Oxidation Process and Changes of Digestion Characteristics on Myofibrillar Protein of Trichiurus lepturus

In order to explore the natural oxidation and the changes of digestion characteristics on myofibrillar protein of Trichiurus lepturus, the myofibrillar protein of Trichiurus lepturus was subjected to natural oxidation at 4 ℃ (37 ℃ was the accelerated control group) through the changes of myofibrillar protein content, carbonyl group, total sulfhydryl group, free amino acids, protein secondary structure and external digestibility. The results showed that with the extension of oxidation time, the contents of myofibrillar protein, total sulfhydryl group and carbonyl group of Trichiurus lepturus had significant changes under two temperature conditions (P<0.05). At 20 d, protein content decreased from 93.97 mg/mL to 40.15 mg/mL (4 ℃) and 11.19 mg/mL (37 ℃), content of total sulfhydryl group decreased from 35.99 μmol/g to 24.57 μmol/g (4 ℃) and 18.23 μmol/g (37 ℃). The carbonyl content increased from 1.32 nmol/mg to 3.85 nmol/mg (4 ℃) and 6.60 nmol/mg (37 ℃). At the same time, external digestibility increased and then decreased, the highest digestibility of protein oxidized at 4 ℃ reached 69.37% at 18 d, and the highest digestibility of protein oxidized at 37 ℃ reached 58.31% at 16 d. The secondary structure of the protein changed. The relative contents of β-turn and β-fold were significantly changed (P<0.05). And β-turn decreased from 37.99% to 28.24% (4 ℃) and 22.53% (37 ℃), while β-fold increased from 22.54% to 35.80% (4 ℃) and 43.40% (37 ℃). The relative content of β-folds of myofibrillar proteins was negatively correlated with the external digestibility. Through 20 d of oxidation, the content of free amino acids was significantly decreased (P<0.05). The content of total amino acid decreased from 9.8940 mg/mL to 0.3132 mg/mL (4 ℃), 0.8438 mg/mL (37 ℃). The specie of amino acids detected decreased by 6 at 37 ℃. In conclusion, the structure of myofibrillar protein of Trichiurus lepturus would change, the content of free amino acids would decrease, the external digestibility would increase first and then decrease in the natural oxidation condition. The higher the temperature, the faster the oxidation would occur. The degree of protein oxidation was negatively correlated with the in vitro digestibility.