| Surimi can be made into many kinds of favourite surimi products. In the past decades, the business on surimi and surimi products was expanding in many countries very fast. In recent years, with the rapid development of the output of freshwater fish in China, the freshwater surimi industry has been developed quickly. While during the rapid development of surimi industry in our country, some related problems also emerged such as surimi adulteration, freeze-thaw cycles during storage and transport, lack of correlation researches on high quality surimi products processing. These have seriously restricted the further development of surimi industry. So achieving rapid detection of surimi quality and studying of surimi protein change mechanism are very important to promote surimi industry development and ensure surimi food safety.The purpose of this study was to research the rapid detection method of silver carp surimi quality and the silver carp surimi protein structure changes induced by process based on ATR-MIR spectroscopy.The main results and conclusions were as follows:1. ATR-MIR spectroscopy was used to classify sliver carp surimi adulterated with different levels of hairtail surimi. Six chemometric methods, including SIMCA, KNN, SVR, PLS-DA, ID3 Decision tree and DA were used to build the classifying models. And the performances of the models were compared. Results showed that when adulterated ratio was greater than 20%, better classifications were obtained using SIMCA and DA model. And the best pretreatment methods for SIMCA and DA were Dl and D2 combined with 11 datas SG filter. When adulterated ratio was less than 20%, the performance of DA and SIMCA model were all unsatisfied. The correct rate of models based on the other four methods (PLS-DA?ID3-decision tree?SVM and KNN) were all very poor. That indicated they were not suitable to classify adulterated surimi samples.2. The traditional modeling methods have some bottleneck, so two-dimensional correlation ATR-MIR spectroscopy, namely the 2D-COS combined with ATR-MIR, was used to quantitative classify sliver carp surimi adulterated with different levels of hairtail surimi. Two useful parameters, Euclidian distance and correlation coefficient between 2D correlation spectra of different surimi samples, were used for this purpose. The classification of surimi samples were carried out through the variance analysis and cluster analysis methods of the Euclidean distance and correlation coefficient. Results showed that for adulaterated surimi samples, variance analysis of the above two parameters all could be used to calssify the sliver carp surimi adulterated with different levels of hairtail surimi (5%,10%,20%,40%,60%,80%). However the classification capability of cluster analysis was not satisfactory, the samples with the adulterate level below 20% were all classified as one kind. So the Euclidian distance and correlation coefficient of two-dimensional correlation ATR-MIR spectroscopy were more suitable to classify adulterated surimi samples than traditional modeling methods.3. ATR-MIR spectroscopy was used to classify sliver carp surimi experienced different freeze-thawed cycles. Six chemometric methods, including SIMCA, KNN, SVR, PLS-DA, ID3 Decision trees and DA were used to build the classifying models. And the performances of the models were compared. Results showed that the model based on ID3-decision tree was the best, the total correct rate was 94.56%, and the correct in calibration set and validation set were 98.48% and 84.61% respectively, and followed by the DA and PLS-DA model. The other three classification models (SIMCA, SVM and KNN) were not suitable to discriminant surimi samples experienced different freeze-thawed cycles. From the detailed classification, we could find that most of the misclassifications were among surimi samples experianced 0 and 1 cycle. The surimi samples experianced more cycles could be classified corectly. So the ID3 Decision tree was more suitable to classify surimi samples experianced freeze-thaw cycles more than 1 times.4. The same two-dimensional correlation ATR-MIR spectroscopy was also used to classify sliver carp surimi experienced different freeze-thawed cycles. Results showed that for surimi experienced different freeze-thawed cycles; the classification capability of variance analysis based on Euclidean distance was less than the variance analysis based on correlation coefficient. Two-dimensional correlation ATR-MIR spectroscopy could discriminate surimi experienced more than 1 time of freeze-thaw cycle. While the surimi samples experienced 1 time of freeze-thaw cycle could not be completely discriminated with urimi samples experienced 0 time of freeze-thaw cycle correctly.5. The changes of secondary structure in silver carp surimi protein were also analyzed in situ using 2D-COS combined with ATR-MIR. The main conclusions were as follows:when silver carp surimi gelled, the hydrogen bond in protein breaked, the content decreased. The polarity of the protein molecules increased and also the hydrophobic force increased. The disulfide bond content in protein also increased. Some a-helix structure changed into P-sheet, ?-turns and random coil respectively. The main change tendencies of P-sheet were the intramolecular ?-sheet converted into intremolecular P-sheet gradually and the parallel ?-sheet converted into anti-parallel ?-sheet. Through the Raman spectra analysis of silver carp surimi protein:changes in the region of 500-550 cm-1 indicated that two disulfide bond with lower energy were mainly transformed into the conformation with high energy conformation. After salt chopping, the value of double bond (I850/830) of surimi samples was higher than that of raw silver carp surimi samples. Salt chopping before surimi gel-forming, could dismantle the myofibrillar proteins, destruct the ionic bonds, let the tyrosine residues be exposed, and also increase surimi protein hydrophobicity. Raw surimi protein secondary structure was mainly composed of a-helix and P-sheet. When the surimi gelled, the a-helix were mainly into ?-sheet and random coil structure. And the content of antiparallel ?-sheet increased significantly.Through two-dimensional correlation ATR-MIR spectroscopy analysis of silver carp surimi, we could draw the following conclusions:The change order of main MIR peaks in silver carp surimi during the gelation process were as follows:2927,2854, 1770 cm-1?1567 cm-1?1630?1457cm-1?1650cm-1?3361?3285?1683?1503 cm-1. The order indicated that during surimi gel process, protein hydrophobic interaction changed firstly, then the secondary structure of the protein were affected and changed, with further changes in the secondary structure of protein, the N-H bond of protein peptide were affected.6. The changes of silver carp surimi protein secondary structure induced by different freeze-thaw cycles were also studied by 2D-COS combined with ATR-MIR. The main conclusions were as follows:freeze-thaw cycles accelerated the oxidation process of surimi protein. The peaks of carbonyl absorption and disulfide bonds were significantly enhanced due to the increased freeze-thaw cycles. In addition, the ?-helix and ?-sheet were all decreased, however the content of the anti-parallel ?-sheet increased. There was no obvious change in the structure of the ?-turn and random coil. The assignments of some undefined absorption peaks were confirmed, for example the absorption at 1223cm-1 of silver carp surimi was assigned to antiparallel ?-sheet,1590 cm-1 was assigned to a-helix and 1639 cm-1 was assigned to anti-parallel ?-sheet.Two-dimensional correlation ATR-MIR spectroscopy analysis under the external perturbation of freeze-thaw cycles showed that the a-helix structure was firstly affected and the content decreased, then the content of antiparallel P-sheet structure increased, and finally the content of carbonyl increased; Two-dimensional correlation ATR-MIR spectroscopy analysis under the external perturbation of tempreture showed that, compared with surimi experienced one freeze-thaw cycles, the auto correlation peaks and cross peaks of surimi samples experienced two and three freeze-thaw cycles all became less obvious, indicating that the protein secondary structure has been seriously damaged. |
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