Decrease in Intramuscular Levels of Phosphatidylethanolamine Bearing Arachidonic Acid During Postmortem Aging Depends on Meat Cuts and Breed
Abstract
The intramuscular phospholipid (PL) level is reported to decrease upon postmortem aging of beef, pork, and chicken, and the change affects meat qualities (e.g., myofibrillar fragmentation and drip loss); however, there is still little information on the effect of muscle position and breed on PL compositions. A preliminary study is performed to investigate the effect of intramuscular PL composition on postmortem aging of beef and venison. Loin and round parts of beef and venison are kept at 4 °C for 28 postmortem days to elucidate the maximum effect of meat aging. The arachidonic acid (20:4n-6) content in PL decreases in round meat during postmortem aging, while the PL 20:4n-6 content is not affected in loin meat. The PL molecular species analysis reveals that such content reduction is consistent with decreases in phosphatidylethanolamine (PtdEtn) levels, especially that bearing 20:4n-6. In addition, the content reduction by aging is affected by the meat cuts and the breed. These results suggest that PL variation depends on meat cuts and breed, and the release of 20:4n-6 from PL has a considerable influence on meat qualities owing to the high reactivity of the polyunsaturated chain. The present results will enable eventual improvement of the taste, smell, and texture of meat during postmortem aging.
Practical Applications: This preliminary study investigates the changes in intramuscular PL composition in beef and venison after postmortem aging. Consequently, PL variation depends on meat cuts and may be induced potentially owing to activity of cytosolic phospholipase A2. Meat properties (e.g., drip, flavor, and taste) are considered as affected by the PL hydrolysis and the 20:4n-6 release. Therefore, monitoring of variation of PL species might be useful to estimate the condition of meat quality by postmortem aging.
Our preliminary study investigates the changes in intramuscular phospholipid (PL) composition in beef and venison after postmortem aging. Postmortem aging decreases the phospholipid levels in meats. The 20:4n-6 is released from the meat phospholipid during postmortem aging. The two above-mentioned are considered due to hydrolyzation of the diacylglycerophospholipids in meats. The hydrolysis of phospholipid may depend on meat cuts and breed.
1 Introduction
Postmortem aging improves the factors including tenderness and contributes to the overall quality of meat.1 Meat becomes tender by the postmortem conversion of muscles to meat, and the meat tenderness and taste are related to degradation of the constituted proteins in muscle cells. It is well known that proteolysis of myofibrils and myofibrillar-associated proteins could be responsible for meat tenderization during postmortem aging.2 Moreover, Nishimura et al.3 reported that the free amino acid content in round meat from Holstein cattle increased during postmortem aging from 4 to 12 days after slaughter, and these increases were caused by the actions of aminopeptidases C and H. Thus, if appropriate postmortem aging is performed, the meat would exhibit good texture, taste, and flavor.
Phospholipids (PLs) are the main components of cell membranes, and the composition in terms of PL classes (e.g., ethanolamine glycerophospholipid (EtnGpl) and choline glycerophospholipid (ChoGpl)) and subclasses (i.e., diacyl, alkenylacyl, and alkylacyl) affects the membrane fluidity and hardness. The PL fatty acid composition also significantly affects the membrane properties. Subcutaneous fat, intermuscular fat, and intramuscular fat are also related to sensory attributes of cooked meat. Further, the microstructure such as myofibrillar Z-line contain the neutral lipid and PLs,4-6 and these PLs are released from myofibrillar Z-lines during postmortem aging7, 8; in addition, some reports have been published on the relationship among postmortem aging, meat quality, and PL content in meat9; PL hydrolysis affects the membrane structure and function, stimulating drip loss in porcine meat,10 and the activity of phospholipase A2 (PLA2) is correlated to drip loss in porcine meat during cooking.11 Therefore, it is important to identify the mechanisms of PL variation during postmortem aging. There are few reports on the effect of the muscle position and breed on PL compositions or PL contents.
The recently developed tandem mass spectrometry (MS/MS) technique offers definite benefits for the analysis of lipids, including PLs.12, 13 In our previous study, we developed a PL analysis method with high specificity and sensitivity based on liquid chromatography (LC)-MS/MS in the presence of alkali metal ions.14, 15 In particular, this method distinguished choline plasmalogen (PlsCho) and phosphatidylcholine (PtdCho) as ChoGpl subclasses.
The aim of the study was to evaluate the effect of postmortem aging on the intramuscular fatty acids and PL composition in cattle and Yezo sika deer primal cuts (round and loin). In this study, we set the aging time of 28 days to elucidate the maximum effect on the aging meat. We performed the detailed analysis of intramuscular PL content in two different cuts, namely loin and round cuts using gas chromatography (GC) and LC-MS/MS.
2 Experimental Section
2.1 Meat Samples and Postmortem Aging
Beef samples were obtained from local meat packers in Hokkaido, Japan. The loin and round (m. quadriceps femoris) beef samples were obtained from three female Holstien cows (delivered cow, 6–7 years old, slaughtered on May) at 3 days after slaughter. On the one hand, because Yezo sika deer (Cervus nippon yesoensis) is a wild animal, venison samples were purchased from special local butcher. The loin and round (m. quadriceps femoris) venison samples were obtained from three wild male Yezo sika deer (about 5 years old, hunted and slaughtered on Oct) at 1 day after slaughter. The age of the deer was judged from the horn. Each primal cut was deboned and cleaned from the external fat and connective tissue, and crude lean meat was divided into ≈300 g of meat, and vacuum packed in a polyvinyl chloride film. Vacuum-packaged beef were stored for 28 days at 4 °C. Vacuum-packaged venison was also stored for 28 days at 4 °C because the postmortem aging time of venison, which become tender at maximum, is unclear at well and we wanted to compare venison with beef. After 28 days, they were transferred immediately to a freezer at −30 °C as packaged, and maintained (maximum 4 months) until the next step.
2.2 Lipid Extraction and Assays
The lean meat was obtained after removal of fat and connective tissue from crude lean. It was minced with an Electric Meat Grinder (MK-63s, Panasonic Corporation, Osaka, Japan). The total lipid was extracted according to the method of Folch et al.16 PL fraction was fractionated on a silica column. The PL content was determined according to the method described by Rouser et al.17 The fatty acids were quantified by GC.18 Total lipid contents were calculated based on fatty acid profiles.
2.3 MS/MS Analysis
The PL molecular species were analyzed by LC-MS/MS operated in the multiple reaction monitoring (MRM) mode.15 A Shimadzu LC system, including a vacuum degasser, a quaternary pump, and an autosampler (Shimadzu, Kyoto, Japan), was equipped with a 4000 QTRAP mass spectrometer (AB SCIEX, Tokyo, Japan). The quantitation of PL species in meat was conducted before and after postmortem aging. Then, eight following species were analyzed: phosphatidylethanolamine (PtdEtn) (18:0/18:1 and 18:0/20:4), ethanolamine plasmalogen (PlsEtn) (18:0ol/18:1 and 18:0ol/20:4), PtdCho (18:0/18:1 and 18:0/20:4), and PlsCho (18:0ol/18:1 and 18:0ol/20:4). All concentrations were calculated based on an equation derived from each of the external standard curve. Each of the standards was obtained from Avanti Polar Lipids (Alabaster. AL, USA). The molecular species whose external standards were not available, such as 16:0/18:1-PtdEtn and 16:0/20:4-PtdEtn were semi-quantitated by comparing the peak area before and after postmortem aging.
2.4 Statistical Analysis
The results in Tables 1 and 2 were expressed as mean ± standard deviation. Individual change was calculated from the variation rate of lipid level after aging to that before aging in the same sample. The Student's t-test was used to compare two groups. Statistical analyses for Tables 1 and 2 were performed using BellCurve for Excel (Social Survey Research Information Co., Ltd., Tokyo, Japan).
| Loin | Round | |||||
|---|---|---|---|---|---|---|
| Before | After | Individual change | Before | After | Individual change | |
| (g or μmol/100 g wet wt)a) | (%)b) | (g or μmol/100 g wet wt) | (%) | |||
| Beef | ||||||
| Total lipid | 12 ± 1 | 11 ± 4 | 91 ± 22 | 3 ± 0 | 3 ± 0 | 108 ± 16 |
| 16:0 | 12994 ± 2156 | 11617 ± 5549 | 87 ± 31 | 2139 ± 254 | 2426 ± 474 | 114 ± 25 |
| 18:0 | 6331 ± 407 | 5880 ± 1121 | 92 ± 12 | 1226 ± 82 | 1440 ± 206 | 118 ± 17 |
| 18:ln-9 | 13569 ± 1823 | 11962 ± 5721 | 86 ± 30 | 2951 ± 179 | 3325 ± 673 | 113 ± 23 |
| 18:2n-6 | 1045 ± 273 | 1632 ± 1237 | 145 ± 75 | 837 ± 112 | 727 ± 175 | 87 ± 16 |
| 20:4n-6 | 90 ± 60 | 68 ± 8 | 100 ± 58 | 148 ± 10 | 99 ± 47 | 66 ± 28 |
| PL | 493 ± 23 | 502 ± 136 | 101 ± 25 | 881 ± 85 | 568 ± 205 | 64 ± 21c) |
| 16:0 | 100 ± 62 | 115 ± 24 | 146 ± 79 | 116 ± 41 | 120 ± 35 | 106 ± 16 |
| 18:0 | 95 ± 4 | 91 ± 11 | 95 ± 7 | 195 ± 32 | 103 ± 34c) | 55 ± 26c) |
| 18:ln-9 | 137 ± 67 | 147 ± 39 | 126 ± 59 | 187 ± 38 | 164 ± 39 | 87 ± 4c) |
| 18:2n-6 | 117 ± 31 | 118 ± 51 | 109 ± 36 | 252 ± 47 | 152 ± 67 | 60 ± 23c) |
| 20:4n-6 | 63 ± 20 | 60 ± 13 | 104 ± 41 | 133 ± 15 | 55 ± 33c) | 44 ± 31c) |
| Venison | ||||||
| Total lipid | 3 ± 1 | 3 ± 1 | 101 ± 37 | 2 ± 1 | 2 ± 0 | 114 ± 21 |
| 16:0 | 214 ± 87 | 160 ± 110 | 70 ± 19 | 230 ± 22 | 249 ± 108 | 107 ± 37 |
| 18:0 | 769 ± 130 | 732 ± 196 | 95 ± 11 | 837 ± 487 | 937 ± 537 | 114 ± 16 |
| 18:ln-9 | 1337 ± 83 | 1374 ± 248 | 103 ± 13 | 938 ± 71 | 1125 ± 370 | 119 ± 29 |
| 18:2n-6 | 326 ± 88 | 344 ± 216 | 99 ± 39 | 358 ± 17 | 434 ± 106 | 121 ± 26 |
| 20:4n-6 | 127 ± 66 | 97 ± 64 | 74 ± 12c) | 171 ± 54 | 162 ± 51 | 95 ± 13 |
| PL | 919 ± 72 | 758 ± 71 | 82 ± 3c) | 1102 ± 74 | 851 ± 99c) | 78 ± 14c) |
| 16:0 | 186 ± 113 | 176 ± 27 | 114 ± 52 | 143 ± 39 | 183 ± 26 | 136 ± 44 |
| 18:0 | 178 ± 22 | 142 ± 34 | 80 ± 15 | 245 ± 44 | 157 ± 26c) | 65 ± 13c) |
| 18:ln-9 | 252 ± 118 | 223 ± 23 | 100 ± 41 | 235 ± 44 | 252 ± 37 | 112 ± 35 |
| 18:2n-6 | 221 ± 104 | 176 ± 14 | 92 ± 43 | 313 ± 9 | 225 ± 39c) | 72 ± 10c) |
| 20:4n-6 | 118 ± 42 | 93 ± 19 | 91 ± 54 | 168 ± 33 | 77 ± 14c) | 48 ± 17c) |
- Mean ± standard deviation, n = 3.
- a) Only the unit of “Total lipid” is g/100 g wet wt.
- b) Individual change was calculated from the variation rate of the same sample during postmortem aging.
- c) These values present significant differences from those before meat aging.
| Loin | Round | |||||
|---|---|---|---|---|---|---|
| Before | After | Individual change | Before | After | Individual change | |
| (μmol/100 g wet wt) | (%)a) | (μmol/100 g wet wt) | (%) | |||
| Beef | ||||||
| EtnGpl | ||||||
| 18:0/18:1-PtdEtn | 45.90 ± 4.88 | 47.31 ± 18.79 | 103 ± 40 | 72.75 ± 29.95 | 46.84 ± 20.03 | 64 ± 4b) |
| 18:0/20:4-PtdEtn | 4.89 ± 0.71 | 7.65 ± 3.99 | 152 ± 60 | 19.18 ± 7.08 | 14.18 ± 4.84 | 76 ± 14b) |
| 18:0ol/18:1-PlsEtn | 2.17 ± 0.32 | 3.44 ± 1.50 | 157 ± 54 | 4.08 ± 1.53 | 3.60 ± 0.73 | 97 ± 42 |
| 18:0ol/20:4-PlsEtn | 5.54 ± 0.30 | 5.30 ± 2.17 | 95 ± 35 | 5.23 ± 2.45 | 4.67 ± 1.51 | 99 ± 36 |
| ChoGpl | ||||||
| 18:0/18:1-PtdCho | 2.43 ± 0.87 | 3.35 ± 3.10 | 179 ± 219 | 12.48 ± 6.14 | 7.62 ± 6.14 | 66 ± 46 |
| 18:0/20:4-PtdCho | 0.20 ± 0.08 | 0.21 ± 0.15 | 142 ± 155 | 0.73 ± 0.38 | 0.56 ± 0.51 | 79 ± 50 |
| 18:0ol/18:1-PlsCho | 0.10 ± 0.03 | 0.50 ± 0.43 | 543 ± 582 | 1.57 ± 0.91 | 1.48 ± 0.98 | 124 ± 125 |
| 18:0ol/20:4-PlsCho | 0.09 ± 0.03 | 0.24 ± 0.24 | 326 ± 394 | 0.47 ± 0.24 | 0.49 ± 0.34 | 126 ± 108 |
| Venison | ||||||
| EtnGpl | ||||||
| 18:0/18:1-PtdEtn | 34.18 ± 0.92 | 40.11 ± 12.18 | 126 ± 56 | 39.93 ± 13.10 | 35.74 ± 10.62 | 90 ± 5b) |
| 18:0/20:4-PtdEtn | 14.45 ± 8.34 | 5.41 ± 1.32 | 47 ± 24b) | 26.21 ± 5.05 | 7.30 ± 1.87b) | 28 ± 7b) |
| 18:0ol/18:1-PlsEtn | 0.76 ± 0.53 | 0.73 ± 0.14 | 122 ± 58 | 1.18 ± 0.14 | 0.85 ± 0.14b) | 72 ± 5b) |
| 18:0ol/20:4-PlsEtn | 2.07 ± 0.93 | 1.64 ± 0.04 | 90 ± 36 | 2.79 ± 0.46 | 1.53 ± 0.08b) | 56 ± 8b) |
| ChoGpl | ||||||
| 18:0/18:1-PtdCho | 2.50 ± 1.23 | 1.14 ± 1.23 | 82 ± 117 | 3.71 ± 2.76 | 6.14 ± 4.07 | 271 ± 266 |
| 18:0/20:4-PtdCho | 0.23 ± 0.11 | 0.08 ± 0.07 | 53 ± 68 | 0.30 ± 0.22 | 0.44 ± 0.30 | 243 ± 240 |
| 18:0ol/18:1-PlsCho | 0.19 ± 0.11 | 0.12 ± 0.13 | 99 ± 130 | 0.13 ± 0.90 | 0.33 ± 0.23 | 356 ± 321 |
| 18:0ol/20:4-PlsCho | 0.22 ± 0.11 | 0.10 ± 0.09 | 66 ± 73 | 0.13 ± 0.11 | 0.27 ± 0.16 | 321 ± 286 |
- Mean ± standard deviation, n = 3.
- a) Individual change was calculated from the variation rate of the same sample during postmortem aging.
- b) These values present significant differences from those before meat aging.
The data in Tables 3 and 4 were analyzed by the two-way factorial analysis of variance (ANOVA) using a Tukey's post-hoc test. These results were expressed as least squares mean and P value for main effects and these interactions. The statistical model included breed, meat cut, and aging as main effects, with slaughter group random variable. Two-way interactions were included in the tables (P value only) and discussed in the text. Further, statistical analyses for the results in Tables 3 and 4 were performed using SAS software (SAS Institute Japan, Tokyo, Japan). In all analyses, P < 0.05 was considered to indicate statistical significance.
| Breed | Cut meat | Aging | Significant differencesb) | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Beef | Venison | Loin | Round | Before | After | Breed | Cut meat | Aging | B × Cc) | B × Ad) | C × Ae) | |
| Total lipida) | 7.0 | 2.6 | 7.1 | 2.4 | 4.9 | 4.6 | *** | *** | ns | *** | ns | ns |
| 16:0 | 7293.7 | 213.2 | 6246.1 | 1260.9 | 3894.1 | 3612.8 | *** | *** | ns | *** | ns | ns |
| 18:0 | 3719.4 | 818.8 | 3428.1 | 1110.0 | 2290.8 | 2247.3 | *** | *** | ns | *** | ns | ns |
| 18:ln-9 | 7951.8 | 1183.5 | 7050.6 | 2084.7 | 4698.9 | 4436.4 | *** | *** | ns | *** | ns | ns |
| 18:2n-6 | 1060.2 | 365.6 | 836.8 | 589.0 | 641.2 | 784.5 | ** | ns | ns | ns | ns | ns |
| 20:4n-6 | 101.1 | 139.3 | 95.4 | 145.0 | 133.8 | 106.6 | ns | * | ns | ns | ns | ns |
| PL | 611.0 | 907.6 | 668.1 | 850.6 | 849.0 | 669.7 | *** | ** | * | ns | ns | * |
| 16:0 | 112.9 | 171.8 | 144.1 | 140.6 | 136.3 | 148.4 | * | ns | ns | ns | ns | ns |
| 18:0 | 121.1 | 180.4 | 126.5 | 175.1 | 178.2 | 123.4 | *** | ** | ** | ns | ns | ** |
| 18:ln-9 | 158.9 | 240.4 | 189.9 | 209.4 | 202.9 | 196.4 | ** | ns | ns | ns | ns | ns |
| 18:2n-6 | 160.1 | 233.7 | 158.2 | 235.6 | 226.0 | 167.8 | ** | ** | * | ns | ns | ns |
| 20:4n-6 | 77.6 | 113.8 | 83.3 | 108.1 | 120.2 | 71.2 | ** | * | ** | ns | ns | ** |
- a) The unit of “Total lipid” is g/100 g wet wt, and units of other lipids are µmol/100 g wet wt.
- b) ns = P > 0.05; * = P < 0.05; ** = P < 0.01; *** = P < 0.001.
- c–e)These letters indicate Breed × Cut meat, Breed × Aging, and Cut meat × Aging, respectively.
| Breed | Cut meat | Aging | Significant differencesb) | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Beef | Venison | Loin | Round | Before | After | Breed | Cut meat | Aging | B × Cc) | B × Ad) | C × Ae) | |
| EtnGpl | ||||||||||||
| 18:0/18:1-PtdEtna) | 53.20 | 37.49 | 41.88 | 48.82 | 48.19 | 42.50 | * | ns | ns | ns | ns | ns |
| 18:0/20:4-PtdEtn | 11.47 | 13.34 | 8.10 | 16.72 | 16.18 | 8.63 | ns | ** | ** | ns | ** | * |
| 18:0ol/18:1-PlsEtn | 3.32 | 0.88 | 1.78 | 2.43 | 2.05 | 2.15 | *** | ns | ns | ns | ns | ns |
| 18:0ol/20:4-PlsEtn | 5.19 | 2.01 | 3.64 | 3.55 | 3.91 | 3.29 | *** | ns | ns | ns | ns | ns |
| ChoGpl | ||||||||||||
| 18:0/18:1-PtdCho | 0.91 | 0.19 | 0.23 | 0.88 | 0.50 | 0.61 | ** | ** | ns | * | ns | ns |
| 18:0/20:4-PtdCho | 0.32 | 0.18 | 0.17 | 0.34 | 0.23 | 0.28 | ns | * | ns | ns | ns | ns |
| 18:0ol/18:1-PlsCho | 6.47 | 3.37 | 2.35 | 7.49 | 5.28 | 4.56 | ns | ** | ns | ns | ns | ns |
| 18:0ol/20:4-PlsCho | 0.42 | 0.26 | 0.18 | 0.50 | 0.36 | 0.32 | ns | ** | ns | ns | ns | ns |
- a) All lipid units are µmol/100 g wet wt.
- b) ns = P > 0.05; * = P < 0.05; ** = P < 0.01; *** = P < 0.001.
- c–e)These letters indicate Breed × Cut meat, Breed × Aging, and Cut meat × Aging, respectively.
3 Results
3.1 Decreases in PL Fatty Acid Content During Postmortem Aging
All the meats examined presented no differences in the total lipid and fatty acid content before and after postmortem aging (Table 1). In the beef samples, lower PL content values were obtained for round meat after postmortem aging (beef round PL, P = 0.07), while the PL content in the loin meat remained unaltered. Further, 18:0 and 20:4n-6 contents in the round meat PL were significantly lower than the values before postmortem aging, while the PL content in the loin did not change. Among the venison samples, the PL content in the round meat was significantly lower than that before postmortem aging and the PL content in the loin meat tended to be lower as well (P = 0.05). Also, the 18:0, 18:2n-6, and 20:4n-6 contents in the round meat PL were significantly lower after postmortem aging. In terms of changes in the individual rates to lipid levels before aging, after postmortem aging, the 20:4n-6 rate in the round meat PL for both beef and venison was significantly lower than 50%, and the 18:0 and 18:2n-6 rates in the round cut PL for both meats were significantly low.
3.2 Decreases in PL Species Contents During Postmortem Aging
Table 2 shows the PL molecular species composition of different meat samples. No differences were observed at the molecular level in the beef loin and round meat before and after postmortem aging because the individual differences of lipid contents were large. However, the individual rates of the levels of 18:0/18:1-PtdEtn and 18:0/20:4-PtdEtn after aging were significantly lower in beef round meat. Also, 16:0/18:1-PtdEtn and 16:0/20:4-PtdEtn in beef round meat were semi-quantitated and significantly lower rates were observed after postmortem aging treatment (63 ± 16%, P < 0.05 and 48 ± 10%, P < 0.01, respectively). No significant changes in the PlsEtn, PlsCho, and PtdCho rates were found in beef round meat before and after postmortem aging. The PL species in beef loin meat also did not change after postmortem aging. On the other hand, venison round meat showed significantly lower rates for four PtdEtn species (18:0/18:1-PtdEtn; 18:0/20:4-PtdEtn; 16:0/18:1-PtdEtn, 66 ± 19%, P < 0.05; 16:0/20:4-PtdEtn, 32 ± 4%, P < 0.01). Moreover, 18:0ol/18:1-PlsEtn and 18:0ol/20:4-PlsEtn showed significantly lower rates although the differences were small compared to those for PtdEtn species.
3.3 Effects of Breed, Cut Part, and Aging on Lipid Composition
To clarify the relationships between the lipid composition and other factors, analysis of variance was performed (Tables 3 and 4). The total lipid and fatty acids were affected by the breed and cut part, but not by aging. On the other hand, PL and the fatty acids were affected by the breed, and PL, 18:0, and 20:4n-6 were affected by the cut part, aging, and their interaction.
The EtnGpl species (except 18:0/20:4-PtdEtn) were affected by the breed, while only 18:0/20:4-PtdEtn was affected by the cut part, aging, their interaction, and the interaction between breed and aging. The ChoGpl species were affected by the cut part.
4 Discussion
The intramuscular PL level is reported to decrease on the postmortem aging of beef, pork, and chicken; this change affects the qualities of meat (e.g., myofibrillar fragmentation and drip loss).4-11 However, there is still little information on the effect of muscle position and breed on the PL compositions. The aim of this study was to evaluate the effect of postmortem aging on the intramuscular fatty acid and PL compositions in primal cuts (round and loin) of cattle and Yezo sika deer.
Table 1 shows that due to postmortem aging, the PL, 18:0, and polyunsaturated fatty acids (PUFAs) such as 18:2n-6 and 20:4n-6 decrease. This decrease is attributed to phospholipase activation. The phospholipase classes may be phospholipase A1 (PLA1) and PLA2, because saturated fatty acids (SFAs) and PUFAs tend to bond at the sn-1 and 2 position of PL, respectively. PL hydrolysis leads to weakened cell membranes and may be induced to tenderize the meat. In addition, the released fatty acids, especially 20:4n-6 being higher PUFA are very reactive, and the resulting aldehyde species also affect the taste, flavor, and color of the meat, even in very small quantities.19 In this study, 20:4n-6 in beef and venison meats existed mainly in the PL form (Table 1). Therefore, monitoring and controlling the PL 20:4n-6 levels during postmortem aging play an important role in meat quality.
PL molecular species analysis revealed a decrease in the 18:0/20:4-PtdEtn levels during postmortem aging for the beef round, and venison loin and round, while a decrease in PlsEtn levels was detected only in the venison round (Table 2). These results suggest that the reduction in PL levels varies for different meats and PL species. The PLA2 family comprises a series of hydrolase enzymes, such as secretory PLA2, calcium-dependent cytosolic PLA2 (cPLA2), and calcium-independent PLA2 (iPLA2).11 PLA2 exhibits structural specificity and fatty acid specificity; cPLA2 recognizes diacylglycerophospholipids, especially those with PUFAs20; while iPLA2 recognizes plasmalogens with PUFAs.21 Therefore, Ca2+ influx and cPLA2 activation are facilitated in round cuts compared to loin cuts during postmortem aging. PLA1 activation in round cuts is also potentially attributed to a decrease in 18:0 levels in PL. Hence, PLs during postmortem aging might be reduced by both PLA1 and PLA2 activation. However, reduction in the 16:0 rate as SFAs was not observed in the PL for the round cut for 28 days after postmortem aging, while there was a rate reduction in 16:0ol/18:1-PtdEtn and 16:0ol/20:4-PtdEtn. PLA2 may strongly contribute to PL hydrolysis during postmortem aging, compared to PLA1.
Analysis of variance suggest that the 18:0 and 20:4n-6 levels in PL are easily affected by differences among the breed and cut part, and decrease with postmortem aging (Table 3). The decrease by aging is also affected by the cut part. Table 4 shows that the level of PtdEtn, which consists of 18:0 and 20:4n-6, decrease with aging, and that this decrease is affected by differences among the breed and cut part. However, the venison and beef used in this study were from male and female, respectively. Therefore, the differences in lipid profiles between venison and beef may be attributed to the sexual difference.
In previous studies, it was reported that PLs in porcine longissimus muscles were hydrolyzed by PLA2 during postmortem aging.10 However, in this study, the reduction in PL content similar to that reported for the longissimus muscle was not observed in beef loin, but in venison loin. Generally, the larger the size of the animal, the longer the required aging process (chicken < pork < beef). Therefore, the PLA2 activity in beef loin may be lower than that in venison and pork during postmortem aging. In the present study, we focused on the differences among the two meat cuts, loin and round. Muscles are classified as red and white muscles according to the myoglobin levels in the muscle fibers, and large-momentum muscles contain higher levels of myoglobin (the oxygen transporter) and PLs. Hence, round cut, which consists of red muscles, may exhibit higher PLA2 activity.
In terms of the plasmalogen content in beef meat reported in the literature, the given values are varied and also differ from those in the present work.13, 22 The reasons for these inconsistencies are believed to arise from differences among the meat cuts, aging conditions, and cattle breed. Improvement in cattle breed is achieved genetically and, such as for Japanese Black among the Wagyu, which have over 40% intramuscular fat,23, 24 and distinct metabolic systems.
5 Conclusion
In this study, we investigated the changes in the intramuscular PL composition in beef and venison after postmortem aging. PL variation depends on meat cuts and breed, and may be induced owing to the activity of cPLA2. Meat properties (e.g., drip, flavor, and taste) may be affected by PL hydrolysis and 20:4n-6 release. Therefore, monitoring the variation in PL species might be helpful in estimating the meat quality during postmortem aging. Further studies on the lipolysis of meats are necessary to understand changes in meat quality under postmortem aging.
Conflict of Interest
The authors declare no conflict of interest.
