Quality Monitoring In Shrimp Meat During Drying Processing And Rehydrating By LF-NMR

The white shrimp was delicious and rich in protein,amino acids and other nutritional ingredient,and it was welcomed by consumers because of its high nutritive and economic value.The traditional method to detect the quality of shrimp was time-consuming,lossy,and not to monitor the change of quality in real time during processing.LF-NMR had been widely used in the research of moisture distribution,moisture mobility and quality prediction due to its advantages of fast,nondestructive,visualization and real-time monitoring.Therefore,in this paper,taking shrimp meat for research objects,the quality of shrimp in the process of drying,rehydration and free-thawing was studied by the determination of LF-NMR and related quality indexes and the application of the chemical metrology.The main conclusions were as follows:(1)The water mobility and distribution in shrimp during drying process at 40 ℃,50℃and 60℃ were investigated by LF-NMR and MRI.The total moisture content and relevant quality indicators including TPA and color parameters were also measured.By establishing Pearson correlation coefficient between magnetic index and quality index,using PCR and PLSR method to establish a prediction model of chewiness,L*and moisture about shrimp meat.LF-NMR results showed that four T2 components assigned to strongly bound,lightly bound,immobilized and bulk water were observed for shrimp sample.With the extension of drying time,significant decrease of the transverse relaxation time of immobilized water(T22)and bulk water(T23)was found,which illustrated their mobility reduced.The signal per mass of the immobilized water(A22)also decreased significantly during the drying process,which demonstrated the lost water mainly was the immobilized water.MRI results revealed the water signal intensity decreased from the external surface to inner regions of shrimp during drying process.The findings of correlation analysis reveal that extremely significant correlations were observed between the moisture content,TPA parameters(hardness,chewiness,cohesiveness),color parameters(L*,a*)and NMR parameters(T22,A2b,A22).Prediction model results showed that LF-NMR combined with PCR and PLSR were able to establish a good prediction model for the dried shrimp of moisture content in drying.(2)The water distribution and state change of dried shrimp meat during rehydrated process at 25 ℃ and 60 ℃ were studied by using LF-NMR and MRI,and the correlation between NMR parameters and texture,rehydration ration was analyzed to construct the predicted mode by linear regression analysis.Three water components bound water,immobilized water and free water were observed in dried shrimp meat during the rehydrated process.The contents and mobility of immobilized water and free water increased as extension of the rehydration time,while that of bound water did not change obviously at 25℃.It made the mobility,freedom and content of immobilized water enhance,but the content of free water decreased at 60℃.The results of MRI indicated the water absorbed by the dried shrimp mainly was components with short relaxation time,and permeated from head to body and tail.The relaxation time of immobilized water and free water increased more obvious,illustrating the mobility and freedom of water enhanced.with the increase of the rehydration time,the content of immobilized water increased,but the content of free water decreased.Good linear correlation was observed between the NMR parameters(T22、T23、A22 and ATotal)and texture profile analysis parameters including the hardness,chewiness and rehydration ratio of dried shrimp meat(R2≧0.884),which could provide a rapid and non-destructive method to predict the quality of dried shrimp meat during rehydrated process.(3)Using LF-NMR to collect the relaxation signal of shrimp in different freezing and thawing time,and PCA method was used to deal with the echo peak data of CPMG sequence.Cooking loss,centrifugal loss,moisture content,PH,shear force and color parameters were also determined.Their correlation was analyzed by Pearson correlation coefficient method.The result showed that with the increase of freeze-thaw cycles,strongly bound and lightly bound did not change evidently,the mobility of immobilized water decreased apparently at the tenth freeze-thaw cycles,the mobility of the free water decreased overall.In addition,combining LF-NMR with PCA could differentiate the shrimp meat of different freeze-thaw cycles.The correlation results showed that T23,A22 and ATotal could monitor the change of shrimp quality during free-thawing except L*.