Study On Separation And Properties Of Protein From Tilapia Muscle

Water-soluble protein, salt soluble protein and insoluble protein were separated using chemical method from Tilapia meat in this study, and nutritional, functional properties and molecular weight of three proteins were determined. The aim of this study was to obtain the information related to processing and utilization of protein from Tilapia. Main results are as follows:1. Proportion of water-soluble protein, salt soluble protein and insoluble protein component in total protein from Tilapia meat were 29.34%, 49.48% and 16.75%. Protein content of water-soluble, salt soluble and insoluble protein powder by freeze drying were 70.55%,45.76% and 64.02%, and essential amino acids content were 41.64%, 41.32% and 37.70%. The first limiting amino acid of three proteins was lysine. Studies of SDS-PAGE showed that salt-soluble protein strips displayed at 200 kDa and 20 kDa, and water-soluble protein strips distributed widely between 200 kDa and 29 kDa. Micromolecule of insoluble protein distributed between 60 kDa and 20 kDa.2. Studies of functional properties showed that no obvious effect of pH value on solubility of water-soluble protein was detected. The solubility was above 79% under pH range from 2 to 10. However, effect of pH value on emulsifying and foaming properties of water-soluble protein was obvious. At 6.0 of pH value, the lowest emulsifying and foaming properties of protein were observed. In addition, the freeze-drying treatment resulted in partial denaturation of water-soluble proteins, and decreased water solubility. These results indicated that isoelectric point of water-soluble protein was maybe about pH 6.0. Studies of thermal stability showed that the turbidity of protein solution gradually increased with extension of the heating treatment time, indicating occurrence of protein denaturation and aggregation of molecules. And the rate of protein thermal denaturation and aggregation was significantly accelerated with increased heating treatment temperature. At 40℃of heating treatment, the turbidity of protein system did not change significantly in 30 min, but turbidity increased significantly in 5min at 50℃heating processing.3. The analysis on pH stability of water soluble protein showed that the pH stability of water-soluble protein was high at low temperature. When the pH value of system was adjusted from 3.0, 5.0, 6.0, 7.0, 9.0 and 11.0 to 7.0, and solubility of water soluble protein were above 70%. However, the hydrophobic groups of protein molecules were exposed by the pH-shifting processing and the relative surface hydrophobicity increased and surface hydrophobicity of protein molecules increased. Especially in the process of extreme acidic pH-shifting at pH 3.0, proteins tryptophan was significantly damaged and the fluorescence intensity of tryptophan decreased. SDS-PAGE electrophoresis analysis further confirmed maromolecule protein was partially degraded in the process of extreme acidic pH-shifting. In addition, effect of pH-shifting treatment on the emulsifying properties of water-soluble protein was obvious, and the emulsifying activity of water-soluble protein decreased significantly in the process of pH-shifting from pH 6.0 to 7.0.4. Effect of pH value on solubility, emulsifying and foaming properties of salt-soluble protein was visible. Solubility, emulsifying and foaming properties of salt soluble protein was higher in condition of neutral and alkaline pH value, and these properties were the worst at pH 4.0. Therefore, the isoelectric point of salt soluble protein fromTilapia muscle was about pH 4.0. In addition, the freeze-drying treatment of salt soluble protein resulted in partially denatured, and its solubility, emulsifying and foaming properties also decreased. The formed ability of gels of salt soluble protein itself was poor, while adding starch and TGase could increase the gel strength and water holding capacity of salt soluble protein.5. Studies of thermal stability of salt soluble protein showed that, the rate of protein thermal denaturation and aggregation was significantly accelerated with the extension of heating time and increased heating temperature. In the range of test temperature, at 40℃of heating treatment, the turbidity of protein system did not change significantly during 30 min, but effect of salt concentration of salt soluble protein on heat denaturation rate was significantly different with the increased heating temperature. The aggregation rate of salt soluble protein in 0.3 mol/L KCl was 6.8 times of that in 0.6 mol/L KCl at 50℃.6. The pH stability of salt soluble protein analysis showed that stability in acidic and alkaline conditions at low temperature was different. pH-shifting treatment from pH 3.0 to 7.0 and from 4.0 to 7.0 reduced the solubility of salt soluble protein, the content of active thiol, the emulsifying and foaming properties, especially stability of salt soluble protein by low concentration extraction was poor. pH-shifting treatment from pH 9.0 to 7.0 and from 11.0 to 7.0 increased the solubility of salt soluble protein, while the content of activity sulfhydryl did not change significantly. Therefore, the stability of salt soluble protein was higher at alkali condition. However, both the acid and alkali adjustment reduced the Ca²⁺-ATPase of salt souble protein significantly, the hydrophobic groups of protein molecules was exposed and the relative surface hydrophobicity increased. In addition, the desalination of salt soluble proteins resultes to the loss of Ca²⁺-ATPase, and the stability of the protein was significantly worse. The analysis of SDS-PAGE electrophoresis further indicated that pH-shifting treatment from pH 3.0 to 7.0 resulted to partially degradation of myosin heavy,desalt soluble protein had less protein band.7. The insoluble myostromin protein from Tilapia muscle dissolved at pH 3.0 or pH12.0,and precipitated at pH 5.5, the protein content of acid-soluble and alkali-soluble protein were 61.02% and 58.17%. The protein content of alkali-soluble and acid-soluble protein was high. The protein content in the dry basis of alkali-soluble and acid-soluble protein was 88%, and was higher than that of untreated myostromin protein. While the ash content of acid-soluble and alkali-soluble protein was significantly lower than that of myostromin. Effect of pH value and ionic strength on the solubility, emulsifying and foaming properties in acid condition was clear. The solubility, emulsifying and foaming of protein in acidic conditions was poor, while was good in the alkaline condition. Comparatively, in the pH range of experiment, the solubility of alkali-soluble protein was better than the acid-soluble proteins. In the ionic strength range from 0.1 mol/L to 0.4 mol/L, the protein solubility, emulsifying and foaming activity increased with salt concentration increased significantly with the increasing of salt concentration, and these properties reached a peak. Consequently, for recovery of insoluble myostromin protein, dissolving at alkali condition and precipitation at pH 5.5 were more suitable.