Heat And Mass Transfer Analysis And Experimental Study Of Infrared Assisted Vacuum Freeze-drying Pet Dog Food

With the development of modern society,pet dogs play an important role of accompanying the elderly and children in the family,and the status of pet dogs are getting higher and higher in the family.Pet dog food has a bright prospect for the development in the future and a higher requirement of drying.Drying is an important process in pet dog food processing.The traditional drying technology has a long history,which is based on the high temperature to dehydrate food.However,the presence of high temperature and oxygen in the drying process has a negative effect on the original color and structure of food and reduces the sensory and nutritional characteristics of pet dog food.The freeze-drying technology is based on the principle of vacuum water vapor sublimation and low temperature.Although freeze drying has a great advantage in maintaining the nutrition of products,it takes a long time and consumes a lot of energy.Based on the above-mentioned problems and technical requirement,the development of novel and efficient freeze-drying technology is of great significance.This thesis took the pet dog food(beef granules)as the research subject,measuring the thermophysical property of the beef granules,designing a novel infrared assisted freeze drying technology and drying strategy,analying the drying kinetics of the beef granules and optimizing the infrared assisted freeze drying process and verifying the rehydration rate and brittleness.The main research work is presented as follows:(1)Measurement of thermal-physical properties and numerical simulation of freezing process of beef granules.Pet dog food was consisted by the beef,grains,vitamins and other substances,which beef was the main component and provided about 50%fat and protein.According to the experments data and the fitted results of curve equations at different temperature segments,thermal conductivity and specific heat of beef granules varied greatly with the temperature.The eutectic point temperature of the pet dog food was at-16.1℃,the eutectic point temperature of the pet dog food was at 2.9℃,and the latent heat of solidification of the pet dog food was at 199J/g.The moisture content of the pet dog food was 73.6%(wet basis)and the density of it was 1066kg/m3.Gambit and Fluent software were used to model the cold trap and mesh structure.Simulation results was used to explore the changes of the freezing temperature and the phase changing interface in the process of freezing.With the comparison of experiment results and simulation temperature results,the overall tendency was consistent and the difference was small between results.It was evided that the finite element model could exactly simulate freezing temperature change and the phase changing interface of frozen beef granules.(2)The drying kinetics of beef granules was analyzed.Through the analysis of the heat and mass transfer of drying process of beef granules,granules thickness,drying temperature and drying strategy were selected as processing parameters in combination with the existing experimental conditions.The effect of the granules thickness,drying temperature and drying strategy on the drying rate was studied.Results showed that with the decrease of the thickness of granules,the increase of drying temperature and the extention of infrared lamp irradiation time,water ratio curve of beef granules slope,drying time and effective diffusion coefficient of water were increased,and drying time of beef granules was decreased.Drying kinetics model could better describe and predict drying process of beef granules under different conditions of the processing predict Four kinds of models:Page model,Henderson and Pabis model,Lewis model and Parabolic model were intended for fitting the relationship between the different granules thickness,drying temperature and drying strategy and the moisture ratio.The parameter of R2,χ2 and SSE were used to evaluate fitness of the model,and results showed that the Parabolic model was the best fittness model.(3)Response surface methodology(RSM)was used to optimize the processing parameter of beef granules.3 factors(granules thickness,drying temperature and drying strategy)were selected as independent variables,and rehydration and brittleness were designed as the response values.The optimizing processing parameter was:the 7.442 mm beef granules thickness,29.16℃drying temperature and adding infrared lamp irradiation after 130.44 min freeze-drying.According to the optimizing response rehydration rate of beef granules was 3.638,the crispness value of beef granules was 139.937N.The experimental results of rehydration rate and the crispness value of beef granules were 3.52 and 135.212N respectively.By comparing the microstructure of beef granules before and after freeze-drying and the central temperature differences of beef granules dehydrated by conventional freeze-drying and infrared assisted freeze-drying,it can be concluded that infrared assisted freeze-drying can not damage the structure of beef granules,shorten the drying time and improve the drying efficiency.Beef granules of dehydrated by this technology has improved water rehydration and crispness.