| Postmortern aging is a crucial method to improve tenderness of yak meat. However, the aging process tend to be accompanied with discoloration. It has been widely demonstrated that the meat color stability is determined by the relationship between lipid oxidation and antioxidant ability. Nevertheless, its mechanism is not completely understood. Thus, it is necessary to conduct comprehensive studies focuing on the whole color stability system. The present paper was designed to investigate the color change rules of yak LD during postmortern aging, the primary causes for discoloration, and the relationship among lipid oxidation, antioxidant ability and meat color stability. Furthermore, the invitro reasearch model was used to reveal the machanism of effects of lipid oxidation products and antioxidatant factors on meat color stability. The results are shown as follows:1. The change rules of meat color from yak were confirmed. The L* values and a* values significantly decreased during 7 d of aging period, meanwhile the H* values significantly increased(P<0.05). The L*, a* and H* values were 38.33, 18.09 and 0.53 on the 0 d, respectively, which showed slightly changing trend over 0 to 3 d. However, the L* values and a* values significantly dropped by 8.22% and 27.87%(P<0.05), respectively, and the H* values increased by 27.66% over 3 to 7 d. Kinetic analysis results of food quality equation showed linear changes in color parameters. The major features of discoloration could be attributed to decreasing in a* value since its rate constant was higher than other color parameters.2. The primary causes for discoloration were validated. The results indicated that the myoglobin mainly existed in the form of OMb(almost 70%) at the begining of postmortern aging. However, the OMb content dropped to 46.25% after 7 d of aging period, meanwhile the MMb content increased to 44.72%(P<0.05). Spectral analysis showed that the characteristic absorption peak of OMb at 540 nm and 580 nm dropped with time increasing, whereas SDSPAGE profile suggested that degradation of myoglobin(17 k Da) was not obvious. Besides, the mitochondria structure was injured during postmortern aging, resulting in decreases of MRA and NADH content(P<0.05), which were significantly related to OMb and MMb content(P<0.05). Therefore, above results suggested that the myoglobin oxidation and loss of mitochondrial metmyoglobin reducing ability were the main reasons leading to discoloration.3. Further studies found that the degree of lipid oxidation increased along with discoloration process of yak meat. The TBARS, POV and CD values were used to evaluate lipid oxidation degree. Results showed that TBARS and POV values increased by 2.32 and 2.08 times(P<0.05) than initial values during 7 d of aging period. The changing trend of CD value indirectly reflected the accumulation of lipid oxidation products. SPME-GC-MS analysis found that aldehydes were the major products of lipid oxidation in yak meat. Ten and twelve aldehydes were detected respectively on the 0 d and 7 d. Total content of aldehydes significantly increased during postmortern aging(P<0.05), and the hexanal, nonanal and heptanal content were higher than other aldehydes.4. Endogenous antioxidant capacity of yak meat decreased significantly during postmortern aging. The TPC, RSA, TEAC and FRAP values were adopted to comprehensively estimate the endogenous antioxidant capacity. Results indicated that TPC, RSA, TEAC and FRAP values significantly decreased by 38.92%, 49.11%, 84.63% and 32.19%(P<0.05), respectively. In general, although yak meat showed a greater antioxidant capacity at the beginning of aging period, it decreased significantly as the aging time went on.5. The relationships among lipid oxidation, antioxidant capacity and color stability were confirmed, and the primary factors were determined. The factors of color stability system, lipid oxidation and antioxidant capacity were analyzed by dimension reduction analysis of principal component. The contribution rate of PC1 and PC2 was 79.337%, which represented the major variation of samples. The factor loadings of TBARS, POV, MMb and H* values were greater than 0.9, these indexs were postively related to PC1. On the contrary, the factor loadings of MRA, OMb, RSA, TEAC, TPC and a* value were greater than-0.9, which were negatively related to PC1. The hexanal, nonanal and propanal were postively related to PC2 since their factor loadings respectively reached 0.961, 0.948 and 0.934. These results suggested that lipid oxidation and antioxidant capacity directly affected meat color stability. Futhermore, the hexanal and nonanal were selected as main factor according to the factor loadings.6. The mechanism that effects of lipid oxidation on meat color stability was revealed in vitro. Firstly, the oxidation rates of OMb were respectively increased by 2.54% and 3.85%(P<0.05) when incubated with fresh- or oxidized-microsome. This result indicated that accumulation of lipid oxidation products accelerated OMb oxidation process. Secondly, the oxidation rates of OMb were respectively increased by 3.41% and 2.05%(P<0.05) when incubated with hexanal or nonanal. The MALDI-TOF MS analysis found that two adducts(17046.3 Da and 17146.8 Da) were formed by reacting hexanal with OMb, and one adduct(17088.5 Da) was formed by reacting nonanal with OMb. This result ulteriorly suggested that aldehydes could accelerate OMb oxidation process by protein modification. In addition, hexanal and nonanal expedited the mitochondria injury, and simultaneously inhibited the electronic respiratory chain mediated MMb reducing process.7. The function of endogenous antioxidant factors in maintaining meat color stability was verified in vitro. As the major endogenous antioxidants, α-tocopherol and carnosine inhibited the lipid oxidation of microsome and decelerated the OMb oxidation process(P<0.05). Besides, carnosine inhibited the OMb oxidation accelerated by hexanal and nonanal(P<0.05). α-tocopherol and carnosine maintained the mitochondrial MRA in short-term, it seems that in addition to lipid oxidation, mitochondria were also influenced by other factors. Moreover, the lipid and myoglobin oxidation rate of yak meat were decreased when adding different concentration of α-tocopherol and carnosine. Above results indicated that endogenous antioxidants could stabilize the meat color by inhibiting lipid oxidation.8. The greater color stability of yak meat benefited from relatively low lipid oxidation rate and powerful antioxidant capacity. When compared with beef cattle, the characteristics of lipid oxidation and antioxidant capacity in yak meat were as follows:(1) The fat content, lipid oxidation rate constant k and aldehydic products were lower.(2) Endogenous antioxidant capacity were greater, and remained at a relatively high level during aging period. According to in vitro results, the myoglobin oxidation rate of yak meat were lower than beef cattle due to above characteristics, which resulted in a greater color stability during postmortern aging. |
PDF Full Text Request