Effects Of Salt On Oxidation And Functional Properties Of Pork Myofibrillar Protein

Cured meat products are an important class of traditional meat products.The quality formation mechanism during the processing is very complex,involving the decomposition and oxidation of various components such as protein and lipid.Among them,protein oxidation affects the functional properties of meat products such as texture and water holding capacity by modifying amino acid side chains,destroying protein backbone,and promoting intramolecular and intermolecular cross-linking.The purpose of this study is to elucidate the effect of salt on the aggregation pattern of proteins in the oxidized environment and the gel densification during curing.The research investigated the conformational changes and densities of myofibrillar protein（MP）gel in cured meat in the hydroxyl radical oxidation system.This study revealed the correlation between quality changes of cured meat and protein conformational changes.Furthermore,the effect of hydroxyl radical oxidation on the oxidative stability of protein molecules,aggregation patterns,gel densification and the mechanism of action of myosin aggregation sites during the curing process was analyzed.We hope to provide theoretical guidance for a comprehensive understanding of the quality formation mechanisms in the processing of cured meat products.The main research contents and results are as follows:（1）Effects of salt on pork quality and changes in muscle protein structure during curing.The pork tenderloin was cured with different salt contents（0%,1%,3%and 5%）at 4℃for three days,and this rapid analysis model was developed to analyze the relationship between muscle water retention and muscle protein oxidation patterns under different curing conditions.The results showed that with the increase of salt content,the water holding capacity of cured meat increased.And the addition of salt significantly improved the chewing performance of cured meat,which increased the hardness and chewiness of the samples（P<0.05）.Furthermore,the addition of salt promoted protein oxidation,which increased the carbonyl value of the protein in cured meat and decreased the content of sulfhydryl groups.The oxidation of the protein reached the highest level at 3%salt content.At the same time,salt promoted protein aggregation,especially myosin heavy chain.The aggregation induced by salt promoted the water holding capacity of cured meat.（2）Effect of salt content on MP conformation and aggregation in oxidation environment.The mechanism of the effect of different oxidation conditions（1 m M,10m M,20 m M H₂O₂）combined with different salt contents（0%,1%,3%and 5%）on the molecular oxidation stability,conformational changes and protein aggregation of MP was analyzed.The results showed that salt could change the equilibrium of the existing Fenton oxidation system.With the increase of H₂O₂,the increase in carbonyl content diminished at low salt levels（1%and 3%）and the decrease in sulfhydryl content strengthened at higher salt levels（5%）.Oxidation converted theα-helix toβ-fold and facilitated intramolecular interactions during salt-induced structural unfolding of MP.1%salt content increased MP hydrogen bonding and partial unfolding of the protein,while 3%salt groups greatly disrupted its hydrophobicity and weakened hydrogen bonding.5%salt treatment group promoted cross-linking of the protein by generating disulfide bonds,resulting in a significant decrease in tryptophan fluorescence intensity.Scanning electron microscopy（SEM）combined with fourier transform infrared spectroscopy（FTIR）results showed that 20 m M of H₂O₂ converted moreα-helices of MPs intoβ-folds,causing the proteins to form larger aggregates and a more porous protein network structure.（3）The effect of salt content on the thermal aggregation behavior and the moisture distribution of gels of MP in oxidized environment.The effects of different oxidation conditions combined with different salt contents on the aggregation pattern of MP and gel densification were analyzed.The results showed that salt promoted the protein aggregation caused by oxidation,especially at low salt content（1%-3%）,resulting in a significant decrease in MP solubility（P<0.05）and a shift of the peak of the particle size distribution toward larger particle magnitude.In terms of the denseness of the formed gels,the addition of salt（especially in the 5%salt treatment group）promoted the denseness of the gels by protein oxidation.1%salt treatment group had no significant effect on the water holding capacity of the oxidized protein gels,while with the increase of salt content,more free water was converted into bound water in the MP gels,which improved the protein-water binding capacity in the gels,resulting in a significant increase in the WHC.In addition,the high salt content made the oxidized protein gel structure porous and the gel more rigid.（4）Effect of different salt levels on myosin conformation and aggregation site changes in oxidized environment.The effects of different salt contents（0%,1%,2%,3%,4%,5%）on oxidative stability,solubility/dispersibility,tryptic rennet digestibility,aggregation sites and microrheological properties of oxidized myosin were analyzed.The results showed that Na Cl promoted the exposure of sulfhydryl groups and inhibited the formation of disulfide bonds at salt contents of 1%-2%.Furthermore,the particle size of oxidized myosin was smaller.Sodium dodecyl sulfate polyacrylamide gel electrophoresis（SDS-PAGE）showed that S1 and heavy meromyosin（HMM）subunits of myosin were highly oxidized and susceptible to reversible assembly.In the 3%salt group,hydrophobic interactions and protein cross-linking increased,but water-protein interactions remained dominant.As the salt content increased to 5%,exposure of tryptophan and disulfide bond formation increased significantly,while the location of reversible aggregation occurred mainly in S2.Microrheology demonstrated that at high ionic strengths,oxidized myosin formed a tighter thermostable network,indicating increased inter-droplet resistance and macroscopic viscosity.