Establishment Of Prediction Model For Ice Crystal Growth And Quality Evaluation System Based On Fractal Dimension Of Frozen Hairtails(Trichiurus Haumela)

Cryopreservation is the most common and effective preservation method,and can effectively inhibit the spoilage of aquatic food.However,during the frozen storage,the characteristics of ice crystals formed has a great influence on the quality of food products such as water retention,lipid oxidation,and protein denaturation,and directly affect the shelf life and commercial value of the foods.The research on the formation mechanism of ice crystals during the freezing process and frozen storage is of important theoretical significance for the improvement and promotion of freezing technology in the field of aquatic foods.Therefore,this study has conducted a comprehensive and systematic study on the methodologies of ice crystal observation formed on different factors such as different storage temperatures,different storage time,and different freeze-thaw cycles in the cold chain logistics.The prediction model of ice crystal growth,and the inhibition of ice crystals by chitooligosaccharides as antifreeze agent,and the antifreeze and water retention effect were also studied.Besides,the fractal dimension was applied as the quality evaluation indicator of hairtails.The conclusions obtained are as follows:(1)The methodology of ice crystals observation in hairtails subjected to different freezing methods was conducted.The four freezing methods were cryosectioning,cryosubstitution,scanning electron microscopy and X-ray micro-computed tomography scanning.Results indicated that there was no significant difference in the characteristics of ice crystals between the cryosectioning method and the cryosubstitution methd(p>0.05).Compared with cryosubstitution,cryosectioning method greatly shortened the time of sample preparation to 20-30 min from 3-5 days.In addition,all the methods showed that samples frozen by liquid nitrogen produced smaller and more regular intracellular ice crystals than other samples,and their muscle fibers maintained better integrity,tightly packed,and higher water holding capacity.For samples frozen by conventional air freezing,large extracellular ice crystals were generated,which caused separation and destruction of muscle fibers with lower water holding capacity,and caused irreversible damage to the final quality of the frozen foods.(2)It was concluded that with the extension of storage time,the 4 groups of samples showed an increasing intermuscle space,which means the ice crystal size was increasing.The obvious separation of muscle fibers was due to the increase of the volume of ice crystals.When the storage temperature was lower,smaller ice crystals were formed at the same time,the growth of ice crystals was not obvious,the damage to the tissue was smaller,and better food quality was produced.In addition,during recrystallization process,the roundness and the longiness of ice crystals did not change significantly,which indicated that ice crystals were uniformly amplified in all directions based on the initial morphology.Through the first order kinetic model and Arrhenius equation,the prediction model for ice crystal size was D=D0 exp(0.5624t exp(-7745.45/RT).According to the verification on the model,the error between the predicted value and the measured value are both within 10%.Therefore,the established model can predict the ice crystal size of frozen hairtails under different storage temperatures and different storage time.(3)With the extension of storage time,the water activity,color,springiness,hardness and chewiness all showed different degree of decline,while the K value gradually increased.In addition,the results showed the lower the storage temperature,the slower the decline of water activity,color,texture and the slower rise rate of K value,and the more beneficial it is to maintain the original quality of the food.Then FD value showed a decreasing trend with the extending of storage time,indicating that the irregularity of hairtails was increased during storage.The lower temperature,the smaller rate of decrease in FD value.Comparing the results of FD and K value,we found that the FD value of first-order freshness was 1.9565 to 1.9175,the FD of secondary freshness was 1.9175 to 1.7705,and the FD of primary corruption was 1.7705 to 1.5302.The freshness critical value of FD was set,so the fish freshness can be directly evaluated by obtaining FD value.Finally,the function model between FD and traditional indicators was established.The correlation coefficient of the function model was good and the degree of fitting was well.Therefore,the method of quality assessment of frozen hairtails based on fractal dimension was established.(4)During the process of repeated freeze-thaw treatment,ice crystals continuously disappear and regenerate.The muscle fibers contract radially and large pores appear between the muscle fibers.The formation and growth of ice crystals will cause certain damage to the fish muscle cells.The porous microstructure was analyzed by fractal dimention,which decreased with the increasing number of freeze-thaw cycles,suggesting that the irregularity of muscle tissue due to the formation of ice crystals increased.At the same time,water holding capacity continuously decreased during freeze-thaw cycles,and the TVBN,TBA,and the total plate count all showed an increasing trend.When frozen at a lower temperature,the decrease in fractal dimension,water holding capacity and texture properties can be suppressed to a certain extent,and the increase in TVBN,TBA,and the total plate count can also be suppressed.It suggested that during the frozen storage of hairtails,the number of freeze-thaw cycle should be reduced as much as possible,and at the same time,the frozen storage temperature should be lower in order to maintain the original fresh quality of hairtails well.This result also further proves the feasibility of fractal dimension in evaluating the quality changes of hairtails during frozen storage.(5)Chitooligosaccharides can effectively reduce the liquid loss of hairtails,and have good effects in maintaining the texture,delaying the decrease of myofibrillar protein content,and protecting the sulfhydryl and Ca2+-ATPase activity.In addition,chitooligosaccharides can inhibit the growth of ice crystals in frozen hairtails,which can delay the decrease of fractal dimension during frozen storage and maintain the microstructure of frozen hairtails.At the same time,higher concentration of chitooligosaccharides have better effects.Therefore,chitooligosaccharides can be used as an excellent antifreeze and water-retaining agent in frozen aquatic products.The large amount of hydroxyl groups contained in it can effectively inhibit the formation of ice crystal,prevent protein denaturation,effectively play a role in antifreeze and water conservation,and ensure good product quality.This study helps to understand the mechanism of ice crystals and related quality changes of frozen hairtails comprehensively,which is useful for optimizing the industrial structure.The fractal dimension theory improves the evaluation system of frozen hairtail quality,providing new ideas and scientific reference to the quality study of other aquatic products and meat.