1. Introduction
There is a worldwide interest in enhancing the use of pea (
Pisum sativum L.) in the diets of ruminants. Using pea in place of soybean meal (SBM) has been encouraged to reduce dependency on imports in Europe [
1]. Moreover, European consumers reject soybean, as it is mainly a genetically modified organism [
2] and is therefore banned by organic production regulations [
3] and because of sustainability issues linked to deforestation in soybean production areas [
4].
Pea has a high crude protein (CP) content [
5,
6] and highly soluble and degradable starch [
7], and the net energy for the weight gain of pea is at least equal to that of corn and is greater than that of SBM [
6]. Up to 15% pea inclusion in the fattening diets of light lambs is recommended [
8], but the use of pea has been studied scarcely. Nevertheless, minor effects were observed when lambs were fattening with greater rates of pea inclusion [
9,
10,
11,
12]. Purroy, et al. [
13] observed greater internal fat depositions in lambs that were fed concentrate including pea instead of SBM, and they related these results to the differences in the net energies for the weight gain of these protein sources. Therefore, the intramuscular fat (IMF) deposition in the studied lambs might also have been altered, although this effect was not studied. The inclusion of pea can also affect the fatty acid (FA) composition of lambs [
6,
12,
14] and improve the health properties of meat because of the deposition of n-3 polyunsaturated fatty acids (PUFAs) is increased when pea is used compared to SBM [
12,
15]. Moreover, pea can be a source of carotenoid [
3], which affects fat colour and lipid oxidation during storage and have a minor effect on meat colour [
9]. Therefore, the aim of this study was to evaluate the effects of increasing the proportion of pea in the fattening concentrate of light lambs on the fat colour and meat quality.
4. Discussion
The inclusion of pea to reduce the proportion of SBM in iso-energetic and iso-proteic concentrates involved a modification of the proportions of other ingredients, but the resulting concentrates had similar chemical compositions [
16]. Numerous studies that have included pea in concentrates have used concentrates that varied by more than two ingredients [
9,
12,
15,
29], and thus, the concentrates varied in some chemical components, such as the FA profile [
5]. In the present study, the FA profiles differed among the concentrates. The differences observed in the content of C16:0 are in line with the proportion of palm oil in the concentrate, which was 1.0%, 2.4%, 2.6% and 1.4% in 0%pea, 10%pea, 20%pea and 30%pea, respectively [
11]. In concordance with the present results, the literature showed that when pea replaced SBM in the diets of lambs, the resulting differences in C16:0, C18:0, C18:1 c9, C18:2 n-6 and C18:3 n-3 ranged from 12% to 32%, 12% to 30%, 3% to 16%, 1% to 24% and 15% to 57%, respectively [
12,
15,
30]. Similarly, the ingredients also contain different contents of carotenoids, especially tocopherols, causing differences in the overall contents of the concentrates. The contents in the concentrates, however, were low, and the differences in the contents of carotenoids and tocopherols among concentrates, although significant, were narrow.
The effects of pea inclusion on the colours of the RA muscle and fat deposits have seldom been evaluated in fattening lambs. The proportion of pea in the concentrate did not affect the RA colour parameters, with similar values to those previously reported in light lambs fed concentrates [
17,
31]. Regarding the fat colour, Lanza, Fabro, Scerra, Bella, Pagano, Brogna and Pennisi [
15] found no differences when lambs were fed 40% pea or 38% fava bean compared to 18% soybean meal for 79 d [
15]. Similarly, Bonanno, Tornambè, DiGrigoli, Genna, Bellina, DiMiceli and Giambalvo [
3] studied the effects of four different protein sources on fat and meat colour and concluded that only fat redness and chroma were affected by the source of protein, and overall, the carcass and meat characteristics were similar to those obtained with conventional SBM. In the current experiment, the LTL muscle did not show any effect from the substitution of SBM with pea on the meat colour; this result was in line with the similar colours reported in lambs that were fed concentrates with SBM or 40%pea for 79 d [
15], concentrates with 18%pea and 39%pea for 43 d [
29], and concentrates with 25%pea for 49 d [
30]. This lack of effect of the proportion of pea on the fat and meat colour may be due to the narrow differences in carotenoid contents among concentrates, regardless of statistical significance, in addition to the short experimental period (41 d). The time of air exposure affected all colour parameters, and haem pigments studied in the LTL, as reported in light lambs of the same breed that were fed commercial concentrates [
17,
32]. Usually, colour variables increase due to blooming and increased MMb contents, a plateau of approximately 5 days follows, and then discolouration occurs in the meat of light lambs [
33]. Accordingly, in the current experiment, the colour variables increased at the beginning, plateaued until day 9 of air exposure when there was an increase of the Hue angle; an abrupt change is a good indicator of discolouration in the meat of light lambs regardless of the absolute value, and the greater MMb content, which indicates discolouration [
34] and the end of the shelf life of meat. The lack of effects observed on either the fat or the meat colour can be considered positive because the meat colour is the main trait influencing consumer choice of light lamb [
35].
The absence of any effect on the texture and shear force was in agreement with the similar values reported for lambs fed 86%pea for 48 d [
3] or for lambs fed concentrates with 40%pea for 42–72 d [
9,
15] when compared to those fed SBM. This result indicates that the ingredients in the fattening diets of lambs have scarce effects on the texture parameters of lamb meat. The oxidative stability of meat depends on the balance between the pro-oxidant compound (i.e., total unsaturated FA, cholesterol and DMb) and antioxidant compound (tocopherols, carotenoids, … [
36]) contents. In the current experiment, the mild differences observed in the unsaturated FAs and antioxidant compound contents among concentrates were not enough to elicit an effect on the lipid oxidation of the meat.
The absence of any effect on the chemical composition of the meat was expected because all diets were iso-proteic and iso-energetic; this result is in agreement with previous experiments that studied the effects of the inclusion of pea in concentrates in several doses [
29,
37]. The greater cholesterol content observed in the 20%pea treatment than in the 30%pea treatment agrees with the differences observed in the concentration of C16:0 in the LTL muscle (see below). This FA increases the plasma total and LDL cholesterol content [
38]. However, this difference did not reach statistical significance in the plasma of these lambs at the time of slaughter [
11].
The differences observed in the FA content of the concentrates were not exactly mirrored in the FA profile of the lamb meat due to the process of biohydrogenation of PUFA conducted by ruminal microorganisms [
39]. The inclusion of pea had a minor effect on individual FAs in the muscle, as reported in light lambs fed pea instead of SBM in concentrates for 42–48 d [
3,
30]. However, when lambs were fed peas or SBM in concentrates for longer feeding periods, the most relevant FAs were affected, but these effects differed depending on the studies. The total replacement of SBM by pea for 72 d increased the C18:1 c9 and C18:3 n-3 contents but decreased the C18:1 t11 content [
15], whereas the total replacement of SBM with pea for 98 d increased the C18:2 n-6 and C18:3 n-3 contents while decreasing the C16:0 and C18:0 contents [
12]. The discrepancies among studies can be partially related to the amplitudes of the differences in the major FAs in the applied diets. The FAs in meat were affected only when the differences in the C16:0 and C18:0 contents in the diets of the lambs were above 30% [
12] or when the differences in the C18:3 n-3 contents among diets were above 28% [
12,
15]. In the current experiment, the slight increase in certain individual SFAs in the meat of the lambs fed 20%pea concentrate concomitantly increased the total SFAs when compared to their counterparts, with no effects on the other FA sums. However, the total replacement of SBM in concentrate by 86%pea or 40%pea had no effect on the SFA, MUFA or PUFA contents in meat [
3,
15], whereas the inclusion of 24–25%pea decreased the total SFA content in meat and increased the MUFA and n-3 PUFA contents [
12,
30]. The slight differences in fatty acids led to differences in the thrombogenicity index, with the 20%pea concentrate yielding the greatest value, making this concentrate the least advisable. In this sense, the consumption of food with a low IA and IT has a better nutritional quality, which may reduce the risk of coronary heart disease, but no organisation has yet provided the recommended values for the IA and IT [
40]. However, the impact of this difference on human health would be mild.