| Meat protein is of high significance for human diet. Myofibrillar protein (MP), the main component of meat protein, could easily undergo attack from reactive oxygen species (ROS) during manufacturing, processing and storage of meat products, which would further influence the functionality and nutritional value of the final products. This PhD thesis is designed to explore a more comprehensive role of oxidation played in meat protein and was mainly divided into two parts. Firstly, different oxidation models relevant to real meat processing procedures were created and were used to initiate oxidation of MP, the consequent structural changes as well as changes in functional properties were well analyzed, aiming to establish a relation between them. In addition, meat homogenate was further used as a model system to explore the oxidation initiated by Fe2+. The corresponding oxidation pathway was monitored at radical scale and the coupling between myoglobin (Mb), lipid, and MP were further discussed, attempting to provide useful guidance when choosing antioxidant strategy. The main contents and results are listed as follows:1. AAPH-derived (2,2’-azobis (2-amidinopropane) dihydrochloride) peroxyl radicals were selected as representative free radicals of lipid peroxidation to investigate the effects of oxidative modifications on MP structure as well as their rheological and gelling properties. Incubation of MP with increasing concentration of AAPH resulted in an increase in carbonyl content and SH→S-S conversion. Results from SDS-PAGE indicated that moderate (～1 mM) and relatively high (> 3 mM) concentration of AAPH induced aggregation of myosin and denaturation of myosin, troponin and tropomyosin, respectively, resulting in changes on gelation of MP. Moderate oxidative modification enhanced the water-holding capacity (WHC) and texture properties of gels, while the further oxidation significantly reduced the gel quality.2. A new type of cold-set MP gel under oxidative stress originated from malondialdehyde (MDA) in the presence of 0.6 M NaCl was discovered. Heating procedure was also applied for further evaluation of gel properties. Protein carbonyl content was used to evaluate protein oxidation. SDS-PAGE was performed to explain the formation of the gel. Gel properties were compared in range of MDA/NaCl concentration with use of gel strength, WHC, scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) measurements. Results revealed that myosin was greatly involved in gel formation through non-disulfide covalent bond. Meanwhile, significant improvements in gel strength, WHC, and the network structure of highly porous with well-defined shapes were observed with increasing addition of MDA/NaCl. Furthermore, with the addition of MDA no more than 10 mM, the gel formed was stable to heat treatment, while the higher MDA could cause gel collapse, which was believed to be due to the excessive covalent bond existed. The gel was formed on the premise of the swelling of MP under certain ionic strength3. This study presented the cold-set gelation of emulsions stabilized by salted MP under oxidative stress originated from MDA. Gel properties were compared in range of MDA/NaCl concentrations by of gel viscoelastic properties, strength, WHC, amount of protein entrapped, and microstructure. The oxidative stability of emulsion gels as indicated by lipid hydroperoxide was further determined and compared. Results indicated that emulsion stabilized by MP at swollen state under certain ionic strength (0.2-0.6 M) was the premise of gel formation under MDA. In the presence of intermediate MDA concentrations (2.5-10 mM), the emulsion gels showed an improved elasticity, strength, WHC, and oxidative stability. This improvement should be mainly attributed to the enhanced protein-protein cross-linkings via MDA, which were homogeneously formed among absorbed and/or unabsorbed proteins, entrapping more amount and fractions of protein within network. Therefore, the oil droplets were better adherent to gel matrix. Nevertheless, addition of high MDA concentrations (25-50 mM) led to the formation of excessive covalent bonds, which might break protein-protein bonds and trigger the desorption of protein from interface. This ultimately caused "oil leak" phenomena as well as the collapse of gel structure, and thus overall decreased gel properties and oxidative stability.4. Effect of oxidized linoleic acid (OLA) on MP digestibility was investigated using an in vitro gastric-intestinal digestion model. The oxidative modifications of MP structure were evaluated by the contents of carbonyl and total sulfhydryls, surface hydrophobicity, SDS-PAGE and FTIR. Based on the results of digestion rate and digestibility, the MP digestion was strongly affected by the oxidation level. With the concentration of OLA below 3 mM, the digestion rate and digestibility of MP were markedly increased mainly due to enhanced protein flexibility and surface hydrophobicity. However, at higher OLA concentrations, the adverse effect in protein digestion was observed, which might be attributed to the accelerated protein aggregation. As for the digest properties, oxidation lowered its overall nutritional value, as revealed by amino acid composition analysis. Nevertheless, digest of MP prior treated with OLA below 1 mM exhibited an enhanced antioxidant activity with a higher ORAC value. This improvement might be mainly attributed to the partial degradation of myosin and troponin-T, which favored their recognition during the intestinal tract, thus increasing the content of Cys and Tyr.5. this study was to investigate the influence of oxidation-induced structural changes of MP on its binding ability with typical aroma compounds such as 2-methyl-butanal, methional, 2-pentanone,2-heptanone, and nonanal. A method using solid phase microextraction (SPME) combined with gas chromatography/mass spectrometry (GC/MS) was used to determine the corresponding binding ability. The binding with aroma compounds was found to be strongly affected by the oxidation levels of proteins. Incubation with oxidants below 1 mM mainly caused the refolding of protein structure and accelerated the protein aggregation, which reduced the affinity of the aroma compounds, thus decreasing the binding ability. Nevertheless, treatment with oxidants over 2.5 mM would cause protein reaggregation and partial degradation. The aggregated protein with winkled surfaces favored the hydrophobic interactions with aroma compounds, forming the protein-aroma compound complex, thus enhancing the resultant binding ability, as evidenced by fluorescence quenching and SPME-GC/MS analysis.6. Fe2+, added as FeSO4, 7H2O, was found to increase the rate of oxygen depletion as detected electrochemically in a pork homogenate from Longissimus dorsi through an initial increase in metmyoglobin (MetMb) formation from oxymyoglobin (OxyMb) and followed by formation of primary and secondary lipid oxidation products and protein oxidation as detected as thiol depletion in MP. Without added Fe2+, under the same conditions at 37 ℃, oxygen consumption corresponded solely to the slow OxyMb autoxidation. Long-lived MP radicals as detected by ESR spectroscopy in the presence of iron(Ⅱ) were formed subsequently to OxyMb oxidation and their level was increased by lipid oxidation when oxygen was completely depleted. Similarly, the time profile for formation of lipid peroxide indicated that OxyMb oxidation initiates both protein oxidation and lipid oxidation. |
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