Effect Of Ph And Salt Concentration On Myofibrillar Protein (Dosidicus Gigas) Aggregation Process

Myofibrillar protein is an important functional material in food processing, and its gel property directly affects the quality of meat. Dosidicus gigas owns poor gel ability due to high moisture content and highly active protease which can break down the myosin.The paper starts with the relationship between protein structure and function, followed by studies on the effects of pHs (acid, neutral, alkaline) and KCl concentrations (low, middle, high) on myofibrillar protein solubility, protein composition, secondary structure and the tertiary structure, respectively. Focus on examining the relationship between myofibrillar protein solu-bility and pHs, KCl concentrations, the SDS-PAGE electrophoresis is applied to analysis of pro-tein compositions, while the Quantity One and Image J softwares are used to analysis the relative content of six major protein components (myosin heavy chain, heavy meromyosin, paramyosin, actin, tropomyosin and myosin light chain) in myofibrillar proteins with vrious pHs and KC1concentrations. Meanwhile, the effects of pHs and KC1concentrations on myofibrillar protein conformation are studied by UV spectrophotometer and circular dichroism.Then we get aggregates under the appropriate conditions by means of both static (pure heating) and dynamic (heating+Oscillation) methods. The rheological properties were recorded in the process of dynamic aggregates generated with vrious pHs and KCl concentra-tions.According to the results of the protein solution, not only the solubility, protein composition, secondary structure, but also microstructure, water holding capacity and water distribution of myofibrillar protein static and dynamic aggregates are studied. Finally light scattering instrument experiments conform that pH and salt ions can cause changes in spatial structure of protein molecules, affect protein intermolecules interaction, and further reveal its relationship between protein structure and function in the process of gelation, which provided a theoretical basis and application guidance for surimi production.The results show that:1. pH and salt concentration have a great impact on the solubility of myofibrillar protein and its aggregates. The effects of KCl concentration on their solubility are the same, all increase with KCl concentration increasing (≤0.9mol/L). But the effects of pH are very different. When the pH deviates from neutral to alkaline, all solubility in neutral pH are less than in alkaline pH. And when the pH changes from acidic to neutral, the solubility of the protein solution and dy-namic aggregate are almost constant, while the solubility of static aggregates keep gradually in-creasing.2. According to the results of SDS-PAGE analysis, UV chromatography and circular dichroism, pH and KCl concentration affect myofibrillar protein solubility mainly in two aspects: First, with different pHs and salt concentrations, the relative content of proteins within myofibrillar protein are various; and second, pH and salt concentration can affect the protein secondary and tertiary structure, chang the protein conformational. Along with pH and salt con-centration increase, the solubility of globulin (paramyosin, tropomyosin, etc.) increases faster, result in an increase of its proportion in the total protein, leading the system to tend orderly.3. pH and KCl concentration also affect the rheological properties of myofibrillar protein dynamic aggregates. With KCl concentration increasing, pH deviating from the isoelectric point, the elastic modulus(G’) of the aggregates raise consistently.4. According to the results of microscopic structure, water holding capacity (WHC) and water distribution of the aggregates:the gel microscopic structure is influenced by salt concen-tration and pH. With KCl concentration increasing, The microstructures of static aggregates vary from solid lumps, mesh-like to granular while dynamic aggregates from solid block at0.1mol/L KCl to mesh-like at0.5mol/L and0.9mol/L KCl. In general, pH increases, aggregates dense. Correspondingly, the water holding capacity of aggregates and the proportion of T2are elevated with pH and salt concentration increasing. 5. Static light scattering experiments further confirmed the above analysis. pH and salt by adjusting and shielding the net charge on particle surface to regulate the intermolecular interac-tion, affect the association between the particles. After heating, the exposed hydrophobic groups enhance attractive force between the particles, tending to aggregate. The increase of net charge result from high pH make attractive force between the particles disappeared and the repulsive force enhanced. Dynamic aggregates have a better network structure with positive or negative A2value both near to zero, especially in high salt (≥0.5mol/L) and high pH (≥7.5) when the interac-tion forces between the particles tend to balance. Since denatured particles timely aggregated, dynamic aggregates also have a greater Rg and Mw than the static aggregates. In addition, the shielding effect of salt to make the decay time of the particles in the three increased with KCl concentration increasement, along with the change of pH is more complex.