Results were analyzed first as a pull, studying the effects of feeding, muscle, packaging and display on all the variables and their respective interactions. However, because of the number of significant interactions between packaging and the rest of the effects, and in order to understand better the behavior of the two muscles and the effect of diet, the results have been separated by packaging.
3.1. Lipid Oxidation
The effects of feeding, muscle and display, and their interactions, on lipid oxidation are shown in
Table 1. Feeding had a significant influence on lipid oxidation when samples were stored under vacuum conditions (
p = 0.045), but did not significantly affect it when the samples were stored under MAP (
p = 0.057), probably because the high presence of oxygen in MAP had the largest influence on lipid oxidation throughout display (
p < 0.001), masking other effects. However, a significant interaction was observed between feeding and display. When samples were stored under vacuum conditions, meat from SIL-fed groups showed higher lipid oxidation (
p = 0.036) at 14 days of storage (
Table 2). Although feeding did not influence oxidation in MAP (
p = 0.057), meat at 7 days of display from CON showed higher lipid oxidation than meat from the SIL group (0.970 vs. 0.672 mg of MDA/kg of meat, respectively). Other authors have reported lower oxidation in meat from grass silage-fed animals with higher vitamin E concentrations when compared to meat from maize-fed animals. [
31,
32]. The use of maize silage could preserve the meat from lipid oxidation more effectively than meat from concentrate-fed animals. However, it is probable that a higher proportion of PUFA could increase the lipid oxidation, especially during storage [
33]. In both types of packaging, lipid oxidation did not differ between muscles (
Table 2).
3.2. Color
The lipid oxidation values of meat stored in MAP packaging were higher than those for meat samples stored in vacuum, and reached 2 mg of MDA/kg of meat (
Table 2), which is considered the limiting threshold for acceptability [
34]. This threshold was reached at 7 days in MAP, but was not reached at all in vacuum-packaged meat. Other researchers have reported an increase in oxidation throughout the display of fresh beef [
35]. Modified-atmosphere packaging resulted in more oxidation than vacuum packaging, limiting the shelf life of MAP packaging [
11], as expected considering that the higher level of oxygen and the prooxidation effect of the light during display boosted lipid oxidation [
36].
Feed significantly affected lightness (L*), redness (a*), chroma (C*) and hue (H°) when the meat was stored in vacuum packaging, but did not affect yellowness (b*) or discoloration (DEC). Furthermore, muscle type affected all color parameters except redness and chroma. Packaging meat in a modified atmosphere had a greater effect on color, and masked any effect that might be derived from the feed. Meanwhile, muscle type affected the rate of discoloration, but none of the other color variables (
Table 1).
Figure 1 shows the significant interactions between muscle and packaging conditions. Meat from animals fed SIL was lighter (L*) than meat from CON-fed animals when stored in vacuum packaging. These results contradict those obtained by Avilés et al. [
2], where the meat was darker from animals fed on total mixed ration than from animals finished on concentrate. The ST muscle presented higher luminosity than samples from LT, independently of the packaging, probably because of the oxidative activity of the semitendinosus, having less color stability than the LT [
37,
38]. Lightness increased as the time of display increased, especially in MAP, due to the higher availability of oxygen (
Figure 1).
Feeding (
p < 0.001) and display (
p < 0.05) had a significant effect on redness (a*) when samples were stored in VAC (
Table 1). Samples from animals fed CON were redder than samples from animals fed SIL, especially after 7 days of storage (
Figure 1). When samples were stored in MAP packaging, both muscle and display time affected (
p < 0.001) redness (
Figure 1). The ST muscle had a similar red color from 0 to 7 days, while that of the LT increased up to 7 days of storage, and then both muscles showed a decline in redness up to 14 days of display. At 14 days of display, MAP packaging resulted in meat with lower red values than VAC packaging, independently of the type of feeding. It is known that oxymyoglobin increases during the first days of storage in MAP because of the presence of oxygen; however, longer display times induce the formation of MetMb, and thus lower color stability, during storage in MAP [
39]. Meat color stability is the net result of autoxidation and the reduction of myoglobin. It is known that in the post-mortem period, the MetMb-reducing activity decreases [
40,
41,
42], which could explain the lowest redness of meat after 14 days of storage in MAP.
Yellowness values (b*) were affected by muscle and display in both packagings, but not by feeding (
Table 1), even though silage might include a higher content of carotenoids that could have been incorporated into the intramuscular fat, changing the color towards more yellow notes [
43]. In older animals, this effect would probably have been more evident. The ST muscle was more yellow than the LT muscle in both packaging systems. However, the behaviors throughout display were different, since MAP meat had higher values of yellowness than VAC meat, especially at 7 days of display (
Figure 1).
Chroma was affected by feeding and muscle in VAC-packaged meat (
Figure 2). Meat from animals finished on concentrates had higher chroma values than meat from animals fed silage. Furthermore, chroma values were higher for ST than LT. The chroma values were influenced by muscle and display time, but not feeding, in meat stored in MAP packaging. At 0 and 14 days of display, samples from the ST muscle presented higher chroma values than LT; however, at 7 days of display, these differences were not shown. Samples in MAP had chroma values higher than samples in vacuum packaging, but after 14 days of display these differences disappeared. This is the consequence of the faster oxygenation rate in samples surrounded by a high oxygen atmosphere, as happens in MAP, and the formation of metmyoglobin (MMb) afterwards. This MMb formation also happens under vacuum conditions, but the residual oxygen in the packaging cannot stimulate the formation of oxymioglobin at the same rate as happens in MAP [
44]. At 7 days of display, samples from both muscles in MAP had chroma values over 18. Chroma values above 18 have been reported as acceptable to consumers [
45]. This suggested threshold was based on older animals than the yearlings used in the present study and meat from grass-fed animals. Nevertheless, our results are in agreement with those of Casasús et al. [
5], who also found that corn silage-fed yearlings had chroma values between 20 and 18 during the 13 days of display. Some authors have reported acceptable chroma values lower than 18 [
46] in young animals, which do not show the degree of color saturation that old or grass-fed animals can reach. Therefore, VAC samples could have chroma values that are also acceptable for consumers.
Hue values were affected by muscle and display time in both VAC and MAP packaging, while feed effect was only significant in samples that were vacuum-packaged (
Table 1). Samples from corn silage-fed animals showed higher hue values, especially in the LT muscle (
Figure 2). Under VAC, hue increased from day 0 to day 7, and after that stayed stable up to 14 days of display. Meat from the ST muscle had higher hue values than meat from the LT muscle, no matter what packaging system was used. A partial explanation for this could be related to the different fiber types in each muscle, with the ST having more fast-switch fibers that would increase the hue [
23]. Besides, some authors have suggested that the ST muscle has greater lightness and hue, but less redness [
47,
48]. Under MAP, the hue angle increased from day 0 to day 14, although that increment was more pronounced in ST than LT.
In samples that had been vacuum-packed, discoloration was significantly affected by muscle (
Table 1). Discoloration occurred faster in the ST muscle than the LT. The VAC system resulted in little discoloration from 0 to 14 days of storage. Feeding had no effect (
Table 1). Without subjecting samples to display, Wales et al. [
49] also reported that increasing the maize silage proportion of a concentrated ration of dry matter from 46% to 96% did not result in a difference either in muscle color or in fat. Similarly, Walsh et al. [
50] observed no differences in fat or muscle color when comparing two rations containing 74% of maize silage and 90% concentrate. On average, maize silage appears to have a similar effect to concentrate on the nutritional quality of beef [
43].
As shown in
Table 1, when MAP packaging was used, significant differences related to the muscle disappeared, and the DEC decreased as time of display increased (
Figure 3). Although modified-atmosphere packaging (MAP) is used to present meat in a more attractive way for the consumer, the shelf life of beef in MAP is limited when it has been previously stored under vacuum conditions, due to the subsequent decrease in color stability during display [
20,
51,
52]. The decrease was more pronounced in ST than LT muscle, because in LT, between 0 and 7 days, there were no differences and no degradations in the color. This might be the effect of the different fiber compositions of these two muscles, with a higher number of white fibers that are more glycolytic and hence more unstable in ST than in LD. Some authors have also reported that muscles such as LT have a more stable color than other muscles such as ST [
37,
38].
The consumer often tends to associate color with several attributes, such as flavor, tenderness, safety, storage time, nutritional value and satisfaction level [
53]. Color allows the detection of certain anomalies or defects that food items may present [
54,
55,
56], and redness in particular is used as an indicator of freshness [
57].
3.3. Visual Color Acceptability and Purchase Choice
Visual color acceptability was influenced (
p < 0.001) by feeding, muscle and display, independently of the packaging used in the display—MAP or FILM. As is shown in
Table 3, meat from the ST of CON-fed animals and kept in FILM packaging presented better color acceptability than meat from the LT muscle during the first 3 days. After that, the LT muscle showed higher scores than ST. In the case of meat from animals fed with corn silage, the better color acceptability of the LT muscle occurred after 2 days of display. In both types of feeding, the LT muscle presented better visual color acceptability throughout display, the scores being above 5 until day 7 of display. In the case of ST, after 4 days of display the scores were below 5. Thus, the LT muscle in FILM packaging had better acceptability than the ST muscle.
The samples in MAP kept their visual color acceptability longer than samples in FILM (
Table 4). Although in the first 3 days, the ST muscle from the SIL group showed better acceptability, at 4 days of display, the LT presented higher scores. The same happened in samples from the CON group after 4 days of display. Besides this, the visual color acceptability of the LT muscle lasted longer than that of the ST muscle, especially in samples from the SIL group. Meat from the LT muscle from SIL animals had scores over 5 until day 7 of display (5.48). In general, the use of modified-atmosphere packaging could preserve the color acceptability of beef more effectively than FILM. Thus, the SIL treatment was the most accepted in the LT muscle.
The percentages of positive assessments of buying meat from the FILM and MAP groups are presented in
Table 5 and
Table 6, respectively. As occurred with the visual color acceptability, in the case of the SIL group, in the first 2 days of display, the percentage of positive assessments of buying the meat was slightly higher in the ST muscle group than in the LT muscle group. However, at 3 days of display, meat from LT had a higher percentage of purchase intention than ST, and this behavior was maintained during the display. In FILM, the LT and ST samples from SIL-fed animals received 77.2% and 6.1% positive assessments of intention to buy at 6 days of display, respectively. LT and ST samples from CON-fed animals after 6 days of display had positive assessments of intention to buy of 50% and 2.3%, respectively. In the case of samples kept in FILM, the decrease was pronounced, especially in ST. At 5 days of display, the percentages of intention to purchase were 32.8% and 35.0% for CON and SIL, respectively, and at 6 days the percentages went down to 2.3% (CON) and 6.1% (SIL).
Meat in kept MAP packaging for 8 days of display had almost 50% positive assessments in both the CON and SIL groups of LT muscle (45.6% and 55.0%, respectively). However, MAP-packaged ST had less than 30% purchase choice after 7 days of display (CON: 28.3% and SIL: 24.4%). One of the most important characteristics of meat is the color. This attribute influences the acceptability of the product and plays a major role in the purchase decision [
44,
58,
59]. The greater deterioration of ST vs. LT implies the lower purchase acceptability of ST. Carpenter et al. [
60] found a strong relationship between color preferences and consumer purchasing decisions, as consumers discriminated against beef that was not red (e.g., purple or brown). Therefore, visual assessments are a gold standard for estimating consumer perception [
44].