Effect Of Water Activity On Oxidative Quality In Beef And Yak Myoglobin Oxidation Related To Its CDNA Sequence

The oxidative quality of foods is a critical factor influencing sensory acceptance. A variety of factors, including water activity, impact the potential for foods to undergo oxidative deterioration. Maximizing sensory and microbiological shelf life of intermediate moisture meat(IMM) may be approached by manipulating water activity(aw). The interactions between lipid oxidation, protein oxidation and Maillard reaction may play together to impair the oxidative quality of IMM. Understanding the molecular mobility of water and its relationship to lipid oxidation would provide critical information related to quality of dehydrated foods. Nuclear Magnetic Resonance(NMR) is a powerful technique that can measure the molecular dynamics of water and can be utilized to address the heterogeneity, be specifically, to measure moisture mobility and migration in rehydrated freeze-dried meat products of different water activity values.The objective of this part of study was to understand the effects of an important meat processing variable, water activity here, on oxidative deterioration of intermediate-moisture meats, and reveal water molecular mobility observed by Nuclear Magnetic Resonance(NMR) in rehydrated freeze-dried meat patties.To evaluate the effect of water activity on oxidative quality in beef, intermediate moisture mega sausage was prepared with glycerol as humectant to reach different water activity, and then subjected to storage at cold storage at 4℃and 49℃, an accelerated condition typical of storage studies for military ration components. Water activity(a_w), total moisture content(ï¼…), lipid oxidation(TBA test), protein oxidation, and non-enzymatic browning reaction at week 0, 4, and 8 were monitored. The values of water activity for fresh prepared samples treated with glycerol 0ï¼…, 25ï¼…, 40ï¼…, 60ï¼…were about 0.98, 0.92, 0.88, and 0.84 at week 0, with 0ï¼…as control. At both storage temperatures, TBARS532 consistently decreased with storage length and there was no significant difference(P＞0.05) between control and treatments as well as among treatment. Interestingly, TBARS532 from control(higher aw) was even higher than those of treatments(lower a_w) at any time points, although there was no differences between treatments(P＞0.05). At 4℃of storage, TBARS450 did not change with storage time or a_w(P＞0.05), while significantly increased with storage time at 49℃(P＜0.05), but no difference between control and treatments was found(P＞0.05). The changes of Maillard reaction(MR@370) showed the same pattern as TBARS450. Protein oxidation decreased with storage time from week 0 to week 4 in samples stored at 4℃, but not further decreased after week 4. No difference between any control and treatments was found(P＞0.05). At 49℃, protein oxidation significantly increased with storage length after week 4. There was no difference between any control and treatments at any time point. Accordingly, in IMF beef system with aw range from 0.98 to 0.84, water activity had no significant effect on lipid oxidation measured by TBARS450, Maillard reaction and protein oxidation. At the conditions described above, lipid-oxidation measured by TBARS450, Maillard reaction and protein oxidation were significantly influenced by accelerated storage condition(49℃) and storage time at this temperature.To study water mobility related to lipid oxidation with using NMR, freeze-dried beef patties were adjusted from 0 to 0.75 a_w and stored at 4℃and 49℃. During 16 weeks of storage, lipid oxidation was monitored by TBARS at 450 and 532 nm, and water mobility by wide-line, solid-state proton NMR. The results showed that water and lipid protons contributed to two peaks; both water and lipid protons spectra showed a line narrowing effect at increasing moisture. ¹H intensity and water T₂ increased with increasing a_w, temperature and storage time. The LW dispersion (decrease with moisture content) occurred at around 0.3 a_w at shorter storage time (0-30 days) and disappeared after extended storage, indicating a complex dynamic behavior even when equilibrium moisture content has been reached. The TBARS increase with decreasing a_w was probably related to the observed molecular and structural changes. The results above indicated the importance of hydration dynamics and its effect on lipid oxidation in muscle foods. A better understanding of water-protein-lipid interaction and the roles of water and lipid mobility in affecting lipid oxidation could also provide the basis for improving product quality and shelf life.Due to the comprehensive study of the effect of lipid oxidation on protein oxidation, especially the specific oxygen-transportative myoglobin oxidation, in fresh meats, this study on the oxidative quality in fresh meat would be focused on myoglobin in yak, including its cDNA sequence, its content and oxidation with related to lipid oxidation, in order to discover the mechanism by which yak adapts to hypoxia condition on high attitude.The mechanism by which yak(Bos grunniens) adapts to the hypoxia environment, especially myoglobin(Mb) kept its role for oxygen transportation and storage in this extreme condition, have not been fully understood. The cDNA sequence of yak Mb, as well as Mb content and oxidation related to lipid oxidation in yak skeletal and cardiac muscle, were investigated to examine the relationship of myoglobin to yak's adaptation to hypoxia environment. The cDNA sequence of yak myoglobin showed that, in the open reading frame coding 153 amino acids of Mb, there was a two-base-difference (cac→cat and gcc→gct, at the 89th and 91th amino acids) between cattle(Bos taurus) and yak, although their protein sequence after translation was the same due to code degeneracy. Mb contents in yak skeletal and cardiac muscles were 488.3 nmol/g and 823.4 nmol/g, respectively, which were 18.8ï¼…and 61.2ï¼…higher than those of cattle. During the storage at 4℃, oxidation of yak Mb was slower by 10ï¼…in skeletal and 30ï¼…in cardiac muscle compared with cattle, while thiobarbituric acid-reactive substance(TBARS) in yak skeletal muscle was 63.5ï¼…lower than that of cattle after 6 days of storage. Our data indicated that yak adapted to hypoxia environment not by the change of cDNA sequence of myoglobin thus the modification of its structure during evolution, but at least partially by the increased content of myoglobin and its improved resistance to oxidation stress, as well as the potential protection effect of lipids in yak muscle.