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Studies on Gelation Mechanism of Tilapia Muscle Proteins Induced by Hydrostatic Pressure
Kuo Chiang, Hsu
本研究係以吳郭魚為材料，探討魚肉蛋白質之加壓凝膠機制。結果顯示吳郭魚肉糊加壓及加熱誘導凝膠之性質不同，與其對蛋白質之變性機制不同有關，加壓誘導凝膠 (≧200 MPa) 主要是以離子鍵、氫鍵、疏水作用力及誘導產生之雙硫鍵構成較柔軟、具黏性且顏色較接近肉糊之凝膠；而加熱誘導凝膠 (suwari) 是以離子鍵、雙硫鍵、疏水作用力及非雙硫鍵之共價鍵等構成較硬、具彈性之白色凝膠。進一步以加壓及加熱交互處理進行凝膠特性之探討，發現單以加壓或加熱處理所形成結構最佳之凝膠條件分別是在 200 MPa 或 50℃；當吳郭魚肉糊先經 200 MPa 加壓再經 50℃ 加熱處理後可得強度最佳之凝膠，此凝膠包含由加壓所形成之氫鍵及離子鍵，亦包含加熱所形成之 GL 鍵結、雙硫鍵及疏水作用力等；而先經 50℃ 加熱再經 200 MPa 加壓處理則只形成與加熱凝膠相似性質之凝膠；在 200 MPa 同時併行 50℃ 加熱處理則會抑制凝膠之形成而呈現極為黏性之凝膠，其性質介於加壓及加熱誘導凝膠之間，但較偏向於加壓凝膠。意即加壓可形成蛋白質間之非共價鍵結而提升凝膠之黏性性質，再經加熱後除可維持前述結構外更可形成大量共價鍵結而提升凝膠之彈性性質。
吳郭魚肌凝蛋白及其斷片在加壓過程中，只有肌凝蛋白及 S-1 會有較明顯之物理及化學性質之改變，rod 則未見變化。肌凝蛋白及 S-1 分別在 150 及 200 MPa 之壓力處理後會有明顯之凝集，此與分子量大小對壓力之敏感度有關。因此可推論吳郭魚之加壓誘導凝膠機制為：肌凝蛋白頭部在較低壓力 (≦100 MPa) 下，會先形成分子內鍵結而使體積縮小，而未有大量凝集團出現；當壓力提高至 ≧150 MPa 時，S-1 開始展開，將疏水基及反應硫氫基暴露出來，最後因分子碰撞產生雙硫鍵及疏水作用力等鍵結而凝集成團，形成極具彈性之凝膠體。但 rod 對肌凝蛋白凝膠之黏彈性質仍具貢獻。
由肌凝蛋白及 S-1 Ca-ATPase 之加壓失活動力學分析得知，若不考慮瞬間高壓破壞 (instantaneously pressure kill; IPK) 時，肌凝蛋白及 S-1 在壓力每提高 185 及 806 MPa 會使 D 值減少 90%，因此肌凝蛋白之加壓變性與瞬間高壓破壞、壓力大小與時間長短有關，而 S-1 之變性只與瞬間高壓破壞與壓力大小有關。
Tilapia was used to investigate the gelation mechanism of fish proteins. Results showed that the properties of pressure- and heat-induced gels were different from the protein denaturation mechanisms. Pressure-induced (≧200 MPa) gels, constructed by ionic and hydrogen bonds, hydrophobic interactions and disulfide bonds, were soft, viscous and similar color to the meat paste. Heat-induced (50℃) gels, constructed by ionic and disulfide bonds, hydrophobic interactions and GL bonds, were hard, elastic and in white color. The stronger strength of gels induced by pressure or heat were obtained by the treatment at 200 MPa and 50℃, respectively. We combined the two treatments (200 MPa→50℃) and got the strongest gel strength which stabilized by all the bonds induced by pressure and heat. However, the properties of gels formed by the treatment at 50℃ and followed by 200 MPa were similar to those only by heat. On the other hand, the gels formed by 200 MPa at 50℃ were very viscous owing to the inhibition of protein denaturation. Therefore, pressure could help viscous properties of gels due to the non-covalent bonds, and the following heat treatment helped maintain the original bonds and constructed the covalent-bonding structures of gels with elastic properties.
Physical and chemical properties of myosin and S-1 changed by pressure treatment (50-300 MPa/4℃), but those of rod did not. Myosin and S-1 molecules occurred apparently aggregation by pressure treatment at 150 and 200 MPa, respectively, due to the different pressure sensitivity on molecular weight. The pressure-induced gelation mechanism of tilapia myosin is as follows: myosin head decreases its volume and forms intramolecular interactions by pressure treatment behind 100 MPa; when pressure increases to 150-200 MPa, hydrophobic groups and reactive sulfhydryl groups of S-1 exposure outward, then myosin forms elastic gels by disulfide bonds and hydrophobic interactions owing to S-1 aggregation.
Due to the analysis of inactivation kinetics of myosin and S-1 Ca-ATPase, the denaturation level of myosin and S-1 decreased 90% of D value with each increase of 185 and 806 MPa, respectively, without considering IPK. Therefore, pressure-induced denaturation of myosin depended on IPK, pressure level and duration, however, that of S-1 depended on IPK and pressure level only.
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