3.1. Milk Production and Chemical Red Deer Milk Properties
Table 1 presents the mean values and lactation period effect for daily milk production (DMP) and the gross composition of the raw milk collected from five individual red deer hinds over the period from May to October 2016. DMP decreased significantly (
p < 0.000) during lactation, with mean values similar to those reported by other authors at the same lactations times [
29,
33]. Mean red deer milk values were 10.4, 7.1, 5.1, 4.3 and 24.2% for fat, protein, CN, lactose and DM, respectively. As for other mammals, fat was the milk component that had higher variations in the 18 weeks of study, increasing significantly by more than 3.5% from the first to the 18th week. Total protein and CN concentrations also increased significantly as lactation progressed, while lactose content remained stable. In the case of urea, with a mean value of 265 mg/100 mL, a significant drop after the tenth week was observed. Considering that, over the study, hinds were fed equally and that protein concentration increased as a consequence of a milk concentration effect, this decrease in urea at the end of the lactation could be explained by a lower protein intake by hinds in these last weeks, coinciding with the hottest period during lactation. Studies on heat stress in hinds and calves at our farm have shown that increased heat stress leads to a lower growth in calves and, in the case of higher values of thermal stress indices, also in lactating females [
36]. The literature on cattle shows feed intake reduction and lower protein in milk during high thermal stress in cows [
37], but while the latter study showed no effect of heat stress on urea, Costa et al. [
38] showed in buffalo that urea had one of the strongest correlations with heat stress indices, although a positive one and not the postulated negative one of our results. Unfortunately, our study was not designed to discern between the mixed effects of greater heat stress in the last weeks of lactation and the effect of reduction in milk production and increase in some nutrients such as protein or fat (maybe paired with a reduction in urea).
The values for milk composition were closer to those described by others [
15,
39,
40] that also previously observed significant increases in fat, total protein, CN and DM over the lactation period. For all of these compounds, the increment was more pronounced in the last eight weeks. The average gross composition of red deer milk was higher in comparison with milk from cow, goat, equids or camelids (
Table 2) and slightly higher than sheep or buffalo milk but similar to other cervids, such as reindeer or fallow deer [
1,
2,
5,
17,
18,
41]. The high content of fat and proteins suggests that red deer milk could be a suitable alternative for cheese production because of its great nutritional value and promising cheese yield.
3.2. Total Bacteria and Somatic Cell Count of Red Deer Milk
The total bacterial count (TBC) of red deer milk showed a mean value of 5.26 log cell/mL (
Table 3). Microbial levels were higher in the first months of lactation and decreased at the end, probably due to the coincidence with the slight decrease in temperatures at the end of the summer period. These levels were slightly higher than the official levels fixed for cow milk [
65,
66] but admissible if comparing with limits established for milk from small ruminant species in the EU [
65,
66].
Regarding the somatic cell count, the data revealed that animals had good sanitary conditions during the study period (
Table 3). Studies about SCC in red deer milk are scarce and limited to a few individuals [
24,
33]. During the interval being studied, an increase in SCC at the end was not observed, unlike the slight increase noticed for a similar period by Pérez Serrano [
33]. Nevertheless, further studies with a higher number of individuals or bulk milk are necessary to have better knowledge of the physiological values for this species. In addition to the sanitary conditions of hinds, SCC is widely used for evaluating milk quality and could provide useful information of the impact on technological properties of milk. Several authors remarked that high SCC in sheep and goat milk has a great impact on the renneting and acidification properties of the milk, on cheese yield and on the composition and sensorial characteristics of cheese and yogurt [
67,
68,
69].
If comparing with cow milk, where the official limit established by the European Union (EU) [
65] and the US [
66] regulations for cows is higher (5.6 and 5.87 log cells/mL, respectively) than counts observed for red deer milk, we could conclude that red deer milk fits within the cow milk standards setting in the EU and the US. Moreover, comparison with the limits established for sheep and goat milk also revealed that red deer milk meets the US standards (legal SCC limit of 1000 × 10
3 cells/mL). Official limits for these small ruminant species in the EU have still not been regulated, but the threshold levels recommended by several European authors [
70,
71] suggest that this species could be considered within the standards of small ruminants.
3.3. Physical Red Deer Milk Properties
Mean values for the pH and acidity of the raw red deer milk were 6.73 and 22.96, respectively (
Table 4). The range of pH (6.79 to 6.65) was narrower than the one described by Krzywinski et al. [
40] during lactation of two hinds (6.25 to 7.15). These values of pH were similar to those found in commercial dairy species [
41,
46,
56] or other mammalian species [
64,
72] (
Table 2). The values for acidity were close to those of sheep milk and higher than those of goat, cow or buffalo milk [
41,
46,
56] (
Table 2). As for other species [
46,
72], the pH and acidity values did not change significantly over the 18 weeks of control, but a slight tendency toward acidification was appreciated when lactation progressed. This evolution of pH was different to that described by Krzywinski et al. [
40], where pH increased with lactation. Bacterial content was normally attributed to milk acidification, but the evolution of pH and titratable acidity do not appear to be related to the bacterial content during lactation, with lower counts at the end of lactation. In addition, as all samples were analyzed in less than twelve hours, we can assume that acidity parameters mainly expressed the natural acidity and undeveloped acidity.
By contrast, milk electrical conductivity (EC) significantly increased, starting from the tenth week during lactation, from 2.31 to 3.56 mS/cm (
Table 4). These values were higher than those described for buffalo [
18], similar to those for sheep [
24,
41,
59] and lower than those for goat and cow milk [
24,
41,
57] (
Table 2). A slight increase in the EC during lactation has also been observed in goat and cow lactations [
57,
58]. The information of electric conductivity of milk has been used to predict and detect subclinical and early clinical mastitis in other ruminant species [
57]. However, as the SCCs observed at the end of lactation in the tested animals tended to decrease slightly as lactation progressed (
Table 3), mineral composition could have more of an influence on the increase in EC than subclinical mastitis. In healthy goats, Díaz et al. [
57] attributed this increase at this phase of lactation to changes in the blood–milk barrier due to a decrease in the tightness of the mammary epithelium that allows a greater permeability of Na
+ and Cl
− from blood to milk. This is in agreement with the significant increment in Na
+ as shown by Vergara et al. [
16] with the progress of red deer’s lactation.
The average density of red deer milk was 1.038 g/mL and significantly decreased during lactation (
Table 4). Comparable density values were reported by Krzywinski et al. [
40]. The density of red deer milk is closer to sheep or buffalo milk, but is higher than goat and cow milk [
41,
46,
56]. As for goat or sheep milk [
56], the evolution with lactation tends to decrease, which could be explained by the significant increase in fat content at the end of lactation (
Table 1).
The analysis of viscosity showed mean values of 3.12 cP (
Table 4), comparable to sheep milk (2.5–3.9 cP) and higher than in dromedary camel (1.7–2.3 cP), cow (1.7–2.5 cP) or goat (2.1–2.2 cP) milk [
49,
64,
73] (
Table 2). The red deer milk viscosity increased significantly starting from the tenth month, in a correlating trend with the increment in fat and protein contents (
Table 1). The change in milk viscosity depended on the contents of the milk: fat, protein and, particularly, the concentration and state of casein micelles [
47], as viscosity is positively correlated with these components.
Regarding color, the mean value for the L* coordinate was 89.94 (
Table 4), similar to values reported by others for deer milk [
24,
42] and slightly higher than the brightness values found in sheep and goat milk [
24,
60] (
Table 2). Brightness increased (
p < 0.002) with lactation, concurring with the increase in fat and protein contents (
Table 1). Several authors have perceived a brightness increment with higher fat content in cow and sheep milk [
48,
60]. A direct influence of proteins, especially caseins, on the increasing brightness has also been reported [
43]. Redness (a* coordinate) values ranged from −3.59 to −2.57 and increased during lactation (
Table 4). These trends are closer to those found in sheep milk with different fat contents [
60]. The mean value for yellowness (b* = 8.38) agrees with those found in other studies for deer milk [
24,
42]. The comparative observation of red deer milk yellowness with other species showed that this milk is yellower than sheep, goat, cow, camel or mare milk [
24,
42,
60] (
Table 2). These differences could be caused by the chemical differences between the milk for these species. In milk from other species such as sheep, a negative correlation for yellowness and fat content was observed [
60], but although a marked increase in fat was observed at the end of lactation, yellowness tended to be stable (
p > 0.05) during lactation in red deer milk (
Table 4). These differences might be due to a different assimilation of carotenes and other natural pigments [
42].
The ethanol stability (ES) did not change significantly during lactation—a mean value of 66.64% was obtained (
Table 4). This value was close to that observed in buffalo and sheep milk (63%), but quite different from cow (83–93%) or goat (44–50%) milk [
18,
24,
50,
51]. The importance of ethanol stability (ES) for the industrialization of red deer milk lies in the fact that it is an indicator of freshness and provides information about heat stability in dairy processes. This is the reason why several countries, such as Spain [
52], have established official limits for this parameter in raw milk. A more detailed understanding of ES for red deer milk has been published by de la Vara et al. [
24].
3.4. Milk Fat Globule Size
The parameters calculated from the size distributions of MFG for red deer milk during lactation are shown in
Table 5. The average volume-weighted mean MFG diameter D
4,3 for this species was 6.12 µm—higher than those found for cow (2.5–5.7 µm), goat (2.76 µm), sheep (4.97 µm), buffalo (5 µm) or yak (4.19 µm) milk [
34,
53,
74,
75,
76] (
Table 2).
The larger size of red deer milk fat globules could be related with the higher percentage of fat found compared to other species [
34]. Menard et al. [
34] explained that the larger size could be due to a limitation of the fat globules’ membranes to envelop the synthesized fat during the secretion of the MFG from the epithelial cells of the mammary gland. This is also in agreement with a significant (
p < 0.001) increase observed in the MFG diameter D
4,3 during lactation as fat content increases (
Table 1). By contrast, the rest of the size parameters were stable during lactation (
p > 0.05). Red deer milk showed a mean D
3,2 of 3.99 µm, a mean span of 1.25 and a mean SSA of 1.73 m
2/g. Values for SSA for red deer milk were similar to those observed by others for cow, goat or buffalo milk that ranged from 1.71 to 2.17 m
2/g [
34,
53] (
Table 2).
3.5. Red Deer Milk Coagulation
Coagulation parameters for red deer milk are shown in
Table 6. A clear difficulty for coagulation has been realized for milk pools corresponding to the fourth week of lactation; one sample did not coagulate after 60 min, rennet coagulation time (r) almost doubled and the time to curd firmness (k20) was six times longer compared with the next lactation time of sampling (
p < 0.001). Consequently, after 30 and 60 min, values for A30 and A60 were also significantly lower (
p < 0.001). These coagulation difficulties were also reflected in a lower curd yield at this lactation time (
p < 0.05), which showed an average yield during lactation of 3.29 g/10 mL. The higher pH and SCC content in the first week of lactation (
Table 3 and
Table 4) could have some influence on this pattern [
35,
61,
77]. After 6 weeks of lactation, the rennet coagulation time was reduced to around 25 min and was 16–17 min at the end of the analyzed period without significant changes. The mean value for
r (25.23 min) was high compared with sheep (6.5–28.1 min), goat (12.9–13.2 min), buffalo (11.6 min) and cow milk (10–19.2 min) [
35,
44,
45,
54,
55,
61,
62,
63] (
Table 2).
In the study period, the average curd firming time (k20) was 7.83 min, a much longer time than that observed in other species with values ranging between 1.57 min for sheep’s milk and 5.2 for cow’s milk [
35,
44,
54,
61,
62]. However, if only milk from the sixth week onwards was considered, the values for k20 ranged from 2.27 to 3.56 min, times closer to those found in other species (
Table 2). Thirty minutes after rennet addition, the mean A30 value was 20.73 mm, a value lower than that found in goat (36–44 mm), sheep (15–59 mm), cow (30–36 mm) or buffalo (40 mm) milk [
35,
44,
45,
54,
55,
61,
62,
63] before the first six weeks of lactation (
Table 2). Nevertheless, after six weeks, the A30 values can be compared to those found in sheep, goat or cow milk. Results for A60 suggested that the red deer milk has different coagulation properties than other species because the distance between the oscillation width continues to increase, unlike what happens with other species where, at this point, the maximum values for A60 were reached and a progressive decrease is observed, indicative of syneresis [
45]. To the best of our knowledge, the only coagulation pattern that is similar to the one previously described for red deer milk is found in Manchega ewes’ milk [
35]. Nevertheless, as for other coagulation parameters, the lactation stage has a high influence as lactation evolves. Further research is needed to investigate factors that have effects on coagulation and cheese yield.