| Bighead carp（Aristichthys nobilis） is one of the most abundant fish species in Chinaand one of the edible fish for cooking. Based on the systematic study on raw materialproperties of bighead carp, this paper was focused on the investigation of the effect ofdifferent heating temperature, heating time on textural properties, color, microstructure, etc.Result showed the change of bighead carp meat quality with the variation of the thermalprocessing conditions. Via the analysis of physical and chemical properties of fish, alternationof the composition and structure in the protein and myofibrillar protein thermal processingcharacteristics, the mechanism of bighead carp meat quality changes was studied. Thisprovides a theoretical foundation for establishing correct thermal processing parameters. Thisstudy has guiding significance and theoretical value for rational design of heating orsterilization procedures and it is important for promoting the development of freshwater fishprocessing industry.Firstly, the raw material properties of bighead carp were studied. Morphology, texturalproperties and nutritional characteristics of bighead carp with different sizes and at differentlocations of the body were compared. It was shown that the difference of dorsal muscles wassmall, but that of head and tail large; protein is a major nutrient and slight differences existedin different parts of fish (about18%); salt-soluble and water-soluble protein attributed to alarger content of about8.4%and4.8%, respectively; tail collagen attributed to the largestcontent; no significant difference in the amino acid composition of proteins was found indifferent parts. The ratio of essential amino acids and total amino acids was the highest,reaching more than40%; the result of AAS, CS and EAAI showed that the first limitingamino acids were methionine+cystine and the second limiting amino acid was valine. Thecomposition of amino acids in fish was balanced and it was easy to digest and absorb;Calcium is the mineral elements with highest content of1195.36mg/kg;16kinds of fattyacids were detected, with unsaturated fatty acids69.77%(polyunsaturated fatty acids:28.80%). The main component of the unsaturated fatty acids was oleic acid (34.66%); DHAand EPA were also high in amount.Secondely, the effect of heating temperature on bighead carp meat quality was studied.The results showed that: weight loss rate, brightness, whiteness, yellowness increased withincreasing temperature, but no significant change occurred after70℃; water holding capacity,pH and other indicators varied slightly; hardness, elasticity, chewiness, plastic viscosity, shearforce and other indicators showed a four-stage change: a double peak occurred at about40,80℃. A minor increase happened before40℃, followed by a rapid decline till the bottom atabout60℃. Afterwards slow rise occurred till peak at around80℃. Then the curve decreased.The second peak is small and not detectable; the trend of cohesion and the recovery propertieswas similar with springiness, but the first peak is not obvious. As the temperature increased,the moisture content of bighead decreased; the corresponding crude protein content rised;salt-soluble and water-soluble protein content significantly declined after40,50℃,respectively. In contrast, alkali-soluble protein content significantly went up. SDS-PAGEelectrophoresis analysis further proved this. The contentn of soluble collagen in meat and collagen in expressible liquid showed an upward trend and began to go flat after70℃.Collagen bands in expressible liquid were visible in the SDS-PAGE.Correlation analysis showed that: the texture index of hardness and shear force weresignificantly positively correlated with water-soluble, salt-soluble protein content (P <0.01),but significantly negatively correlated with meat soluble collagen and the alkali-solubleprotein content (P <0.01); whiteness was positively correlated with soluble collagen andalkali-soluble protein in meat (P <0.05) and collagen content in expressible liquid (P <0.01),but was negatively correlated with the water-soluble protein, salt-soluble proteins shear force(P <0.01) and hardness (P <0.05). These results showed that the dissolution of collagen anddenatured protein significantly affects the texture and color of the meat during heating.Thirdly, the effect of thermal intensity on quality of bighead carp muscle wasinvestigated in this study. Results showed that after cooking at different temperatures, qualityof bighead carp muscle, characterized by texture and color, experienced significant changeswith the increase of cooking time. And the effect of cooking became more significant with theincrease of temperature. Generally, trend of shear force, hardness, chewiness andadhesiveness exhibited fluctuation as damped vibration，the process of which could be dividedinto four phases, i.e., increase followed by decrease, and then increase followed by decrease.There were two peaks in this process, the second of which was relatively lower and obscure.Additionally, in the relatively low temperature including90,100℃, with the increase ofcooking time, the changes of color was not significant, and the yellow value, b, slightlyincreased in the later stage of cooking, accompanied by a slight decrease of white value.However, in the relative high temperature of110and120℃，with the increase of cooking time,L value and white value both experienced significant decrease, but b value drasticallyincreased. Overall, in all cooking temperatures, the cooking loss rapidly increased in the earlystage, and tended to plain off in the subsequent stages.In addition, in the relatively low temperature including90,100℃, with the increase ofcooking time, changes of protein solubility and-amino nitrogen were not significant, withonly slight increase in the later stage of cooking. However, in the relative high temperature of110and120℃，with the increase of cooking time, protein solubility increased significantlyand-amino nitrogen dramatically increased. This indicated that cooking at hightemperature during long time lead to degradation of muscle proteins. Analysis of SDS-PAGEshowed that with the increase of cooking temperature and cooking time, intensity of myosinheavy chain (MHC) band gradually decreased and finally diminished. And intensity of theupper part of electrophoresis lane increased, indicating the aggregation or degradation ofmyosin in cooking process. Furthermore, the intensity of1,2and β bands of collagegradually decreased and finally diminished, suggesting the degradation of collagen in cookingprocess.Analysis of correlation showed that after cooking of120℃, hardness and water solubleprotein (WSP), protein exudant in cooking loss (PE), cooking loss (p<0.01) and-aminonitrogen (NH2-N) were negatively correlated (p<0.05). This suggested that cooking loss anddegradation of protein probably cause the quality deterioration of bighead muscle after cooking at high temperature with long during time. Also, the yellow value (b value), watersoluble protein (WSP)(p<0.01),-amino nitrogen (NH2-N) and protein exudant in cookingloss (PE) were positively correlated, indicating that the degradation of protein, accumulationof free amino acid and exacerbation of Maillard reaction was the mainly responsible for theincrease of yellow color of muscle.Fourthly, the thermal properties of myofibrillar protein were studied. Results showed thatadhesiveness and turbidity and rheological properties significantly changes with the increaseof temperature. And these properties were simultaneously influenced by pH of system. Thechanges of adhesiveness could be divided into four phase: firstly decrease, followed byincrease, and then decrease, and finally maintained the plain trend. From pH6.0-7.0, peak ofadhesiveness gradually decreased, and the temperature of maximum adhesiveness increased.The turbidity of system rapidly increased in the stage of40-50℃, followed by slow increase,and no changes was observed after60℃. Moreover, storage modulus (G’) exhibited changeswith five phases. G’ remained unchanged or slightly increased at first, and rapidly increaseafter34℃, peaking at45℃, and subsequently rapidly decreased before50℃, reaching theminimum value at50℃, and then rapidly increased until56℃, and finally maintainedunchanged. The peak at pH6.0was the maximum among all pH systems. As a large number of-helix exists in myofibrillar proteins, with the increase of temperature,-helix graduallyunwound into β-sheet and β-turn and random coil, and sustained unchanged after45℃.Furthermore, with the increase of temperature, the hardness, chewiness and adhesivenessof myofibrillar gel all increase first and subsequently decreased, peaking at80℃. At90℃,with the increase of cooking time, the hardness, chewiness and adhesiveness of myofibrillargel exhibited a general decreased trend, with a slight peak in cooking after70min. themyofibrillar and sarcoplasma degraded after cooking at relative high temperature (110and120℃) for long time. And the protein solubility and-amino nitrogen significantlyincreased with the increase of cooking time. The-amino nitrogen in sarcoplasma andmyofibrillar increased by8-10mg/g protein and3-5mg/g protein, respectively, indicating thatprotein in sarcoplasma was more extensively degraded than that of myofibrillar. |
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