Carcass characteristics and meat quality
Linseed supplementation had no effect (P > 0.05) on carcass weight, average daily food intake, average daily gain, dressing loss, backfat thickness, however, it did increase (P < 0.05) loin-eye area which is mainly used to evaluate carcass meat production (Table 3). It was also observed that linseed supplementation significantly reduced pH24h (P < 0.01), L* (P < 0.05) and shear force (P < 0.01), but had no effect (P > 0.05) on pH45min, a*, b* and cook loss (Table 4). Additionally, crude protein and ash significantly increased (P < 0.01) in the linseed group (Table 5).
Table 3
Effect of dietary linseed supplementation on carcass traits of sheep
Item | Control group | Linseed group | P-value |
Initial weight, kg | 16.69 ± 1.80 | 16.03 ± 0.91 | 0.272 |
Final weight, kg | 31.96 ± 4.82 | 33.51 ± 2.41 | 0.331 |
Carcass weight, kg | 14.07 ± 2.27 | 14.08 ± 1.38 | 0.983 |
Dressing percentage, % | 43.94 ± 1.66 | 42.04 ± 2.85 | 0.883 |
backfat thickness, mm | 3.97 ± 0.83 | 4.43 ± 1.29 | 0.331 |
Loin-eye area, cm2 | 12.24 ± 2.42 | 14.78 ± 2.10 | 0.028 |
Average daily food intake, kg/d | 1.90 ± 0.13 | 1.89 ± 0.15 | 0.258 |
Average daily gain, kg/d | 0.17 ± 0.04 | 0.19 ± 0.03 | 0.225 |
Table 4
Effect of linseed supplementation on sheep muscle quality
Item | Control group | Linseed group | P-value |
pH45min | 6.40 ± 0.20 | 6.32 ± 0.39 | 0.555 |
pH24h | 5.73 ± 0.16 | 5.45 ± 0.18 | 0.001 |
L* | 35.13 ± 0.85 | 33.78 ± 1.41 | 0.026 |
a* | 17.14 ± 1.13 | 18.09 ± 1.43 | 0.093 |
b* | 2.92 ± 0.53 | 2.77 ± 0.55 | 0.526 |
Shear force, N | 86.23 ± 15.76 | 69.78 ± 7.41 | 0.005 |
Cook loss, % | 39.58 ± 3.42 | 39.82 ± 7.44 | 0.922 |
Table 5
Effect of linseed supplementation on the chemical composition of sheep muscle
Item (%) | Control group | Linseed group | P-value |
Intramuscular Fat | 2.53 ± 0.97 | 2.03 ± 0.85 | 0.211 |
Crude Protein | 21.69 ± 1.11 | 22.92 ± 0.97 | 0.01 |
Moisture | 76.63 ± 2.46 | 75.76 ± 1.26 | 0.296 |
Ash | 1.64 ± 0.27 | 2.11 ± 0.33 | 0.001 |
Several reports have shown that dietary supplementation with linseed had no significant effect on growth performance, carcass traits, meat quality and chemical composition[20–24]. In the present study, however, linseed supplementation did exert a significant effect on carcass traits, meat quality and chemical composition. Conflicting results might be related to the level of linseed supplementation and the duration of the feeding trial [25]. For example, A Guerrero, C Sañudo, MM Campo, JL Olleta, E Muela, RMG Macedo and FAF Macedo [26] reported that a diet supplemented with 10% linseed and fattening for either 30 or 50 d could improve the fatty acid composition, productive, carcass characteristics and meat quality of cull ewes. According to R Marino, M Caroprese, G Annicchiarico, F Ciampi, MG Ciliberti, Ad Malva, A Santillo, A Sevi and M Albenzio [4], dietary linseed supplementation improved the meat tenderness and agrees with our current observations, which showed that dietary supplementation with linseed exhibited a lower shear force. The pH of muscle after slaughter is an important indicator of glycolytic activity; and was the basis for identifying potential PSE and DFD. In agreement with CE Devine, AE Graafhui, PD Muir and BB Chrystall [27], the pH45min and pH24h fell within the normal range. The pH24h in the linseed group was lower than that of the control group. This difference in pH may account for the difference in muscle fiber composition between the two groups since muscle fiber type is related to the ultimate pH [28, 29]. In the case of meat color, supplementation with linseed only reduced L* values, which is contrary to previous [4, 30].
Major volatile flavors in LL muscle including alcohols, aldehydes, ketones and hydrocarbons are given in Table 6. Aldehydes were the most, prominent followed by alcohols. A total of 47 volatile compounds were detected in current study, of which 33 volatiles were detected in the control group. In contrast, the linseed group contained 38 volatiles. The relative content of 1-pentanol, hexanal, nonanal, 2,3-octanedione and allyl 2-ethyl butyrate in the control group were significantly higher than that in the linseed group (P < 0.05). Dietary linseed supplementation significantly increased the relative content of Z-10-Pentadecen-1-ol, pentanal, 2-Octenal, (E)-, decanal, butane and 2-heptanone (P < 0.05).
Table 6
Effect of linseed supplementation on volatile flavor substances in sheep muscle
Item | Compound | Control group (%) | Linseed group (%) | P-value |
Alcohols | | 39.48 ± 6.12 | 44.04 ± 8.14 | NS |
| 2-Penten-1-ol, (E)- | 0.58 ± 0.35 | ND | ND |
; | 1-Pentanol | 7.30 ± 1.49 | 4.50 ± 1.72 | 0.003 |
| 3-Methyl-2-butanol | 3.06 ± 1.88 | ND | ND |
| Cyclopentanol, 3-methyl- | ND | 1.83 ± 0.77 | ND |
| 1-Hexanol | 4.10 ± 1.62 | 4.08 ± 1.33 | NS |
| 1-Heptanol | 4.47 ± 1.03 | 4.63 ± 1.57 | NS |
| 3-Octyn-2-ol | ND | 0.22 ± 0.52 | ND |
| 1-Octen-3-ol | 10.41 ± 4.81 | 9.07 ± 5.08 | NS |
| 2-Octen-1-ol, (Z)- | 1.90 ± 0.64 | 2.04 ± 0.79 | NS |
| Cyclohexanol, 2,4-dimethyl- | ND | 0.74 ± 0.37 | ND |
| 1-Hexanol, 2-ethyl- | 1.66 ± 0.86 | 2.06 ± 0.80 | NS |
| 1-Octanol | 7.18 ± 2.09 | 8.11 ± 1.98 | NS |
| 2,4-Decadien-1-ol | ND | 1.23 ± 0.56 | ND |
| 4-Methyl-5-decanol | ND | 2.03 ± 1.53 | ND |
| Z,Z-2,5-Pentadecadien-1-ol | 0.71 ± 0.44 | 0.50 ± 0.24 | NS |
| Z-10-Pentadecen-1-ol | 0.66 ± 0.22 | 1.50 ± 0.20 | 0.005 |
| 2-Hexadecanol | 1.52 ± 0.80 | 1.82 ± 0.99 | NS |
Aldehydes | | 39.22 ± 11.29 | 37.44 ± 9.71 | NS |
| Butanal, 3-hydroxy- | 0.89 ± 0.36 | 2.37 ± 0.91 | NS |
| Glutaraldehyde | ND | 1.67 ± 0.80 | ND |
| Pentanal | 1.02 ± 0.26 | 4.37 ± 2.09 | 0.003 |
| Hexanal | 12.38 ± 5.61 | 0.88 ± 0.46 | 0.000 |
| Benzaldehyde | 1.71 ± 0.97 | ND | ND |
| 2-Heptenal, (Z)- | ND | 3.38 ± 0.79 | ND |
| Heptanal | 4.72 ± 1.31 | 4.81 ± 1.31 | NS |
| 2-Octenal, (E)- | 0.78 ± 0.28 | 1.41 ± 0.47 | 0.005 |
| Octanal | 4.27 ± 1.79 | 4.08 ± 1.39 | NS |
| 2,4-Nonadienal | ND | 0.62 ± 0.31 | ND |
| 2-Nonenal, (E)- | 0.44 ± 0.12 | 0.48 ± 0.09 | NS |
| Nonanal | 11.12 ± 2.60 | 7.73 ± 3.24 | 0.012 |
| 2,4-Decadienal, (E,E)- | ND | 0.62 ± 0.19 | ND |
| 2-Decenal, (E)- | 0.96 ± 0.32 | 1.08 ± 0.36 | NS |
| Decanal | 0.97 ± 0.35 | 1.42 ± 0.45 | 0.023 |
| Undecanal | 0.73 ± 0.15 | ND | ND |
| 2,4-Dodecadienal, (E,E)- | ND | 1.31 ± 0.83 | ND |
| Dodecanal | ND | 0.87 ± 0.28 | ND |
| Tetradecanal | ND | 0.93 ± 0.25 | ND |
Hydrocarbons | | 5.41 ± 1.26 | 10.31 ± 3.72 | 0.004 |
| Butane | 3.18 ± 0.84 | 9.57 ± 1.26 | 0.001 |
| Pentane, 3-methyl- | 1.85 ± 0.74 | ND | ND |
| Cyclopentane, methyl- | 1.99 ± 0.63 | ND | ND |
Ketones | | 5.01 ± 2.65 | 4.35 ± 1.89 | NS |
| 2-Heptanone | 1.10 ± 0.16 | 1.42 ± 0.40 | 0.047 |
| 2,3-Octanedione | 3.47 ± 1.56 | 1.45 ± 0.66 | 0.002 |
| 3-Undecanone | ND | 0.65 ± 0.18 | ND |
Others compounds | | 4.77 ± 1.49 | 1.13 ± 0.37 | 0.000 |
| Phenol | ND | 1.03 ± 0.65 | ND |
| Hexanoic acid, ethyl ester | ND | 0.54 ± 0.38 | ND |
| Allyl 2-ethyl butyrate | 3.07 ± 1.50 | 1.13 ± 0.38 | 0.000 |
| Formamide, N, N-dibutyl- | 0.68 ± 0.24 | ND | ND |
| Cyclohexaneacetic acid, 2-phenyl- | 0.52 ± 0.15 | ND | ND |
Note: ND, not detected; NS, nonsignificant |
Aldehydes, which had a low threshold and were mostly derived from fat oxidation and degradation, moreover, it played an important role in the formation of mutton flavor [31]. Linseed significantly increased the relative content of pentanal and enhanced the fruity odors of mutton [32]. However, the relative content of hexanal was significantly reduced in linseed group, indicating that the control group had stronger grass like odors [32]. Hexanal was derived from the oxidation of linoleic acid and arachidonic acid. The reason why the relative content of hexanal in the control group was higher than that in the linseed group may be related to the antioxidant capacity of meat. The increase of antioxidant capacity can reduce the degree of oxidation of unsaturated fatty acids, thereby reducing the content of hexanal, the product of linoleic acid oxidation [3, 33]. Linseed significantly increased the relative content of 2-octenal, (E)-,which provided a stronger meat and fatty and green odors [32]. Relative content of nonanal in the control group was significantly higher than that in the linseed group. Nonanal is associated with various odors in mutton including fat, floral and citrus [34]. 2,4-Decadienal, (E,E)- was a product of linoleic acid, which had a meaty and grilled odors, and was a unique flavor substance in the linseed group [33]. The relative content of decanal in the linseed group was significantly higher in the control group (P < 0.05), contributing to stronger odors of soap, orange peel and tallow in mutton [32]. Undecanal were the special flavor substances in the control group, which exhibited fat, wax and soapy odors [34]. Dodecanal was endemic to the linseed group and has been reported to have an onion and yeast odor [34].
The formation of meat flavors is generally related to low thresholds of volatile compounds. For example, aldehyde compounds, produced by the oxidation of fat were reported by [35] to have low thresholds with respect to smell. Compared with glycolytic muscle fiber, oxidized muscle fibers had higher a content of phospholipid [36] which were one of the main substances affecting meat. flavor Free amino acids are also vital flavor precursors in meat [37]. D Mashima, Y Oka, T Gotoh, S Tomonaga, S Sawano, M Nakamura, R Tatsumi and W Mizunoya [38] reported that there was a positive correlation between the MyHC Ⅰ and total free amino acid concentrations. These results revealed that muscle fiber type had a strong effect on the flavor of meat. In addition, PUFA in meat were prone to oxidation and were important precursors of volatile flavor substances. But excessive oxidation could cause bad smell. According to ND Cameron and MB Enser [39], PUFA in meat was easily oxidized which was inversely related to flavor. Therefore, the difference in flavor may be related to the antioxidant capacity. The antioxidant system and the oxidizing system restrict each other to prevent the occurrence of lipid peroxidation.
Muscle fiber characteristics and MyHC isoforms mRNA expression level
Representative photomicrographs of control and linseed fed groups are shown in Fig. 1. Muscle fibers were classified into types: I (black), IIA (white) and IIB (brown) using myosin ATPase staining. Muscle fiber characteristics are shown in the Table 7. Linseed supplementation decreased CSA (P < 0.01), increased total fiber number and density (P < 0.01). The fiber diameters of types I, IIA and IIB were observed to decrease (P < 0.01) with linseed supplementation, but had no effect on muscle fiber numbers and area composition (P > 0.1). According to the polymorphism of the myosin heavy chain (MyHC), muscle fibers can be divided into four types: MyHC I (slow-twitch oxidative), MyHC IIa (fast-twitch oxidative), MyHC IIx (fast-twitch oxidative-glycolytic) and MyHC IIb (fast-twitch glycolytic) [40]and serve as a fast, reliable and accurate method to identify muscle fiber types. As shown in Fig. 2, linseed supplementation increased the mRNA expression of MyHCⅠand MyHC Ⅱx (P < 0.01), but had no effect on the mRNA expression level of MyHCⅡa and MyHC Ⅱb. These results suggest that the number of oxidized muscle fibers were higher in the linseed group. Muscle from the linseed group contained increased (P < 0.01) activity of SDH which is a key enzyme involved in aerobic metabolism. In addition, a decrease (P < 0.01) in LDH activity, major enzymes involved in anaerobic glycolysis, were observed (Fig. 3). These results suggest that a diet supplemented with linseed increased the oxidative metabolism of muscle. Several reports have shown that the of number muscle fibers contributed to no significant changes in postnatal mammalian muscles [41, 42]. Therefore, muscle fiber hypertrophy in postnatal lies with the total number of muscle fibers within a muscle [42]. This finding was consistent with that of AJ Fahey, JM Brameld, T Parr and PJ Buttery [43] who found that maintenance, nutrition, and mobility of lambs during the postnatal period had no effect on the muscle fibers numbers and fiber types However, nutrition did have a significant effect on muscle fiber diameter. Fewer number of muscle fibers may lead to an increase in the muscle fiber diameter [44]. This finding was also reported by F Gondret, L Lefaucheur, H Juin, I Louveau and B Lebret [45]. In current study, linseed supplementation resulted in higher total number of muscle fiber and lower muscle fiber diameters. In the control group lower numbers of muscle fiber and higher muscle fiber diameters were consistent with previous research results. A relationship between the number and size of muscle fibers and muscle mass has been reported [15, 46]. Several reports have shown that mean CSA fibers [47] and muscle fibers diameter [48] were negatively related to tenderness. Moreover, a previous study has shown that shear force was negatively related to tenderness [49]. All in all, as the muscle fiber diameter and CSA increased, the shear force increased, and the tenderness decreased. In our study, dietary linseed supplementation decreased CSA and the diameter of muscle fibers, meanwhile the lower shear force value was observed in linseed group. These results fit well with the previous several studies. According to G-D Kim, J-Y Jeong, E-Y Jung, H-S Yang, H-T Lim and S-T Joo [50], the higher proportion of fast-twitch glycolytic (IIB) fibers may lead to L* values and the rate and extent of pH decline were increased. A study in Simmental hybrids found that MyHC I and MyHC IIa were negatively correlated with and intramuscular fat (IMF) content and meat shearing force (MSF), MyHC IIx was inverse associated with MSF [51]. Our present study indicated that dietary supplementation with linseed increased the mRNA expression level of MyHC I and MyHC IIx, along with decreased CSA and muscle fiber diameter, meanwhile enhanced the activity of SDH and decreased the activity of LDH, which may partly explain why linseed can improve the meat quality of sheep as demonstrated in the current study.
Table 7
Effect of linseed supplementation on muscle fiber characteristics
Trait | Control Group | Linseed Group | P-value |
Mean CSA fibers (µm2) | 1435.09 ± 172.79 | 1104.31 ± 215.03 | 0.001 |
Total muscle fiber number (× 103) | 853.46 ± 238.02 | 1200.89 ± 154.23 | 0.001 |
Density of muscle fibers (mm2) | 688.01 ± 111.13 | 901.41 ± 167.08 | 0.003 |
Muscle fiber number composition (%) | | | |
Type Ⅰ | 8.89 ± 2.45 | 9.80 ± 3.34 | 0.485 |
Type ⅡA | 32.51 ± 5.87 | 35.73 ± 7.18 | 0.274 |
Type ⅡB | 57.68 ± 5.64 | 55.39 ± 6.78 | 0.274 |
Muscle fiber area composition (%) | | | |
Type Ⅰ | 8.11 ± 2.18 | 6.91 ± 1.72 | 0.139 |
Type ⅡA | 38.19 ± 9.41 | 40.67 ± 9.50 | 0.554 |
Type ⅡB | 53.71 ± 8.52 | 52.42 ± 9.40 | 0.942 |
Mean diameter of muscle fibers | | | |
Type Ⅰ | 41.85 ± 5.90 | 34.04 ± 5.24 | 0.005 |
Type ⅡA | 44.95 ± 3.18 | 40.21 ± 4.24 | 0.009 |
Type ⅡB | 38.28 ± 3.43 | 32.67 ± 3.69 | 0.002 |
Muscle antioxidative capacity
Dietary supplementation with linseed had no effect (P > 0.10) on muscle CAT and GSH-PX enzyme activity but did result in greater (P < 0.01) T-AOC activity (Table 8) and the mRNA level of GSH-PX (P < 0.01) and CAT (P < 0.05) (Fig. 4), and decreased T-SOD enzyme activity and mRNA level (Table 9, Fig. 4). Moreover, MDA content was decreased (P < 0.01) by dietary supplementation with linseed. Lipid peroxidation had a negative effect on meat quality [52]. Therefore, it is common knowledge that inhibit oxidation and reduce lipid peroxidation by dietary natural antioxidants were effective for improving meat quality. Previous studies had shown that flaxseeds are rich in phenolic substances which can directly participate in anti-free radicals active [13]. I Moñino, C Martínez, JA Sotomayor, A Lafuente and MJ Jordán [53] reported that sheep which are rich in polyphenols in the diet, free radical scavenging ability will also be enhanced. Because of MDA was a secondary product of lipid oxidatirron, MDA content could reflect the degree of lipid peroxidation in the tissue [54]. Therefore, MDA may indicate an antioxidant capacity. In current study, T-AOC activity and the mRNA level of GSH-PX and CAT were increased, MDA was decreased in linseed group, which results indicated that dietary linseed supplementation can enhance antioxidant capacity of sheep. This finding was consistent with that of LB Pouzo, AM Descalzo, NE Zaritzky, L Rossetti and E Pavan [55] who found that by supplementing with low levels of flaxseed, the antioxidant capacity of beef can be improved. SOD was the first enzyme to fight oxidative stress, the content of flaxseed oil and fat were as high as 30–45%, and the oil and fat may act as a pro-oxidant to inhibit the activity of SOD, which may be the reason for the decreased SOD activity of linseed group [56, 57]. Moreover, at the same time point, aldehyde compounds which was considered to be a characteristic product of the oxidation process [35] had changed was observed in linseed group, which may be related to increased antioxidant capacity. In general, we speculate that the improvement of meat quality and the change of flavor substances may be attributed to the enhancement of antioxidant capacity.
Table 8
Effect of linseed supplementation on antioxidant activity
Item | Control group | Linseed group | P-value |
T-SOD, U/mg prot | 41.86 ± 2.35 | 37.53 ± 3.79 | 0.008 |
CAT, U/µg prot | 4.86 ± 0.61 | 4.41 ± 0.46 | 0.564 |
GPH-PX, U/mg prot | 36.99 ± 3.23 | 38.83 ± 2.92 | 0.678 |
T-AOC, U/mg prot | 0.35 ± 0.04 | 0.53 ± 0.03 | 0.001 |
MDA, nmol/mg | 2.96 ± 0.13 | 1.23 ± 0.11 | 0.001 |