| The manufacture of surimi and surimi-based products is an important developing direction for Chinese freshwater fish processing. At present, conventional washing processing is widely used in the production of surimi, but this technique brings about some problems to be solved, such as the heavy labor intensity, the big water consumption, the loss of protein yield and the production of a large number of waste water containing high solid content, nitrogen content and chemical oxygen demand. Many reports have shown that pH-shift processing (acid- and alkali-aided processes) can be used in the recovery of fish muscle proteins and then the production of surimi. Usually, pH-shift processing can promote the utilization of raw materials, improve the protein yield and reduce the labor intensity and the production cost. Currently, the research works about pH-shift processing mainly focus on the marine fish species rather than the freshwater fish species. There is no study to utilize pH-shift processing to produce surimi from bighead carp although many Chinese reports are concerning about pH-shift processing in the production of surimi from some low-value freshwater fish species, such as silver carp and grass carp. Therefore, the aim of this study is to investigate the feasibility of pH-shift processing on the production of minced bighead carp and the effect of pH-shift processing on gel quality of minced bighead carp. It provides the potential for the creation of a new technique in the production of freshwater fish surimi as well as the development of the novel surimi-based products. The main results are as follows:1. The preliminary comparisons of conventional washing processing and pH-shift processing were carried out by determining the basic indicators of minced bighead carp made from wash surimi and protein isolates. Results indicated that the ash and crude fat contents of minced bighead carp were decreased significantly (P<0.05) and the crude protein content increased significantly (P<0.05) comparing with the raw fish muscles. The solubility of bighead carp muscle proteins was maximum at pH 3.0 and 12.0, respectively, while the isoelectric point of bighead carp muscle proteins was 5.5. Moreover, pH-shift processing led to the higher protein yield and fat reduction (P<0.05) than conventional washing processing. The water consumption of pH-shift processing was significantly lower than that of conventional washing processing (P<0.05). The water- and salt-soluble protein contents of minced bighead carp were decreased remarkably (P<0.05) as compared to the raw fish muscle, and pH-shift processing resulted in the lower water-and salt-soluble protein contents comparing with conventional washing processing (P<0.05).2. The gelation characteristics of bighead carp protein isolates prepared by pH-shift processing were investigated as compared with those of wash surimi prepared by conventional washing processing. Results showed that the gel from alkali-aided protein isolate exhibited a higher breaking force than that from conventional wash surimi (P<0.05), while the gel from conventional wash surimi exhibited the higher deformation and gel strength. In textural profile analysis (TPA), the highest hardness and cohesiveness were observed in the gels prepared by the conventional washing and alkali-aided processing, respectively, while the lowest TPA values (except the springiness) were obtained in the gels prepared by acid-aided processing. Conventional wash surimi gels exhibited a higher whiteness than acid- and alkali-aided protein isolate gels (P<0.05) due to the higher lightness (L*) and lower redness (a*) and yellowness (b) values (P<0.05). In addition, the highest water-holding capacity was found in the gels from conventional wash surimi. From the microstructure, acid-aided processing could lead to a coarser and more unordered gel network. The folding test score of surimi gel was higher than that of protein isolate gels and the lowest score was obtained from acid-aided protein isolate gels. Rheological study showed that alkali-aided protein isolate exhibited the highest final storage modulus (G’) during heating and the protein isolates exhibited a total different loss tangent (tanδ) pattern from conventional wash surimi.3. The changes on the structure of bighead carp muscle proteins were determined after conventional washing processing and pH-shift processing. Result showed that the activity of Ca2+-ATPase found in acid- and alkali-aided protein isolates was significantly lower (P<0.05) than those from conventional wash surimi. The acid- and alkali-aided protein isolates showed a total different pattern of circular dichroism (CD) spectrum and the lower a-helix content (P<0.05) as compared to conventional wash surimi. In addition, the result of sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) demonstrated that the intensities of myosin heavy chain and actin bands from protein isolates were lower than those from conventional wash surimi. The intensities of myosin heavy chain bands decreased remarkably but the intensities of actin bands rarely changed after the thermal gelation. Moreover, two endothermic peaks were found at approximate 35℃ and 67℃ in the differential scanning calorimetry (DSC) thermogram of conventional wash surimi, while no endothermic peak was found in the DSC thermogram of protein isolates prepared by pH-shift processing. Finally, the surface hydrophobicity of acid- and alkali-aided protein isolates increased significantly (P<0.05) as compared to conventional wash surimi. The hydrophobic interactions and disulfide bonds were the main bonds to maintain the stability of gel matrix. To sum up, the major reason leading to the lower gel properties was the denaturation and aggregation of myosin in minced bighead carp after treated by pH-shift processing.4. The proteomic technique was applied to investigate the specific changes of bighead carp muscle proteins treated by conventional washing and pH-shift processes. Result demonstrated that the proteins from minced bighead carp could all exhibit the clear and high resolution two-dimensional gel electrophoresis (2-DE) maps. There were 102 differential protein spots obtained in 2-DE maps in terms of significantly statistical differences (P<0.05). Among them, there were 71 differential protein spots identified by mass spectrometric analysis. Most of the identified differential protein spots were obtained from the cytoplasm, cytoskeleton and myosin complex. These protein spots could be classified into two types, one had the binding functions and another participated in the enzyme activities of the organism. The differential protein spots mostly involved in the metabolic process, phosphorylation, glycolytic process and oxidation-reduction process. Conventional washing processing led to the lower densities of protein spots related to some enzymes, including pyruvate kinase, enolase 1, phosphorylase and creatine kinase M2 as compared to the raw fish muscles and pH-shift processing. There were 14 protein spots related to bighead carp muscle tissues and the structure of muscle proteins. Moreover, conventional washing processing resulted in the lower densities of protein spots for myosin heavy chain fast skeletal type 3 and actin 3b, while pH-shfit processing resulted in the lower densities of protein spots for low-temperature light meromyosin and desmin. The wash surimi exhibited the higher densities of protein spots for myosin heavy chain fast skeletal type 1, myosin low-temperature type S1 heavy chain and tropomyosin alpha-1 chain comparing to the protein isolates. Moreover, pH-shift processing contributed to the higher densities of protein spots for Sk-tropomodulin and myosin regulatory light chain. Especially, the acid-aided processing led to the higher density of protein spot for myosin heavy chain fast skeletal type 3, while the alkali-aided processing led to the higher densities of protein spots for myosin heavy chain fast skeletal type 1 and 10C light meromyosin, respectively.5. The effects of freeze-thawing cycles on gel properties of minced bighead carp were investigated after conventional washing and pH-shift processes. Result showed that the increasing freeze-thawing cycles resulted in the decreasing breaking force, deformation and strength of gels from minced bighead carp. Moreover, gel hardness, springiness, cohesiveness and chewiness were also decreased along with the increasing freeze-thawing cycles. Freeze-thawing cycles could lead to the lower water-holding capacity as well as the higher cooking loss. In addition, freeze-thawing cycles could lead to the lower gel whiteness due to the decreased lightness and the increased redness (a) and yellowness (b). The folding test scores of surimi gels were declined during the freeze-thawing cycles. The structure changes of myosin during the freeze-thawing cycles were studied in this part by determining the salt-soluble protein content, Ca2+-ATPase activity, carbonyl content, total sulfydryl content, surface hydrophobicity and thiobarbituric acid reactive substance (TBARS) value and carbonyl content. Result showed that the salt-soluble protein content, Ca2+-ATPase activity and total sulfydryl content is on the decline while the surface hydrophobicity, TBARS value and carbonyl content is on the rise along with the increasing freeze-thawing cycles. |
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