ovine milk were -0.544°C and -0.542°C, respectively [23]. Lower values were obtained for ovine milk from two other studies (-0.578°C and -0.561°C, [20,40] respectively).
pH
The pH of milk is naturally slightly acidic. A reduction in milk pH demonstrates that the milk is souring. Bacteria play a very important role in milk pH. Certain types namely lactobacilli, convert the sugars in milk into acids hence reducing its pH. On the other hand, there are other bacteria that convert milk components into ammonia products hence increasing the pH. Therefore, milk pH can also serve as an indication of livestock health.
There was no significant difference in mean pH of ovine and caprine milk between Malta and Gozo (p>0.39); however a significant difference was observed in bovine milk samples between the two localities (p<0.05). Interspecies differences were significant, with bovine milk pH (6.42±0.14) being higher as compare to ovine and caprine (6.35±0.21 and 6.32±0.18) milk pH (p<0.05).
Similar patterns were also observed in similar studies with pH values of 6.7, 6.64 and 6.61 for bovines [29,15,41], ovines and caprines [29], respectively. In another study [42], pHs ranged between 5.75 and 6.72 in bovine milk, which is an indication of milk freshness, according to the authors. Other studies show minimal interspecies pH differences [16,17,23]
Conductivity
Conductivity can be a potential indicator for subclinical mastitis as the conductivity in milk increases with infection. From the values obtained in this study, it can be demonstrated that milk samples were collected from mastitis-free herds.
There is a significant difference in the mean conductivity of bovine milk between Malta and Gozo (p<0.05) however no difference can be observed in the mean conductivity of ovine and caprine milk between the two islands (p>0.78). The mean values for conductivity of bovine (4.7±0.1mS/cm), ovine (4.8±0.1mS/cm), and caprine (4.5±0.1mS/cm) milk are significantly different (p<0.05).
In another study [42], the conductivity ranged from 2.36 to 5.70 mS/cm in nine milk samples resulting in a mean value of 4.20mS/cm. Several studies [43-45] concluded that conductivity can be a potential indicator for subclinical mastitis as this increases with infection. Hence, results in this study demonstrate that the fresh milk samples were collected from mastitis-free herds.
Principal component analysis
The main objective of running a factor analysis (the screen plot) is to reduce the large number of variables to ease interpretation of results. Figure 1 reveals that the first four factors explain most of the variability in the original eight factors hence the first four factors are a simpler substitute for all eight. However, since the program is a 2-dimensional model, the next highest value of some variables in F3-F8 is compressed in F1 and F2 and hence only F1 and F2 are plotted in the variables plot. Pearson correlation (Table 2) and the variables plot (Figure 2) reveal that the milk fat content, milk pH, and conductivity are unrelated to all other variables. Conversely, there is a good relationship between proteins, lactose, SnF and salt content in milk and milk Fp. The positive relationship between proteins, lactose, SnF and salts (r>0.974) demonstrate that these variables are directly related with each other but are inversely related to the Fp (r < -0.941). The solid-non-fat portion of milk is primarily constituted of proteins, lactose, and minerals (salts: including Ca and P) [46]. Hence serves as an indication of the positive relationship between these four variables. The negative correlation of various parameters with Fp confirms that with respect to these parameters, the freezing point is a negative indicator for milk quality [47]. State that freezing point is proportional to the concentration of milk’s water-soluble characteristics. The same authors also mention that the milk fat globules, casein micelles and whey proteins are not particularly associated with a lower Fp. However, according to [48], the Fp of milk is determined by water-soluble constituents (including Ca, K and Mg), and lactose. Figure 1 demonstrates that ovine milk in both Malta and Gozo has distinctive characteristics as compared to bovine and caprine milk. This relates to the discussed physicochemical parameters, also confirming the statistical analysis obtained through the Analysis of Variance.
Figure 1: Principal Component Analysis: (a) the factor loading plot demonstrating the different groups of variables; (b) the factor scores of the two latent factors.
Figure 2: Visual representation of the interspecies differences for the parameters under study.
Variables
|
F
|
SnF
|
D
|
Fp
|
P
|
L
|
Sl
|
W
|
pH
|
Co
|
Temp
|
0.137
|
0.229
|
0.215
|
-0.230
|
0.238
|
0.227
|
0.197
|
-0.036
|
-0.236
|
-0.129
|
F
|
|
0.541
|
0.184
|
-0.624
|
0.497
|
0.554
|
0.536
|
-0.233
|
-0.197
|
0.362
|
SnF
|
|
|
0.831
|
-0.993
|
0.996
|
0.999
|
0.978
|
-0.232
|
-0.197
|
0.446
|
D
|
|
|
|
-0.789
|
0.843
|
0.827
|
0.807
|
-0.191
|
-0.186
|
0.326
|
Fp
|
|
|
|
|
-0.983
|
-0.994
|
-0.973
|
0.226
|
0.198
|
-0.465
|
P
|
|
|
|
|
|
0.993
|
0.974
|
-0.225
|
-0.196
|
0.439
|
L
|
|
|
|
|
|
|
0.976
|
-0.232
|
-0.201
|
0.451
|
Sl
|
|
|
|
|
|
|
|
-0.237
|
-0.179
|
0.448
|
W
|
|
|
|
|
|
|
|
|
0.130
|
-0.120
|
pH
|
|
|
|
|
|
|
|
|
|
-0.178
|
Table 2: Pearson correlation matrix for the studied parameters.
Temp (temperature), F (fats), P (proteins), L (lactose), Fp (freezing point); SnF (solid non-fat), D (density), Sl (salts), W (added water), pH, Co (conductivity).