Physicochemical And Rheological Changes Of Myofibrillar Proteins From Tuna (Thunnus Obesus) During Frozen Storage

Tuna and tuna-like species are important fish species due to their high globaleconomic value and their frequent use for canning and sashimi production ininternational trade. Myofibrillar protein, the most abundant protein group in meat, canform a viscoelastic gel capable of physiological function.The gel-forming propertiesof myofibrillar proteins are essential to the develop of muscle-based products for itscontribution to textural properties, shaping the product, retaining water， etc.Rheological techniques can be used to determine subtle changes in muscular tissue.The gelation characteristics of myofibrillar proteins are indicative of the texture of themeat product.Sea foods are highly perishable and usually spoil faster than other muscle foods.They are more vulnerable to texture deterioration than other meats. Frozen storagebecame one of the most important techniques for long-term preservation of mincedfish muscle. Nevertheless, frozen commonly damaged muscle protein, induced proteindenaturation and resulted in a loss of protein functionality. Structural andphysicochemical changes accompanied by deterioration of muscles still could happenduring frozen storage. In order to know the mechanism of protein denaturation duringfrozen storage, the physicochemical changes of muscle protein as well as therheological properties of tuna during frozen storage at-18and-30°C wereinvestigated.The main research contents and conclusions are as follows:（1）Chemical factors including ionic strength, pH value and SDS concentrationwere investigated for their roles in the rheology of myofibrillar protein. The higherNaCl concentration would be able to hold linear viscoelasticity steadily. The gelationcharacteristics of myofibrillar proteins are indicative of fish product texture. Theeffect of NaCl concentration （0.3⁰.6mol/L） on tuna myofibrillar protein gelation andrheological properties were studied using dynamic oscillatory rheometer of both small-amplitude oscillatory shear （SAOS） and large-amplitude oscillatoryshear（LAOS）. The results showed that during the gel heating-induced process, the G‘of myofibrillar protein increased with the increase of NaCl concentration before47℃.But this general trend changed after47℃, G‘reached a maximum value at NaClconcentration0.4M. The influence of NaCl was stronger during the period of gelformation. The relationship between its storage modulus and strain within could bedescribed by the logarithmic model. The LAOS tests showed that the higher NaClconcentration would bring closer network structure and larger energy dissipated （Ed）during a sinusoidal stress cycle.The effect of pH （5.6–7.0） on tuna myofibrillar protein gelation and itsrheological properties were studied.The temperature of inflection point1significantlyincreased （P <0.05）, while that of inflection point2decreased （P <0.05）, as pHincreased. The myofibrillar protein at the inflection point exhibited markedlynonlinear characteristics as the strain increased, indicating unstable proteinconformation. LAOS tests showed that a higher pH leads to a closer network structureand greater energy dissipation during a sinusoidal stress cycle. Higher pH valueswould maintain a steady linear viscoelasticity. Thus, we found that proteindenaturation and gel formation were pH dependent. Furthermore, pH also appeared toinfluence the water-holding capacity and surface hydrophobicity of tuna myofibrillarproteins.The SDS experiment showed that hydrophobicity played a key role in thegelation of myofibrillar proteins.（2）Physicochemical changes of muscle from tuna were monitored during30daysof storage at-18and-30°C. Ca²⁺-ATPase activity of myofibrillar protein （MP） storageat both temperature decreased continuously during storage （P<0.05）. A decrease insulfhydryl group content was observed during the storage （P<0.05）. TBA valueincreased with the storage time. Ca²⁺-ATPase and sulfhydryl content can be used asindicators for the integrity of myosin molecules.The temperature of frozen storageexhibited significant influence on denaturetion of actomyosin. Frozen storage of tunaresulted in protein denaturation and tissue disruption. Disulfide bond formation andexposure of hydrophobic residues was associated with the loss in protein integrity.During the storage at-18℃and-30℃，the freshness index, K value, of tuna meatmaintain below20%indicating that the meat was in the first grade of freshness in30d.Protein denaturation and lipid oxidation are of the most significant impact. Changes in the physical and chemical properties of extracted fish proteins give morein-depth information on the changes that have occurred at the head of myosin duringfrozen storage. Cryoprotectants can suppresse the protein fozen denaturation. Weinvestigated the effect of trehalose on the denaturation in tuna myofibrillar proteinduring frozen storage at18°C and-30°C for30days. The changes in the SH andCa²⁺–ATPase activity were examined during frozen storage. The addition of trehalosemarkedly decreased the inactivation rate of the SH contents and Ca²⁺–ATPase. Theeffect was greater than that of sorbitol.Trehalose significantly suppressed the inactivation of Ca²⁺-ATPase and SHcontents of tuna myosin.Trehalose and its mixtures are characterized in respect to theother disaccharides and their mixtures by a superior structural resistance to thermalstress which allows them to create a more rigid environment to protect biologicalstructures.（3）Effects of different storage conditions on rheological properties of myofibrillarproteins were investigated. Results showed that rheological properties were highlycorrelated with storage temperature. Storage temperature showed significant impacton protein denaturation, including changes of SH contents and disulfide bonds whichmay damage the structure of heavy meromyosin. Rheological properties ofmyofibrillar proteins relies heavily on the physiological function of myosin.Myofibrillar protein at-18°C showed higher storage modulus （G‘） than-30°C.Tuna myofibrillar proteins proved to belong to non-Newtonian pseudoplasticfluid, its viscosity distinctly reduced with increasing shear rate. Measurement of theapparent viscosity of fish muscle homogenates has been proposed as an indicator ofquality. Inherent physico-chemical properties, e.g, molecular weight, polydispersity,hydrophobicity, and the conformation of each protein species, affect the viscosity ofthe protein solution. Thixotropy myofibrillar protein increased with the decrease ofstorage temperature, indicating that the system could have a strong synergisticinteraction and a more stable structure at lower temperature.The higher temperaturewould lead to loose structure and the strand break of protein chains.Inflection points （IP） of both storage temperature differed significantly （P <0.05）,especially IP2,which unfolding and association of heavy meromyosin （HMM）. Thedecrease in sulfhydryl groups with a concomitant disulfide bond formation may causethe change of IP2. The loss of free sulphydryls associated with the decrease inCa²⁺-ATPase activity could result in an ascent in the storage modulus （G‘）. The denaturation of the head of myosin may have an influence on the gelling phase ofmyofibrillar proteins.