This study was conducted in order to investigate the mechanism of the effect of adding different concentrations of L-lysine (L-Lys) on the gelling properties of porcine myofibrillar protein (MP) damaged by repeated freeze-thaw cycles. The changes in the conformation, solubility and gelling properties were analyzed by means of circular dichroism (CD) spectroscopy, intrinsic tryptophan fluorescence spectroscopy, a rheometer, a texture analyzer and a scanning electron microscope. The results showed that the addition of L-Lys caused an increase in the α-helix content and intrinsic tryptophan fluorescence intensity of freezing-damaged MP. The solubility of freezing-damaged MP gradually increased with increasing L-Lys concentrations, the storage modulus (G’), gel strength, cooking loss and gel whiteness gradually decreased, and the gel microstructure became gradually more uniform. On the whole, the addition of L-Lys significantly reduced the gel strength and cooking loss of repeatedly frozen-thawed MP gel by changing the conformation, and could thus improve the water-holding capacity and tenderness of freezing-damaged meat.

The effects of punicalagin (P) and sodium pyrophosphate (SPP) and their combination (P + SPP) on the oxidative stability and gel properties of myofibrillar protein (MP) under oxidative stress were studied. The results showed that the addition of P and SPP effectively inhibited the increase of carbonyl content and the decrease of free amino content induced by oxidation, and the combination of P and SPP had the best effect; however, neither single nor combined additives effectively inhibited the decrease of sulfhydryl content induced by oxidation. The addition of P promoted protein unfolding induced by oxidative stress and significantly inhibited the increase of protein particle size (percentage inhibition of 10.50%) induced by oxidation. It had no significant effect on protein solubility or rheological properties during heat-induced gelation, but significantly improved the cooking loss and texture deterioration of heat-induced MP gels under oxidative stress. The addition of SPP also promoted protein unfolding induced by oxidation, inhibited the increase of protein particle size (percentage inhibition of 17.65%), increased the solubility of oxidized MP by 9.78%, significantly changed the rheological behavior of MP during heat-induced gelation, and resulted in a 42.56% reduction in the cooking loss of heat-induced gels, while texture indexes such as gel strength were significantly reduced. The combination of P and SPP had a similar effect on the heat-induced gel properties of MP as the addition of SPP alone. Moreover, the antioxidants used in this study significantly increased the oxidative stability of MP and the water-holding capacity of its heat-induced gel, but the effects on the gel texture characteristics were closely related to the type of antioxidants.