Study On Frozen Storage Technology And Thawing Method Of Squid

Squid has many advantages,such as low fat content and high quality proteins,which has positive impacts in the health of consumers.At present,squid is easily susceptible to deterioration after being caught through long-distance fishing.In order to inhibit the decline in quality,freezing is a widely used technique in squid preservation.The freezing rate affected the the size and distribution of ice crystals,affected the oxidation proteins of squid.A layer of ice on the surface of frozen fish could significantly delay the deterioration of quality in aquatic products which was caused by dehydration,lipid oxidation and protein denaturation and helped to prolong the shelf-life during frozen storage.Meanwhile,the quality characteristics and shelf life of the product are significantly different during frozen storage at different temperatures.The squid sample need to be thawed before entering the people's mouths,but thawing can cause the loss of juices and nutrients in the squid,which may lead to deterioration of squid quality.Firstly,this paper studied the effect of different freezing methods on quality of squid and selected the appropriate freezing method on squid.Secondly,the squid was glazed before frozen storage.Glazing with sodium polyacrylate,rosmarinic acid,D-sodium erythorbate and their hybrid were also applied on frozen squid,and selected the best ratio of compound ice-glazing preservatives.Storage time of glazing squids was predicted based on neural network.Finally,the effect of different thawing methods on quality of squid was researched.The specific research contents and conclusions of the paper are as follows:1.It is very important to study the proper freezing method to maintain the quality of squid.Fresh squid was frozen by four different freezing methods?refrigerator freezing,plate freezing,spiral freezing and ultra-low temperature cold storage freezing?.The influence of different freezing methods on quality of squid was analyzed through the freezing time,physiochemical indexes.Water migration and microstructure of interstitial muscle fibers was conducted by low-field nuclear magnetic resonance?LF-NMR?and optical microscope,respectively.The results indicated that,the formation time of maximum ice crystal of squid of the four different freezing methods respectivelyrequired171 min,56 min,26 min and18 min.Colour value,p H value,texture properties,MDA value,water migration and muscle microstructure of the squid by spiral frozen were relatively closer to those of the fresh sample.Among the four freezing methods,the squid frozen by the ultra-low temperature cold storage had the best water-holding capacity,and the springiness of the squid by refrigerator freezing was the best.Therefore,the spiral freezing method showed better frozen squid quality than other freezing method,which could provide a reference for optimizing the frozing process of squid in the future.2.In order to investigate the influence of glazing on the quality of squid,the unglazed samples were selected as the control group?CK?,pure water ice-glazing group?IG?,ice-glazing with sodium polyacrylate,rosmarinic acid,different concentration of D-sodium erythorbate?0.1,0.3 and 0.5%?and their hybrid were also applied on frozen squid which were stored at-18?for 6 months.For all samples,the quality parameters,including water-holding capacity,colour,p H,texture properties,MDA value,sufhydryl content?SH?,water population and free amino acids?FAA?were tested periodically.Results showed that squid exhibited significant quality decline during frozen storage.Glazing treatment significantly reduced quality loss,lipid oxidative and protein degradation,compared to the unglazed sample.Glazing with sodium polyacrylate was not easy to broken,and effectively maintain the quality of the squid.The glazing containing D-sodium erythorbate exhibited better preservative effects than that of glazing containing rosmarinic acid.The addition of 0.3%D-sodium erythorbate to 0.1%sodium polyacrylate is more effective than both 0.1 and 0.5%D-sodium erythorbate in delaying the decline the squid quality and better maintaining the quality of squid during frozen storage3.In order to obtain the best ratio of compound ice-glazing preservatives,L₉?3⁴?orthogonal array design was used to optimize the amounts of sodium polyacrylate,rosmarinic acid and sodium erythorbate based on water-holding capacity,total sulfhydryl content and intrinsic fluorescence intensity.Meanwhile,combined with colour,texture properties and free amino acids test,the preservation effect of the compound ice-glazing preservative was verified.Results showed that the most important factor affecting the preservation effect was sodium erythorbate,followed by sodium polyacrylate and rosmarinic acid.According to mean analysis,the best ratio of the compound preservative was 0.10%of sodium polyacrylate,0.15%of rosemary acid and 0.5%of sodium erythorbate?A₁B₃C₂?,while the best ratio of compound preservative was 0.10%of sodium polyacrylate,0.10%of rosemary acid and 0.5%of sodium erythorbate?A₁B₂C₂?for comprehensive evaluation.In the verification experiment,compared with the pure water ice-glazing group,these two compound ice-glazing preservatives could effectively maintain the quality of the squid,especially A₁B₃C₂can significantly inhibit the oxidative degradation of protein,maintain the quality of squid during frozen storage.4.The objective of this study was to develop prediction modelusing neural network for prediction of storage time of glazed frozen squids.In this study,quality changes in water holding capacity?WHC?,weight loss,color,texture properties and total sulfhydryl?SH?content of glazed frozen squids during frozen storage at-5,-10,-20,-30 and-40?,were determined.In addition,back propagation neural network?BP-NN?and long short-term memory neural network?LSTM-NN?model were established to predict storage time of glazed frozen squids during frozen storage,and then these two models were performed with a comparative study.The results showed that the influence on the quality of the squids during frozen storage at different temperatures had significant difference?P<0.05?,and at the lower storage temperatures,the declined rate of squids'quality was slower,especially.However,during frozen storage at-30 and-40°C,changes in the total SH content of squid were not significant difference in the first 60 days.Both BP-NN and LSTM-NN model ware reliable models for predicting storage time of glazed frozen squid.Experimental results of LSTM-NN model provided an improvement in the accuracy of prediction compared with those obtained by using BP-NN model,which the mean absolute percentage error?MAPE?is 5.01%that lower than the BP-NN model?7.67%?.The prediction RMSE using LSTM-NN decreases by 32.46%against BP-NN and 54.39%for MSE.However,the LSTM-NN model has some shortcomings in term of training time compared with BP-NN model.The use of LSTM-NN provides a technique to accurately predict the storage time of glazed frozen squid.5.This chapter evaluated different thawing methods including natural air thawing,water soaking thawing,flowing water thawing,microwave thawing and ultrasonic thawing The deference of thawing time,water-holding capacity,colour,p H value,MDA value,sufhydryl content,texture properties were analyzed.Meanwhile,low-field nuclear magnetic resonance?LF-NMR?was used to determine the relaxation time T₂and nuclear magnetic resonance imaging?MRI?and optical microstructure observation of thawed squid was conducted.The results indicated that the thawing time of microwave thawing method was the shortest,followed by flowing water thawing.Water holding capacity of flowing water thawing was the highest than that of other thawing methods,which was also confirmed by LF-NMR.Squid after thawing in flowing water greatly maintained microstructure.The results of colour,p H value and the oxidation degree of protein and fat showed that flowing water thawing were the best in the five thawing methods.Although texture property of squid after the ultrasonic water thawing was the best,the local gap of the muscle fiber bundle in microstructure became bigger and because of the concussion of water during the thawing process.Overall,flowing water thawing is an ideal thawing method for frozen squid.