| The processing methods of surimi products are mainly based on water bath heating and steam heating in the industrial application,while the traditional two-steps water bath heating mode is usually used as research method.Although the traditional two-steps water bath heating methods pass through gel degradation zone quickly,the gel strength of surimi is still very low.In addition,water bath produces large amount of sewage,the energy consumption and energy loss are also high during the heating treatment.As a kind of clean energy,microwave heating pomotes fast heating rate and high energy efficiency,which has the possibility of replacing traditional water bath heating in surimi industry.Due to the special heating mode of microwave,the responses of various components in microwave field are also different.Furthermore,the vibration of polar groups and the rotation of chemical bond sites among surimi components results in the change of molecular conformation,thus affecting the reaction process.However,the intervention of microwave intrinsic characteristics on the surimi gelation process is mainly reflected by the behavior of myosin and endogenous enzymes responding to electromagnetic field.Therefore,this paper takes the special heating mode of microwave as the dominant factor,the regularity of microwave heating on gel formation of surimi products was studied.On this basis,the internal mechanism of microwave heating surimi products was explored.Finally,microwave was applied to the actual surimi production after energy saving and quality research.Comparing with the traditional two-steps heating method,it is found that using microwave heating instead of the traditional water bath heating in the first step would lead to the deterioration of surimi quality.However,using microwave heating to replace the second step can significantly improve the gel strength and water holding capacity of surimi.Microwave promotes the crosslinking of protein gel mainly through disulfide bonds and non-disulfide covalent bonds.The results of scanning electron microscopy(SEM)about microstructure of surimi gel showed that denser gel network structure is formed by microwave.Although both temperature preservation mode and the time dependent mode can obtain favorable gel strength,the three-dimensional confocal laser scanning of surimi gel showed that the gel surface of time dependent mode is rough and shrinking.Fast heating rate of the microwave makes the protein molecules aggregate to form superior gel structure,and temperature preservation mode increases the quality of surimi gel while improving the morphology of surimi gel.The increase of myosin turbidity caused by microwave heating is greatly influenced by temperature,and surface hydrophobic interaction is the main force during the process of myosin aggregation.However,the disulfide bonds have particularly significant effects on the formation of myosin aggregation in the range of 60-90°C.The results of circular dichroism showed that the secondary structure of myosin was changed by microwave irradiation.In addition,the characteristic peaks at 202 and 222 nm gradually disappeared,which indicated that the structure ofα-helix of myosin molecule decreased.Furthermore,the analyses of particle size distribution demonstrated a more rapid myosin aggregation in response to the microwave treatment at 60 and 90°C.Finally,atomic force microscopy was used to characterize the aggregation behavior of myosin.It was found that microwave heating was more conducive for the aggregation of myosin to form macromolecular protein clusters.The structure and activity of endogenous enzymes were studied at the same heating rate.The activity of TGase(transglutaminase)was significantly increased by microwave irradiation and reached maximum at 20 min,while the activity of TGase was less affected by traditional water bath heating.The results of electromagnetic characteristics show that microwave has no obvious effect on the dielectric properties of TGase,but remarkably increases the conductivity of TGase.The conformational transformation of TGase under microwave field was investigated by means of ultraviolet and intrinsic fluorescence spectra,which clarify that microwave irradiation changed the spatial structure of TGase owing to exposed and accumulated chromogenic amino acids such as Trp and Tyr.Accordingly,spectral characteristics of TGase were enhanced.The change of microenvironment in the active region affects the ability of TGase to bind with substrates,thus altering the activity of TGase.Secondary structure of TGase was further studied by circular dichroism,which proves that microwave irradiation reduced theα-helix of TGase,but increased theβ-folding andβ-turn.Finally,the dynamic and static light scattering method was used to investigate the changes of the molecular chain configuration of TGase.The increasing R_g/R_h indicated that the molecular configuration of TGase was gradually extended and the active center was exposed under microwave irradiation.Taking gel strength and water holding capacity as indicators,the effects of microwave heating and traditional water bath heating on the gel degradation of surimi were compared,microwave heating can maintain better quality characteristics of surimi gel.The results of SDS gel electrophoresis and TCA soluble peptide showed that microwave could inhibit the decomposition of myosin heavy chain.Based on the overall evaluation of endogenous protease activity in surimi,Cat L(Cathepsin L)was choosen as the research object,and the effect of microwave heating on the activity and conformation of Cat L was investigated.The decrease of total sulfhydryl content and the increase of free sulfhydryl content after microwave heating indicated that the newly formed disulfide bond may shield the active center of Cat L.On the other hand,the newly exposed sulfhydryl group combined with the free sulfhydryl group of the active center,and reduced the activity of Cat L.In addition,the results of circular dichroism showed that the contents ofα-helix andβ-sheet of Cat L were decreased,which indicated that microwave induced the destruction of secondary structure and reduced the activity of Cat L.The structure of Cat L molecule chain was characterized by laser light scattering,the R_g/R_h of Cat L molecule heated by microwave increased first and then decreased,which suggested that the molecular structure was curled up,and therefore the activity of Cat L was decreased.The dielectric and microwave absorbing properties of surimi with high moisture content were studied,which demonstrated that the penetration depth of surimi was less than 1 cm,and the maximum microwave absorbing thickness of surimi was about 8 mm.Further temperature profile showed that the temperature distribution was the most uniform,and the quality was the best when the surimi thickness was 2 cm,however,the water loss rate was higher.Therefore,actual product fish tofu(2cm)was used as study subject,microwave combined steam method can not only improve the gel strength of the surimi,but also improve the microstructure of the surimi gel.In conclusion,the results of this research will help to improve the regulation mechanism of surimi processing and provide theoretical basis for microwave technology to better serve food processing. |
