There were no significant differences between the diet treatments in terms of birth weight, slaughter live weight, and cold carcass weight of suckling kids (on average 2.84 kg, 3.06 kg, and 2.95 kg; 8.98 kg, 9.03 kg, and 8.65 kg; 4.74 kg, 4.82 kg, and 4.65 kg, for DOP-0, DOP-40 and DOP-80, respectively; unpublished data). The cold carcass weights ranged from 4.65 kg to 4.82 kg and are in line with commercial carcass weights in the southern European market [
16].
Mother diet had no significant effect on milk yield (the average daily milk yield and the total yield during the suckling period were 1.6 l/d and 60 l, respectively; unpublished data), chemical components, or ME, except for fat (
p < 0.05;
Table 2), which was significantly higher for the DOP-40 diet than for the DOP-80 diet (the DOP-0 diet was not significantly different than the other two diets). Although increased milk fat content is common when dietary fiber concentrations rise at the expense of starch [
28], small differences were found in our study, probably due to the small differences in fibre content between the three diets. Furthermore, we observed significant effects of diet treatment on the FA profile (
Table 2) for only two acids: C18:0 (
p < 0.001; higher for the DOP-0 diet than DOP-80 diet, while the DOP-40 diet did not differ from the other two diets) and C20: 4 n-6 (
p < 0.05; higher for DOP-40 diet than DOP-0 diet, while the DOP-80 diet did not differ from the other two diets). Concerning C18:0, the higher percentage of this acid in the DOP-0 diet (
Table 1) may explain its increased abundance in DOP-0 milk.
3.1. Chemical Analysis and Physical Properties of the Meat
The chemical and physical characteristics of the meat are shown in
Table 3. The pH values found were in line with several studies [
14,
29] and higher than those reported by Ripoll et al. [
16] (pH values around 5.70) for European goat breeds. High ultimate pH values for goat muscles are common in the literature, suggesting that, unlike other species such as sheep and cattle, kids are generally highly prone to stress [
30]. In the present study, no significant differences (
p > 0.05) between the three treatments were observed in meat pH values after 24 hours. The use of DOP in the goat feed does not seem to affect the pH values of the kid meat, and so alterations to the properties of the meat during maturation are not expected.
The values for the chemical composition of the meat were within the ranges described by Guzmán et al. [
14] in Payoya kids and Ripoll et al. [
16] for kids of native Mediterranean goat breeds. The chemical analysis of the kid goat meat (
Table 3) did not reveal significant differences between the treatments in terms of moisture, protein, fat, or ash content (
p > 0.05). There were also no significant differences in the main descriptors of goat milk composition between the three diet groups (
Table 2).
Meat tenderness is one of the most important attributes for consumer satisfaction. All the meat samples had shear force values in a similar range to the values reported by Marichal et al. [
2] (5.5 to 8.1 kg/cm
2) for suckling kid goats slaughtered at similar weights and fed exclusively with milk, which are representative of the kid goat market in Mediterranean Europe. The mother’s diet did not significantly affect shear force values in the kids’ LT muscles (
Table 3). We found no significant difference (
p > 0.05) between the groups in WHC, measured as expelled juice and cooking loss (
Table 3). These observations are in line with Caparra et al. [
7], who did not observe a dramatic effect on the main physical properties of meat after incorporating dried citrus pulp into the diet of lambs.
With regard to meat colour (
Table 3), the meat was observed to have a reduced myoglobin content as compared to adult animals, as corresponds to suckling animals [
29]. As the animals’ diet consisted exclusively of breast milk and this milk does not contain the iron necessary for myoglobin, it was expected that the meat would exhibit a reduced myoglobin level. In the present study, no differences between the treatment diets (
p > 0.05) were observed in myoglobin content. Although orange pulp includes trace amounts of iron (0.009 mg/100 g), the lack of a dietary effect on myoglobin may reflect the absence of iron in mother’s milk, which makes it necessary to introduce other more iron-rich feeds (such as grass or cereal) to maintain meat myoglobin levels. The instrumental meat colour values were within the ranges observed by other authors using dairy goat kids slaughtered with similar weights [
29] (ranges 50.07–56.93, 9.08–11.50 and 36.83–43.99 for L*, C* and H
0 respectively), while they were lower than those reported in meat from heavier kids from meat goat breeds [
16] (ranges 52.95–54.76, 18.05–23.65, and 44.30–59.90 for L*, C*, and H
0 respectively). Mother’s diet significantly affected L* (
p < 0.01), a*, and H
0 (
p < 0.05) values. The inclusion of DOP in the goats’ diet decreased the L* and H
0 parameters of suckling goat meat, which were darker than those observed in the case of traditional diets. In fact, an increase in a* was observed in suckling goat meat when DOP was added to the goat feed. In general, the addition of DOP in the mother’s diet can lead to a decrease in the proportions of light pink meat and increase the proportion of darker meat in suckling goat meat.
3.2. Fatty Acids and Nutritional Properties of the Meat
While a total of 37 FAs were identified in the LT muscle, only the main FAs are shown in
Table 4. In general, the results for the three treatments were within the ranges reported in kids of the Payoya breed [
31]. The SFA group was the most prevalent (around 43% of total FAs detected), while MUFAs and PUFAs comprised approximately 32% and 25% of total FAs detected, respectively. Similar results have been reported in Spanish breeds of suckling kids [
18].
There was no significant difference in SFA content between the treatments (
p > 0.05), including major saturated palmitic FA (C16:0) (
Table 4). Mother’s milk is rich in saturated short and medium-chain FAs (mainly C16:0). The use of concentrates rich in orange pulp in the dam’s ration had no effect on the C16:0 content of the suckled milk nor on the composition of the kids’ meat. Differences in the MUFA content of the kids’ meat (
p < 0.05) were observed among the treatments. In fact, higher C18:1 fatty acid content in meat from the DOP-0 and DOP-40 diets (
p < 0.05;
Table 4) were observed. However, we did not observe significant changes in the content of the main unsaturated FA (C18:1) in mother’s milk when orange pulp was included in the goats’ diet (
Table 2). Our results are in line with those of Wang et al. [
32], who found higher C18:1 content in the meat of animals raised on cereal-based diets. On the other hand, the lower C18:0 content observed in the meat from DOP-80 kids as compared to the DOP-0 group (
Table 4) reflects the fact that the milk of the DOP-80 group had the lowest C18:0 content (
Table 2). This observation relates to the idea that the fat composition of the meat of lactating kids depends mainly on the characteristics of the milk they ingest because the ruminal processes of fat biohydrogenation are not evident in lactating animals.
In all the proposed treatments, the PUFA content in the kids’ meat was lower than the SFA and MUFA content (
Table 4). This observation was expected because the PUFA content of the milk ingested was lower (
Table 2) than the SFA and MUFA content. The PUFA content is important in meat because these FAs are highly oxidable during cooking [
33]. Moreover, previous studies have indicated that SFAs (e.g., C16:0 and C18:0) and MUFAs (e.g., C18:1) are positively correlated with meat flavour [
34], while PUFAs are negatively correlated. The effect of including DOP in the dam diets was insignificant for most PUFAs in the kid goat meat. Only two PUFAs were affected by the diet treatments. In fact, significant decreases of the most important PUFAs, C18:2 (
p < 0.01) and C20:3 n-6 FA (
p < 0.05) were observed in meat from diets that included orange pulp.
Consumers are increasingly interested in the lipid content of edible meat due to its relationship to human health. In a healthy human diet, the PUFA/SFA ratio should be as low as possible to reduce the risk of heart disease, among other health problems [
35]. In our study, significant differences were observed in the PUFA/SFA ratio (
p < 0.01;
Table 4). The most favourable PUFA/SFA ratio for human health (0.60 index) was observed in the case of kid goat meat from treatments using the highest concentration of DOP in the dam’s diet (DOP-80). The reduced C18:0 content observed in kid goat meat from the DOP-80 treatment (
Table 4) improved the PUFA/SFA ratio. Replacing cereal with DOP reduces the C18:0 content of the meat and therefore reduces the PUFA/SFA ratio.
A relationship between human health and the n-6/n-3 PUFA index has been proposed. Some clinical studies have recommended a n-6/n-3 ratio of less than four in order to reduce the risk of coronary disease [
35]. The use of DOP in goat feed had a significant positive effect on the n-6/n-3 ratio (
Table 4). The values obtained for the n-6/n-3 ratio in the three diet treatments (range 7.0 to 8.4) were higher than those recommended to prevent coronary heart disease. However, replacing cereal with DOP improved the n-6/n-3 ratio (
p < 0.01) of kid goat meat, which may be due to the decrease of the most prevalent n-6 PUFAs, C18:2 and C20:3 n-6 FA, observed in meat from diets using orange pulp. Recommendations to consume foods with reduced thrombogenicity indices have also been suggested as a measure to improve human health [
19]. The thrombogenicity index in the meat of the kids from DOP treatments was lower than that observed in the meat of kids whose goats ingested mainly cereal concentrate (DOP-0) (
Table 4). This observation is related to the lower content of C18:0 and the higher content of C20:4 n-6 in the milk of the DOP-40 and DOP-80 mothers (
Table 2).
3.3. Volatile Compounds and Aromatic Properties of Meat
The flavour of cooked meat play a major role in determining consumers’ acceptance of meat and their meat preferences [
36]. As is reported in the literature, the analysis of volatile compounds in raw meat is complicated by the low percentage of intramuscular fat distributed heterogeneously in meat [
37], as volatile compounds are stored in muscle fat only at trace levels. This observation is especially important for suckling kid goat muscle, as the average fat content is usually less than 2% in fresh meat. A total of 21 volatile compounds were tentatively identified in grilled goat kid meat for the three diet treatments (
Table 5).
All the volatiles that we identified were clustered in the following chemical families: aldehydes (3), aliphatic ketones (4), aliphatic alcohols (1), furans (1), sulphur compounds (4), aliphatic aldehydes (5), aliphatic hydrocarbons (2), and aromatic hydrocarbons (1). Aldehydes were the main chemical family in cooked kid loins (around 55%), followed in decreasing order by aliphatic ketones, sulphur compounds, aliphatic aldehydes, aliphatic alcohols, furans, aromatic hydrocarbons and aliphatic hydrocarbons (around 19%, 13%, 11%, 0.8%, 0.7%, 0.4%, and 0.1%, respectively). The main chemical families of the volatile compounds found in this study are in accordance with those reported for cooked meat in the literature [
40]. The generation of these volatile compounds in cooked meat is dependent upon the concentration of carbohydrates, amino acids, and lipids in the raw meat and the cooking conditions [
37].
The effect of including orange by-products in goat feed was significant in a low number of individual volatile compounds (
Table 5), including ethyl furan, dimethyl disulphide, and heptane. No significant differences were observed in other carbonyl compounds, such as unsaturated aldehydes or aliphatic ketones, among the three treatment groups. This observation reflects the small contribution of lipid oxidation to the development of aldehydes and ketones in suckling kid goat meat because of the meat’s low fat content.
Including orange pulp in the dams’ feed caused significant differences (
p < 0.05) in the ethyl furan content of kid meat (
Table 5), as the highest values were observed in kid steaks from DOP-40 and DOP-80 (0.20 and 0.30 AU × 10
6). Furan is mainly associated with an aroma described as sweet and caramel-like [
36]. Limacher et al. [
41] found that the main formation pathways for furans are from the intact sugar skeleton of fruit. Under the roasting conditions used in our study, furans would mainly be formed from the intact sugar skeleton (mainly hexoses) of the orange pulp. Including DOP in goat feed also affected the meat’s sulphur compounds, causing significant differences (
p < 0.05) in dimethyl disulphide in cooked suckling kid goat meat (
Table 5), with the highest amounts of dimethyl disulphide occurring in kid goat steaks from DOP-80 (2.32 AU × 10
6). While DOP is rich in sulphur compounds (0.11%/dry weight), the sulphur compound content of cereals varies [
42]. Sulphur compounds were found to contribute to the meat odour of lambs [
43], but there are no cases in the literature regarding sulphur compounds in suckling kid goat meat. Farmer et al. [
44] found that sulphur compounds in meat are products of reactions involving sulphur-containing amino acids. Therefore, it is to be expected that the meat of kids from goats fed with DOP would have a higher content of sulphur-containing amino acids than the meat of kids from goats fed using cereal diets.
Madruga et al. [
45] reported that hexanal is an important aldehyde in the aroma of goat meat. Aldehydes are derived from the lipid-oxidation Maillard reaction and the breakdown of amino acids through transamination and decarboxylation [
46]. In fact, during cooking, hexanal is derived from the thermal degradation of C18:1. The lower C18:1 content observed in the meat of the goats from the DOP diet groups does not seem to have resulted in differences in meat hexanal content between the three treatment groups. A significant decrease in C18:1 content (
p = 0.015;
Table 4) was observed when increasing DOP in the ration of the mothers, while diet treatment was not observed to have a significant effect on hexanal content (
p > 0.05;
Table 5). This observation may relate to the low fat content in the meat of the suckling kids in all three treatment groups.
Aliphatic hydrocarbons are especially important for the typical aroma of meats and contribute to the characteristic flavour of several animal species [
47]. With regard to these compounds, the highest heptane content (
p < 0.05,
Table 5) was observed in suckling kid goat meat from DOP-80. Elmore et al. [
48] suggested that a diet high in PUFAs leads to an increase in heptane in grilled lamb meat. However, this relationship was not evident in the case of the kid goats used in the present study, since no significant effect of PUFA content in mother’s milk on heptane levels was observed (
Table 2). The three diet treatments also had no significant effect on PUFA content in kid meat. It is likely that these results are related to the low fat content of the kid meat.
3.4. Sensorial Properties of Meat
The untrained panellists’ perceptions of the acceptability of suckling kid goat meat depending on the mother goat’s diet are shown in
Table 6. In general, the scores ranged between seven and eight on a 10-point scale measuring tenderness, juiciness, flavour quality, and overall appraisal of the meat.
There was no specific preference between diet treatments concerning the tenderness or juiciness of the meat because the animals were young and the meat in all the treatments was tender and juicy (
p > 0.05,
Table 6). This result may be related to the fact that no significant differences were observed among the three treatments for the shear force or water losses of the meat (
Table 3). However, the flavour quality of the meat was influenced by the dam’s feed (
p < 0.01). Meat from treatments including DOP was considered to have a more desirable flavour than the meat from the cereal-based diet (
Table 6). There was no general agreement regarding the desirability of the flavours. There was considerable disagreement among the subjects about this factor. However, consumers from a specific geographic area tended to like the same kind of meat, probably due to the influence of previous experiences or the gastronomic culture in local areas, which are partially determined by the products available on the market [
24].
Fat content can affect the aromatic characteristics of meat. However, no significant differences were observed among the three diet treatments in terms of the fat content of suckling kid goats (
Table 3). Accordingly, the fat content of the meat should not be considered to have been a factor in the untrained panellists’ evaluation. Alternatively, the higher flavour values likely relates to the specific content of aromatic compounds detected in the meat. Resconi et al. [
49] identified several volatile sulphur compounds that contribute to the development of meat flavour. In our study, the highest content of dimethyl disulphide was observed in kid goat meat from treatments that included DOP in the goat feed (
Table 5). Dimethyl disulphide is a volatile compound derived from the oxidation and degradation of cysteine and methionine (sulphuric amino acids) that can be aromatically detected at low concentrations [
49]. Ethyl furan and aliphatic hydrocarbons such as heptane could also contribute to the differences in flavour quality reported by the untrained panellists (these compounds occurred at the highest levels in meat from the DOP treatments).
Many individual sensory attributes influence the acceptability of meat (e.g., juiciness, colour, greasiness, etc.). However, an overall appraisal provides an idea of whether the consumers liked the meat. The effect of including DOP in the dam’s feed on the overall acceptability of kid goat meat was insignificant (
Table 6). The low fat content in suckling kid goats is likely to have influenced this assessment for untrained panellists.
3.5. Discriminant Analysis
A linear discriminant analysis model was built to determine the relationship between groups of variables and the three feeding groups for datasets that were significantly different of suckling kid goat meat (
Figure 1).
We performed a discriminant analysis to quantify the contribution of each variable to the differences observed among the three treatments. Since the number of variables analysed is very large, only those variables that were significantly different (
p < 0.05) according to a permutation ANOVA test were selected in the model. The resulting model was used to identify the key variables that contribute to the differences in the meat suckling kid goats based on meat quality parameters, FA profile, and volatile compound content. Function 1 of the discriminant analysis accounted for 72.9% of the total variation among feeding strategies and it was mainly determined by the absence (DOP-0) or presence of DOP in goat feed (DOP-40 and DOP-80). This function clearly discriminated the effect of including DOP in goat feed, with meat from goats fed using DOP on the right and meat from goats fed with concentrate without DOP on the left. The parameters of colour L*, a*, and hue angle determined the variability assigned to LD1 (
Table 3).
On the other hand, Function 2 discriminated three groups (27.0%) taking into account the level of DOP included in the feed. The high-DOP concentrate group (DOP-80) was in the upper quadrant with the highest aromatic compound content (i.e., ethyl furan, dimethyl disulphide and heptane;
Table 5), whereas the groups of concentrates not including DOP (DOP-0) and with low DOP (DOP-40) were in the lower quadrant with the highest monounsaturated FA content (
Table 4). The ability of LDF1 and LDF2 to separate the three groups suggests that using DOP in the feeding regime of goats could affect the kids’ meat colour, the prevalence of some volatile compounds (ethyl furan, dimethyl disulphide, and heptane), and MUFA content.