Oxidation and decrease of gelling properties for meat myofibrillar protein induced by hydroxyl radical

Abstract: Oxidation and changes of gelling properties of myofibrillar protein (MP) induced by the hydroxyl radical (·OH) were studied in this paper. Pork myofibrillar protein was suspended in 15 mmol/L piperazine-N, N-bis(2-ethane sulfonic acid) (PIPES) buffer (pH 6.0), and 0.6 mol/L NaCl, and incubated at 4℃ for 24 h with ferric ion (Fe3+) and ascorbic acid (Vc) at six concentrations of hydrogen peroxide (0, 0.5, 1.0, 5.0, 10.0, 20.0 mmol/L H2O2). The indexes including protein carbonyl content and whiteness, water-holding capacity (WHC), and texture profile analysis (TPA) of MP gel were measured. The ability of MP to form a fine gel network was described in terms of shear storage modulus (G′), and it was analyzed by small strain oscillatory rheological testing. The microstructure of MP gel was observed by scanning electric microscopy (SEM). The transverse relaxation time (T2) and water distribution of MP gel were analyzed by low-field nuclear magnetic resonance (low-field NMR). Correlation analysis between indexes of MP gel was also performed to establish possible linkages between concentration of H2O2 and different parameters of MP gel in this study. The results showed that carbonyl content steadily increased with H2O2 concentration, increased from 1.17 nmol/mg protein (non-oxidized MP) to 2.82 nmol/mg protein (20.0 mmol/L H2O2). With increasing H2O2 concentration, the whiteness and WHC of MP gel decreased significantly (p<0.05) (89.89, 64.17% respectively for non-oxidized MP gel, 87.35, 54.77% respectively for 20.0 mmol/L H2O2). The texture (hardness, springiness, cohesiveness and chewiness) and shear storage modulus (G′) also attenuated with increasing H2O2 concentration. The SEM results demonstrated that oxidation of protein could significantly affect the microstructure of MP gel. The non-oxidized MP gel exhibited a compact and homogeneous fine network microstructure, whereas the oxidation process produced empty spaces and changed the compact and fine gel structure to a coarser network, and the effect increased with higher H2O2 concentration. The low-field NMR relaxation measurement results indicated that the oxidation degree of protein had no effect on the bound water (T21) of MP gel but had significant effects on immobile water (T22) and free water (T23) of MP gel. T22 water content (P22) decreased with increasing H2O2 concentration (94.03% for non-oxidized MP gel, 91.16% for 5 mmol/L H2O2, 88.14% for 20 mmol/L H2O2). T23 water content (P23) increased with H2O2 concentration (1.37% for non-oxidized MP gel, 4.25% for 5 mmol/L H2O2, 7.56% for 20 mmol/L H2O2). Taken together, these results demonstrate that with increasing H2O2 concentration, some part of the immobile water of MP gel shifts to free water, and the free water would become the potential centrifuge drip loss. Correlation analysis results showed that concentration of H2O2 was highly negatively correlated with whiteness, WHC, hardness and chewiness (p<0.05). Based on the synthetic evaluation on the testing results of the MP gel, it can be concluded that the ·OH will result in oxidation of MP, and have detrimental effects on the gelling property of porcine MP. Therefore, it is imperative to inhibit the oxidation of MP during storage and processing of meat to avoid losing the functional properties of porcine MP.