4.1. Diet Intake and Physicochemical Milk/Cheese Composition
The slight decrease in the intake of the SO diet was not statistically significant. However, the studies carried out showed that feed consumption decreased by 6% in goats in response to a mixture of extruded linseeds and fish oil (representing 14.7% and 1.7% of the DM in the respective diets), while adding 21% extruded linseed alone did not affect the intake, although both diets had the same ether extract content (6.9% DM [
24]. In goats, the inclusion of 3% unprotected fish oil reduces feed intake, but supplementation with protected fish oil has no effect on the same parameter [
25]. Thus, the effect of adding fatty supplements on intake depends on many factors, including ruminant species, the amount included in the diet, and the type and composition of any supplement.
The dietary treatments had no significant effect on daily milk yield despite the substantial reduction in feed intake when a mixture of extruded linseeds and fish oil was included in the diet [
24]. As shown in
Table 2, the supplementation of goat diets with fats with a higher level of polyunsaturated fatty acid had no effect on the physicochemical parameters of the resulting milk. In goat milk, unlike in cow milk, there is no decrease in the milk protein and fat content when the goat diet is supplemented with PUFA-rich vegetable oils, which can partially be explained by the fact that the inhibition of acetyl-CoA carboxylase and de novo lipogenesis is less strong in the goat mammary gland [
26]. However, significant differences were observed in the dry matter content of the milk, which was lower in the milk from animals receiving the SO diet than in the corresponding milk from FL diet, in agreement with studies carried out in goats [
27] and in ewes fed diets supplemented with fish oil [
28]; one possible explanation may be the hypophagic effect of long-chain PUFA from fish oil [
9,
29].
Regarding fresh goat cheese, no influence of either diet supplementation was observed on the physicochemical parameters when linseed supplementation was used [
30,
31]. Regarding the technological suitability for cheese-making, no significant differences were observed in the clotting time or cheese yield between diets. Milk clotting time is associated with the milk physicochemical composition, mainly its protein concentration; thus, as expected, no differences between milks were observed in this parameter [
32]. The protein content, especially, would explain why no differences were found in the cheese yield.
Our results agree with those obtained in Padraccio cheese derived from a dietary supplementation with extruded linseed [
33] where no significant differences were observed in any rheological characteristics. However, cheese texture derived from cows supplemented with extruded linseed and vitamin E, and plant extract produced a softer, more uniform, meltable, and fatty texture than in the control [
34]; these results could be explained in reference to the lower fat melting point of the cheeses due to the higher PUFA content. In our study, although no significant differences were found regarding hardness, a softer texture was correlated to a higher unsaturation level [
35]. However, an increase in hardness was observed with linseed supplements associated with the lower moisture of supplemented cheeses [
31]. Changes in the textures of different types of fortified cheese could be explained by the interactions between milk components, enzymes, and sources of fat [
36] and by the different technological treatments applied during cheese-making.
4.2. Fatty Acid Profile
Diets rich in n-3 FAs affect the FA composition not only by direct assimilation into milk, but also by modulating the expression of lipogenic enzymes [
37]. Following the same pattern, as was described by several researchers of dairy cows, supplementation with sunflower oil decreased the SFA and increased the total n-3 FA. However, FL supplementation enhanced the n-3FAs, especially ALA, in the diet supplemented with FL, which would contribute to a decrease in cardiovascular disease risk factors due to reduced levels of serum low-density lipoprotein cholesterol. In addition, the FL diet decreased the levels of palmitic acid (C16:0) and hypercholesterolemic saturated acid [
27]. The RA level was highest in the diet supplemented with FL, in accordance with the findings obtained in Manchega ewes when their diet was supplemented with extruded linseed [
38]. The greater increase may have been due to the levels and form of the linseed because the extrusion process increases the accessibility of ALA to rumen microbiota. The resulting alteration of the rumen metabolism could make biohydrogenation less efficient and may also decrease the saturation ratio, increasing the concentration of C18:3 and trans-fatty acids in milk from diets rich in linseed oil [
27]. The isomer trans-10 cis-12 (another CLA) always remains at trace levels in goat milk because this CLA is converted into C18:1, trans-10 in the rumen, and an increase in this FA was only observed when it was infused postruminally [
39]. However, diets with extruded linseed had a minimal effect on this isomer, which agrees with our results [
38]. On the other hand, a diet supplemented with fish oil increased the concentrations of CLA and long-chain PUFA and decreased C18:0 in milk, as also shown in our study [
40].
Goat seems to be less sensitive than cows to the shift from C18:1, trans-11 to trans-10, which would explain the stability of the cis-9, trans-11 CLA determined in our study. However, increases in the percentages of C18:2, trans-9, and trans-12 were observed in the milk of animals receiving FL/SO diets, as supported in previous studies using diets supplemented with extruded linseed [
38].
In our study, we found that the highest DHA values are associated with an increase in SFA concentration and a higher saturated/unsaturated fatty acid ratio, in contrast to that observed in milk derived from ewes supplemented with tuna oil [
25]. In a study considering the influence of the type of diet in goat and ewe milk [
27] suggested that the biohydrogenation of PUFAs in soybeans or linseed would occur slowly, producing SFA and less C18:1 or CLA; however, this effect was not observed in our study, possibly due to the way that the seeds had been treated, since the process used to obtain flaked linseed breaks the seed and increases the accessibility of ALA. Therefore, the diet supplemented with FL significantly improved the nutritional value of the subsequent milk due to the reduction in SFA and increased levels of PUFA and CLA isomers [
4]. A diet rich in linseed oil decreased the n-6/n-3 ratio and significantly increased the CLA levels, which agrees with our results [
41]. A nutritional improvement was also observed in milk from goats given the diet supplemented with SO due to the significant increases in PUFA and CLA. Although several studies have stated that the n-6/n-3 PUFA dietary ratio is of no relevance for modifying the risk of cardiovascular disease, there are studies which have determined that the conversion of long-chain omega-3 PUFAs (n-LCPUFAs), such as EPA and DHA, is reduced by a high ratio of linoleic/linolenic acids [
42]. Thus, an increase in the dietary intake of pre-formed n-LCPUFA or reducing the n-6 PUFA intake is recommended, or a combination of both; however, direct DHA intake is more efficient. Indeed, this was the case with our results for the fatty acid profile and n-6/n-3 ratio of the milks from the diets enriched with flaked linseed or fish oil, and for the increase in DHA with the SO diet. In addition, the decreases in the atherogenicity index observed in milk and cheese resulting from the diet enriched with FL, due to decreases in the saturated/unsaturated ratio, confirmed the results obtained in goats using a diet supplemented with unsaturated plant oils and those obtained in grazing goats with a diet supplemented with extruded linseed [
43]. A decrease was also observed in milk and cheese derived from Manchega ewes fed extruded linseed [
38].
Unlike milk, few studies have investigated the effect of diet supplementation on FA profiles in cheese. The similar FA patterns observed in milk and cheese agree with the results determined by Gebreyowhans et al. [
4]. The FA profiles of cheeses, mainly FAs associated with potential benefits to human health, primarily depend on the FA composition of the milk used rather than the cheese-making technology [
44].
It should be highlighted that none of the ALA values obtained in our study for milk or cheese derived from an FL-supplemented diet were higher than the overall mean value found across European countries [
45].
The CLA determined in cheese was observed to be primarily dependent on the CLA level of the unprocessed milk. In our study, increases compared with the control were observed in the PUFA and CLA contents, as were decreases in the n-6/n-3 ratio and atherogenicity index, particularly in the cheeses made with milk from the animals receiving FL. Notably, although our supplementation-control periods were short, significant changes in the fatty acid profile were evident, which indicated that it is possible to change the nutritional value of animal products using short time intervals, such as that defined in our trial. However, longer studies should be conducted to evaluate their effects on animal health or further changes in the nutritional value of products.
4.3. Milk/Cheese Sensory Profile
No significant differences were observed by the trained panel between the milk and fresh goat cheese resulting from the different diets in any of the determined sensory attributes. These results confirm that it is possible to obtain milk/cheese with better nutritional characteristics without altering the sensory profile. These results agree with those obtained by Dauber et al. [
46] in goat cheese derived from milk supplemented with sunflower oil. Our results are in agreement with those observed by Nguyen et al. [
44] in ewe cheese, although they observed that levels of MUFA exerted a strongly negative effect on cheese edible quality, which can partially explain the lowest overall acceptance of cheeses derived from the SO diet. However, in commercial CLA-fortified dairy products, some defects or losses in flavor have been determined [
47]. Differences were determined in Pecorino cheese odor, flavor, and toughness as a result of a diet supplemented with extruded linseed; lower odor and higher toughness and flavor values were found for a CLA-enriched cheese [
48]. Thus, in our study, although no significant differences were found, the highest overall acceptance was obtained from the milk and cheese corresponding to the control group in accordance with the results observed by Santurino et al. [
30], where the least acceptable was the milk/cheese from goats given the SO diet.
Furthermore, this study was designed as practical applied research in nutrition; thus, the omega-3-rich supplements selected for evaluation were commercially available flaked linseed rich in ALA (Agrocava S.L., Caravaca de la Cruz, Murcia, Spain), and a salmon fish oil product rich in DHA and EPA (Optomega-50, Optivite International Ltd., Nottinghamshire, UK). Therefore, the milk obtained from animals receiving dietary supplementation can be provided to the dairy industry, mainly goat production.