4.1. Growth Performance
To date, the use of by-products in animal nutrition is widely documented. In fact, it is also thanks to the inclusion of by-products in animal feeding that strategy have been pursued to improve environmental and economic sustainability. Ali et al. [
25] observed that using by-product, as citrus pulp (dried), in the diets of pigs has the potential to reduce environmental impact of pork production in terms of global warming potential and allows for the use of land for the food crops production intended for human food.
As claimed by Kyriazakis and Emmans [
26], an abundant presence of dietary fibre in pig diet can lead to a reduction of feed intake, due to the specific polysaccharides such as pectins, which absorb water and form a gelatinous compound within the intestinal lumen. Other authors [
27] showed that the inclusion of 5% and 10% of ensiled citrus pulp in the diet for growing pigs resulted in a reduction of animal growth performance. This negative effect may derive from the reduction of DM feed intake during the first 4 weeks of the experimental trial, but the differences subsequently disappeared.
In the current study, no significant differences were observed in terms of DMI between treatments. Probably, supplementing the bergamot by-product as dried, ground and mixed with the remaining ingredients of the diet affected this result.
4.2. Fatty Acid Composition
Apulo-Calabrese, similarly to the other Italian local pig breeds [
28], is characterized by reduced growth and carcass performance [
29]. Growth is slow, and this was probably the main cause that led breeders to prefer other breeds that are earlier and with a better feed conversion index. Actually, this slow growth, if associated with a balanced diet, could have an effect on the fatty acid profile of the meat and therefore affect the nutritional quality of the food itself. However, in our trial, we also evaluated the fatty acid composition of meat, because Aboagye et al. [
29] showed that, when the Apulo-Calabrese pigs are managed in the same rearing conditions as crossbreeds, their muscle fatty acid composition was comparable; therefore, the fatty profile of the meat seems to not be influenced by the genetic type. Conversely, the fatty acid composition of meat can be influenced by the fatty acid composition of the experimental diet. Wood and Enser [
30] as early as 1997 asserted that, in all species, meat fatty acid composition can be modulated by the diet, especially more easily in pigs, monogastric animals, where the linoleic, α-linolenic and long-chain PUFA respond quickly to high dietary concentration.
Despite the fact that meat has been and still is criticized because of its undesirable fatty acid profile, due to the high proportion of SFA rather than PUFA, meat is an essential food for the human diet. Meat is a valuable source of high biological value protein, iron, vitamin B
12 and other B complex vitamins, zinc, selenium and phosphorus. In fact, the elimination of meat from the diet could increase the risk of severe nutritional deficiencies and impair human health and nutritional status [
31]. Moreover, pork has a favorable balance between polyunsaturated and saturated fatty acids (PUFA/SFA), and our data were in agreement with these authors.
It is well-known that pork meat in general has an unacceptably high ratio of
n-6 and
n-3 polyunsaturated fatty acids [
32].
The World Health Organization [
33] recommended a reduction in the intake of SFA in favor of the n-3 polyunsaturated fatty acids (
n-3 PUFA) for their beneficial effects on human health. Essentially, SFA are known to increase low-density lipoproteins and, in turn, the risk of cardiovascular disease. In our trial, despite that no significant increase in the concentration of ALA in EB pork meat was observed, the proportion of DPA was affected by the dietary treatment. The DPA is a long-chain
n-3 PUFA that derives from ALA [
34] thanks to the action of desaturase and elongase enzymes that catalyze the reactions; its beneficial effects on human health are well-known [
35,
36,
37,
38,
39].
Furthermore, the longissimus thoracis et lumborum muscle from pigs fed with the EB diet tended to have a greater proportion of
n-3 PUFA, while the proportion of n-6 PUFA was comparable between treatments and, consequently, the
n-6/
n-3 ratio was the lower in the EB treatment compared to control. A similar trend was observed by Scerra et al. [
13] in meat by pigs fed a diet that included ensiled bergamot pulp supplement at the level of 15% dry matter.
The thrombogenic index (TI) was calculated to assess the potential for platelet aggregation. The TI tended to be reduced when the pigs were fed the EB diet compared with the control diet, and this can be explained by the fact that bergamot pulp affected some desirable fatty acids, such as those belonging to the n-3 family.
An accumulation of αlinolenic acid and long-chain fatty acids derived from it in the pig’s back fat is generally related to the levels of ALA in the diet provided. In this trial, exhausted bergamot by-product supplementation in the finishing diet of pigs significantly increased the concentration of ALA in the back fat. Consequently, the concentration of total n-3 PUFA tended to increase in back fat from EB fed pigs. However, no differences were observed on long-chain n-3 PUFA between the two groups.
Fatty acid composition determines the firmness/oiliness of adipose tissue and the oxidative stability of muscle, which, in turn, affects flavor and muscle color [
40].
The Apulo-Calabrese breed is among those authorized for the production of the four protected-designation-of-origin cured meat products: “soppressata”, “salsiccia”, “pancetta” and “capocollo” of Calabria, all certified by the “Consortium for the protection of Calabria PDO cured meats”. In our study, we analyzed the salami, and we noticed that the use of exhausted bergamot pulp influenced, coherently with that observed in the meat and back fat, the fatty acid profile of the salami, enhancing some desirable fatty acids and improving indexes related to a lipid nutritional quality.
4.3. Oxidative Stability
Lipid oxidation is one of the main factors responsible for the loss of quality of meat and meat products. Following lipid oxidation, a series of unpleasant tastes and odors develop, as well as changes in color and texture. It is a rather complex process, where unsaturated fatty acids are involved and react with molecular oxygen to form peroxides, from which aldehydes, ketones and acids derive, many of which are responsible for the unpleasant rancid smell of oxidized fats [
41].
Tocopherols are fat-soluble antioxidants and have a protective role against lipid oxidation [
42]. Animals are unable to synthesize tocopherol, and its concentration in tissues is, therefore, strictly dependent on the diet. In the present study, although the concentration of vitamin E, especially α-tocopherol, in exhausted bergamot pulp was much higher than in the concentrate, no difference in vitamin E content was observed in meat of the two experimental groups. While for retinol, defined as a lipophilic scavenger, its concentration tended to be influenced by diet; in fact, it was greater in the meat of the EB group. As claimed by Halliwell and Gutteridge [
43], in conditions of oxidative stress, there is a shift in the pro-oxidant/antioxidant balance in favor of the former in animal tissues. Despite the beneficial effects,
n-3 PUFA are subject to oxidation during the processing and storage phases, inducing a potential alteration of the nutritional composition and product quality [
44], and as the degree of unsaturation increases, the tendency toward oxidation also increases [
45]. The results of the present study showed that the highly peroxidizable (HP)-PUFA ÷ Tocopherols ratio tended to increase (
p = 0.057) in meat when exhausted bergamot pulp was integrated in the pig diet. It means that other possible compounds came into play, meaning that the use of exhausted bergamot pulp did not alter the shelf-life in raw meat. Agro-industrial by-products are rich in secondary compounds with antioxidant properties, such as phenolic compounds and essential oils, which, when used in animal nutrition, can affect the resistance of meat to oxidative deterioration. The peel of bergamot fruit contains a significant amount of flavonoids, in particular, naringin, neoeriocitrin and neohesperidin [
46], compounds that have been found to have health-related properties, especially based on their antioxidant activity. Moreover, for their content of polyphenols and other bioactive phytochemicals, several agro-industrial by-products can be considered as functional feedstuffs [
47]. The exhausted bergamot pulp used in our study showed a considerable residue of phenolic compound, as shown in
Table 1 (>6 g TAe/kg DM). This supports the thesis that these compounds have come into play. Above all, phenolic compounds act against the oxidation of myoglobin by extending the shelf life of the product [
48]. The discoloration of the meat is mainly due to the oxidation of myoglobin and the consequent accumulation of metmyoglobin [
49]; this process causes the decrease in meat of redness (a*) and saturation (C*) values and the increase in hue angle (H*) during storage time. In the present study, the main descriptor of meat discoloration changed over the time of storage, as expected. Indeed, the meat yellowness (b*) and the hue angle (H*) increases during storage time. Dietary treatment significantly affected meat lightness (L*). This finding was consistent with the results from Priolo et al. [
50] and Inserra et al. [
51], who observed higher meat lightness in lambs and pork, respectively, fed with a diet containing carob pulp and therefore rich in tannins. Conversely, Crosswhite et al. [
52] concluded that the inclusion of citrus pulp did not affect the main color descriptors.
In the study conducted by Luciano et al. [
53], the authors reported that differences in oxidative stability between meat samples were evident with strong oxidative challenges, such as cooking, compared to samples stored in a refrigerated aerobic in darkness environment. In the present study, we also evaluated cooked meat by subjecting it to strong oxidative stress factors in order to highlight the influence of dietary treatment. Moreover, under these conditions, lipid oxidation (TBARS values) was not affected by the dietary treatment. Conversely, the dietary treatment significantly influenced the extent of lipid oxidation in salami, and the TBARS values were lower in the EB diet (
Figure 1). The TBARS assay detects the level of malondialdehyde (MDA), which is the major lipid oxidation product [
54]. The limit value that distinguishes the condition of rancidity is indicated in terms of MDA and is a maximum of 2 mg/kg [
55]. In our study, in salami from the EB group, this value did not exceed this limiting threshold, while in salami from the control group, the level of MDA was already beyond of 2 mg/kg on the second day of monitoring. Furthermore, the TBARS values increased during aerobic refrigerated storage, and, after 5 days, the salami of the EB group showed lower values compared to the salami of the control group.
In salami, the concentration of the HP-PUFA increased when the pigs were fed the EB diet compared with the control diet, leading to an increase of the HP-PUFA ÷ Tocopherols ratio in salami from the EB group compared to the control group.
Despite the high levels of HP-PUFA, the TBARS values in the salami of animals fed with the EB were lower during the storage period, highlighting how salami from animals treated with bergamot responded better to the strong oxidative stresses, such as the grinding and the long storage time. The study of Luciano et al. [
42] showed that vitamin E is the greatest contributor in the improvement of the antioxidant capacity of tissues. However, in our study in both raw meat and salami, no difference was found in the content of vitamin E between the two dietary treatments. Phenolic compounds may explain this result, as it was reported that dietary administration of hesperidin and naringin exerted a significant effect on the antioxidant capacity of broiler meat [
56]. Therefore, it might be speculated that these phenolic compounds may have contributed to the delay in the lipid oxidation in salami.
Furthermore, Scerra et al. [
13], in a study on salami, observed that by feeding pigs with a diet containing ensiled bergamot by-product, TBARS values were below the value of 2 mg MDA/Kg for the monitoring period. However, after 5 days of refrigerated storage, the TBARS values observed by Scerra et al. [
13] in salami by feeding pigs with ensiled bergamot by-product were lower than the TBARS values observed in this trial by feeding pigs with a diet containing exhausted bergamot by-product. The highest amount of antioxidant compounds, such as phenols in the bergamot by-product used in the trial of Scerra et al. [
13], could influence these differences (14.15 vs. 6.39 g TAe/kg DM, respectively, in ensiled bergamot by-product and exhausted bergamot by-product).