Theoretical And Experimrntal Study On A Novel Vacuum Sublimation-rehydration Thawing Method

Frozen livestock and poultry meat products are the important bargaining chips for the national strategic reserve and the regulation of meat market(balancing supply and demand,stabilizing prices),are the main forms of meat products in the import and export trade and market circulation,and play the key roles in the development of national economy and the maintenance of national security and stability.At present,the quality of frozen meat products in our country is not optimistic and the phenomenon of serious thawing loss,cell oxidation and protein denaturation are prone to occur during the freezing and thawing process.In particular,the thawing process is slower than the freezing process and is more susceptible to the microbial damage.Most of the existing thawing methods have some disadvantages such as the long thawing time,large thawing loss,high cost and poor handling.In view of the above,a novel vacuum thawing method based on the principle of vacuum sublimation and rehydration was proposed and applied to the rapid thawing process for the frozen pork.Through the theoretical and experimental research,the mechanisms of vacuum sublimation-rehydration thawing(VSRT)were deeply analyzed and revealed to provide a new idea and method for the development of food thawing technology and efficient thawing equipment in our country,which had the important academic significance and engineering application value.The specific research contents and conclusions are as follows.(1)Experimental study on the single influencing factor of the novel VSRT systemPreliminary experimental research on the key parameters(sublimation time,heating plate temperature,rehydration water volume and temperature)was conducted to verify the feasibility and operability of the new VSRT method.The influences of key parameters on the thawing efficiency(thawing time,thawing rate),thawing effect(thaw loss and total color difference)and system specific energy consumption were also analyzed.The results showed that during the whole process of the VSRT,the pressure in the vacuum chamber could be controlled between 30 Pa and 3000 Pa,and the temperature in the vacuum chamber could be controlled in a lower range(about 20°C).At that time,the vacuum chamber was full of low-temperature and low-pressure steam,and the vacuum pumps were only operated in the sublimation stage(20 min)and closed in the rehydration stage.Therefore,the VSRT process could be carried out under the conditions of low oxygen,low temperature,high humidity and low energy consumption.And within a certain range,there were optimal sublimation time(about 20 min),heating plate temperature(about25°C),rehydration water volume(1500～2000 m L)and rehydration water temperature(25～30°C)so that the thawing time was shorter,the thawing rate was faster,and the thawing loss and the system specific energy consumption were relatively low,and the color of the thawed pork was closer to that of the fresh meat.(2)Establishment of correlation equations for the porous media(frozen products)thermal physical property parameters in the sublimation and rehydration stagesThe thermal and physical properties of porous media(frozen products)are the basic parameters that affect the energy transfer in the thawing process.To strengthen the energy transfer process in sublimation zone and sublimation interface by properly regulating the thawing process,thus effectively improving the thawing efficiency,the water content,initial ice content and thermal physical property parameters(density,specific heat capacity and thermal conductivity)of porous media(frozen product)were measured.And the correlation equations between the thermal physical property parameters of porous media(frozen product)and the ice crystal saturation,temperature in the sublimation and rehydration stages were established.These correlations were applied to the UDF to study the temperature and quality of frozen pork during the vacuum sublimation-rehydration thawing process.The results showed that the water content of pork before freezing was75.6%,and the initial ice content after freezing was 91.9%.The density,specific heat capacity and thermal conductivity of the frozen porous media were 0.99×10~3 kg/m~3,2302J/(kg·K)and 2.30 W/(m·K),respectively.Those of the solid matrix were 1.782×10~3 kg/m~3,6182 J/(kg·K)and 0.78 W/(m·K),respectively.In the vacuum sublimation stage,the density,specific heat capacity and thermal conductivity of frozen products gradually reduced with the ice crystal saturation decreasing.In the rehydration heating stage,the density,specific heat capacity and thermal conductivity of frozen products had the nonlinear monotonic relations with the ambient temperature in the vacuum chamber.With the increase of ambient temperature,the density and specific heat capacity of frozen products showed an upward trend,while the thermal conductivity showed a downward tendency.This work could provide a basis for the establishment of thermo-mass coupling dynamic model.(3)Analysis of the heat and mass transfer in porous media(frozen products)and establishment of the thermo-mass coupling dynamic modelsIn the engineering practice,the thawing process generally is carried out in accordance with the field experience and is lack of the specific scientific theoretical guidance.In this work,the thermo-mass coupling dynamic models of the VSRT process were stablished,the changes of temperature field,velocity field and the water vapor flow field were analyzed,the dynamic advancing process of phase transition interface was studied,the process and the thawing process was predicted.The theoretical results of thawing time and thawing loss in the thermo-mass coupling dynamic model were in good agreement with the experimental results.The simulation results showed that in the vacuum sublimation stage,when the heating plate temperature was the same(25°C),with the gradual progress of sublimation process,temperature of the frozen product lower surface changed little due to the thermal radiation of the heating plate below the frozen pork,but the rest part temperature decreased gradually.There was a clear boundary between the sublimation layer and the frozen layer(sublimation interface).With the extension of sublimation time,the sublimation interface moved from the upper surface to the center of lower surface.When the sublimation time was consistent(20 min),the temperature of frozen products increased gradually with the heating plate temperature increasing,the sublimation interface was closer to the lower surface of frozen products,and the thickness of sublimation layer increased gradually,while the thickness of frozen layer decreased gradually.The simulation results showed that in the rehydration heating stage,there was little difference between the upper and lower surfaces temperature of the frozen product,and two vortices appeared near the lower surface.The temperature of water vapor near the frozen product right surface changed greatly,and the temperature of the frozen product right surface was slightly higher than that of the left surface due to the closer distance to the air inlet,more water vapor condensation,and the released latent heat.There were three vortices in the back right side and the lower left side of the vacuum chamber.The vortex areas could cause momentum loss and reduce the flow velocity of water vapor,but also because of those,the surfaces of the frozen products were protected from the scouring of air flow and the quality of thawed meat could be better guaranteed.In addition,the water vapor had enough time and space to condense and release heat on all other surfaces of the frozen product,rather than just on the right and underside surfaces near the vapor inlet.(4)Multi-parameter and multi-objective coupling optimization of the novel VSRT processThere were the certain coupling relationships among the key parameters in the new VSRT process,and in the practical application of engineering,thawing time,thawed meat quality and the system energy consumption were all the indicators to be measured.Therefore,through the design and analysis of the quadratic regression rotation combination experiment,the influence orders of factors were determined.For the thawing time,sublimation time was the most important factor,rehydration water volume and heating plate temperature were the secondary factors,and rehydration water temperature was the least important factor.For the thawing loss,the order of influencing factors was heating plate temperature,sublimation time,rehydration water temperature and rehydration water volume.For the total color difference,the order of influencing factors was rehydration water temperature,sublimation time,heating plate temperature and rehydration water volume.For the system specific energy consumption,the order of influencing factors was rehydration water temperature,sublimation time,rehydration water volume and heating plate temperature.For the comprehensive objective,the four influencing factors were basically equally important.The sublimation time and the heating plate temperature had the relatively significant influences on the comprehensive objective.It can be concluded that the sublimation time,heating plate temperature,rehydration water volume and temperature were the very meaningful factors for optimizing the VSRT system,and the adjustments of the sublimation time and the heating plate temperature were relatively effective to improve the comprehensive performance of the VSRT system.In this paper,frozen pork with the size of 300 mm×100 mm×80 mm(length×width×height)and weight of 2 kg was studied.Based on the experimental data,the optimal conditions of the multiple parameters and multiple objectives in the thawing process were obtained quickly and accurately by the response surface analysis methodology and programming solution.When the sublimation time was 19.6 min,the heating plate temperature was 25.9°C,the rehydration water volume was 1634 m L,and the rehydration water temperature was 28.8°C,the thawing time was 0.90 h,the thawing loss rate was0.72%,the total color difference was 1.02,and the specific energy consumption of the system was 0.026 k Wh/kg.Each evaluation index of the system reached a better value.Under these optimal conditions,the VSRT method water could shorten the thawing time of the natural air thawing and vacuum steam thawing time by 55%and 35%respectively,speed up the thawing rate to 5.1 and 2.4 tims,reduce the thawing loss by 86%and 79%respectively,and reduce the total color difference by 87%and 83%respectively.The color and texture parameters of the pork thawed by the VSRT method were closer to the fresh meat and more acceptable to the consumers.In addition,the vacuum sublimation-rehydration thawing could reduce the system specific energy consumption of the vacuum steam thawing system by 41%.