The Interaction Mechanism Of Porcine Myofibrillar Proteins With Typical Flavor Compounds

Muscle protein is an important source of the dietary nutrition for consumers.Among them,myofibrillar proteins?MPs?are the main component of the muscle protein and act as the structural protein.MPs contain many kinds of essential amino acids for people,and largely determine the texture properties and sensory qualities of meat products.Flavor perception in food is one of the important determinants of food acceptance for consumers.There are many kinds of flavor compounds in meat products,such as aldehydes,ketones,alcohols,esters,carboxylic acids,alkanes,benzene series,pyrazines,furans,pyridines,sulfur-containing compounds and so on.Different meat products almost own the similar kinds of flavor compounds,but the content of each flavor usually differs greatly.This is closely related to the source of meat,food components,additives and the processing conditions.The interactions between flavor compounds and matrix components have important influence on food flavor perceptions.The adsorption and release of proteins to flavor compounds can regulate and control the flavor perception in meat products.Therefore,the studies on the binding mechanism of MPs to flavors can guide the food producer to provide the meat products with pleasant flavors.The adsorption of proteins to flavors is affected by many influence factors.Protein conformation and molecular structure of the flavor compounds both can affect the adsorption effect of protein to flavors.In general,the binding manners of protein to flavors can be divided into two types,reversible binding and irreversible binding.The reversible binding types include hydrophobic interactions,hydrogen bonds,van der Waals forces and ionic bonds?electrostatic interactions?.The irreversible bonding is usually through a covalent bond.The binding manner of protein to flavors is largely depended on the molecule activity of the flavor compounds themselves.Based on the above considerations,this paper was focused on the following points:1.The influence of pH-induced structural modifications of myofibrillar proteins?MPs?on their interactions with heterocyclic compounds?pyrazines?were investigated.At a lower pH?4.9,5.5?,MPs could aggregate to larger particles due to the enhanced protein-protein interactions.The binding phenomena of MPs to pyrazine compounds were significantly affected by the pH and the nature of flavor compounds.Due to the protein-protein interactions and the decreased surface tension,MPs could exhibit flavor releasing behavior.Fluorescence analyses revealed that the interactions of MPs with pyrazine compounds followed a combination of the static and dynamic quenching procedure.The changes in quenching constant(K_sv)might be attributed to the dynamic quenching process,probably related to the aggregation behaviors within MPs.The SPME-GCMS results shown that the percentage changes of free 2,5-Dimethylpyrazine?2,5-DMP?under different pH were similar to that of K_sv.Thermodynamic parameters indicated that electrostatic interactions and hydrophobic interactions were the major acting forces in the binding of MPs to 2,5-DMP.The binding of2,5-DMP increased the?-helix content of MPs.2.Typical ketone flavors?various in chain length,the position and the number of keto group,branched chain?were selected to investigate the influence of molecule structure of flavor compounds on their interactions with MPs.Results showed that 2,3-pentanedione quenched the fluorescence of MPs more effectively than 2-pentanone and 3-pentanone due to its much number of keto group.There was no significant difference in K_sv between 5-methyl-2-hexanone and 2-heptanone,which was attributed to their similar molecular sizes and polarities.The quenching effect of homologous ketone flavors increased with the carbon chain growth,which contributed to the higher hydrophobic interactions.Dynamic procedure played a major role in the fluorescence quenching process of MPs by 2-pentanone,3-pentanone,5-methyl-2-hexanone,2-heptanone and 2-octanone.The content of?-helix structure in MPs decreased gradually with the increase of ketones concentrations,implying the unfolding of MPs.Generally,SPME-GCMS results accorded with the fluorescence quenching performances,while 2,3-pentanedione and 2-nonanone exhibited some differences due to their higher steric hindrances.This work suggested that the nature of ketone flavors,such as molecular size,molecular polarity,carbon chain length,the position and the number of keto group,had significant influence on the protein-flavor interactions.3.By establishing the hydroxyl radical-generating system consisting of FeCl₃,ascorbic acid and H₂O₂,the effects of oxidation induced by hydroxyl radical on the gel properties and the emulsifying properties of MPs were investigated.The binding abilities of MPs gels under different oxidation levels to aldehydes and ketones were also monitored.With the increase of H₂O₂ concentrations,the sulfhydryl contents in MPs decreased and changed into disulfide linkages?S-S?;the surface hydrophobicity and turbidity of the MPs suspension increased;the salt solubility of MPs enhanced at lower oxidation levels?0?0.2 mM H₂O₂?and then decreased with further oxidation?0.5?5.0 mM H₂O₂?.The particle size distributions of MPs exhibited an obviously shift toward the larger particles with the increase of H₂O₂concentrations.SDS-PAGE shown that there were other covalent bonds existed in MPs except S-S.The intrinsic fluorescence intensity gradually decreased with the increase of H₂O₂concentrations,while the firmness and viscosity of MPs suspensions enhanced upon addition of 0–0.2 mM H₂O₂ and then decreased with the further oxidation.The hardness of MPs gels would significantly enhance under mild oxidation levels?0–0.2 mM H₂O₂?,and then significantly weakened at higher oxidation levels?2.5–5.0 mM H₂O₂?;the water-holding capacity of MPs gels gradually decreased with the increase of H₂O₂ concentrations.Scanning electron microscope results exhibited that MPs gels could form larger aggregates at higher oxidation levels?2.5–5.0 mM H₂O₂?,and the relaxation time T₂ results suggested that the water mobility in MPs gels increased at higher oxidation levels.MPs suspensions owned the highest emulsifying activity and emulsion stability under 1.0 mM H₂O₂ oxidative stress;the storage stabilities of MPs emulsions significantly decreased at higher oxidation levels?5.0–10.0 mM H₂O₂?.SPME-GCMS results stated that the binding abilities of oxidized MPs gels?0–5.0 mM H₂O₂?to aldehydes were stronger than to ketones;the binding abilities of oxidized MPs gels to aldehydes and ketones decreased with the carbon chain growth;there were no significant differences in the binding abilities of oxidized MPs gels to one certain flavor.Raman spectra analyses indicated that the oxidized MPs gels exhibited the similar secondary structure and partly similar tertiary structure.Thereby,there might exist the similar amount of binding sites for the flavors among these gels.