Study On Freezing Characteristics Of Shrimp During Immersion Freezing Process

As one of the three most extensively cultured shrimp in the world,South American white shrimp(Penaeus vannamei)is very popular in China because of its delicious taste and rich nutrition.Fresh shrimp is perishable due to its high water content.In order to preserve the nutritional value of shrimp and extend its shelf life to the maximum extent,the freezing processing is the most simple and effective way to keep food fresh.Immersion freezing,as an efficient freezing technology,has the advantages that the traditional freezing methods do not have,but it has some problems,such as poor comprehensive performance of liquid refrigerant solution.In order to further optimize the immersion freezing technology,a novel liquid refrigerant solution was developed in this paper.Taking South American white shrimp as the research object,the freezing rate was enhanced by liquid jets,and the freezing characteristics of shrimp during the immersion freezing process were studied in depth.The main research contents and results are as presented follows:(1)Optimization of liquid refrigerant solutions.The suitable substitute was selected through a numerous preliminary experiments.The single factor test was carried out,and the optimal liquid refrigerant solution formula LR-1 was optimized by response surface design by taking the absolute value of the freezing point and viscosity as the response values.The thermophysical properties(viscosity,density,thermal conductivity and specific heat,etc.)of the optimal liquid refrigerant LR-1 were measured by experiments.The freezing point of LR-1 could reach to-42.63℃,and the viscosity of LR-1 was 3.705mPa·s at 20℃,which both satisfied the practical production requirements.(2)Calculation of thermophysical properties of shrimp.By referring to literature,the composition of shrimp meat was determined,and the appropriate thermal properties calculation model was selected to calculate its thermal properties,such as density,thermal conductivity,specific heat,etc.The density and specific heat of South American white shrimp were verified by experimental verification and literature comparison and were compared with the theoretical value.The relative error of the model was very small,and the high accuracy of the model provided a basis for the following research.(3)Establish the physical model and the mathematical model of freezing tank in immersion freezing system.The process of immersion freezing was simulated by ANSYS-FLUENT.The mesh independence of models with different mesh sizes was verified to determine the optimal result.A small freezing system was built,and experiments were carried out to verify the accuracy of the numerical model.The results show that the numerical model has high accuracy.(4)The effects of operative variables(the refrigerating liquid temperature,the flow velocity of the liquid at orifice,the distance between the orifice and the shrimp samples,etc.)on freezing characteristics of shrimp were studied.The results show that when the inlet flow rate is constant,the total freezing time decreases significantly with the decrease of inlet temperature.The faster the freezing speed,the smaller the unevenness of the temperature distribution and the more uniform the temperature distribution in the whole freezing space;when the inlet temperature is constant,the total freezing time decreases with the increase of the inlet flow rate.The faster the freezing speed,the higher the unevenness of temperature distribution,and the more non-uniform the temperature distribution in the whole freezing space;Under the condition of the same arrangement of shrimp samples,the increase of the distance between the orifice and the shrimp samples has little effect on the freezing characteristics of shrimp.The above research results provide a theoretical basis for liquid jets enhanced immersion freezing,optimize the immersion freezing technology,and promote its development in frozen food processing.