Preparation Of Functional Food Products From Foamed Blueberry Puree By Microwave Freeze-drying

Freeze-drying is a process to remove moisture by sublimation and desorption at low temperature and pressure.Freeze-drying effectively retains the flavor and volatile components of raw materials,reduces the degradation of bioactive components,and provides excellent product quality.However,freeze-drying is a process with long drying times and high energy consumption,which hinders its large-scale application.In order to enhance the freeze-drying process,shorten the drying time and obtain blueberry powder products of high quality,blueberry puree was foamed to enhance mass transfer,and microwave heating assisted by silicon carbide(Si C)as wave-absorbing material was used to enhance heat transfer in this paper.In addition,the quality evaluation system of blueberry powder was established to evaluate the product qualities.Firstly,eutectic temperature of blueberry puree was measured to determine the pre-freezing temperature.Appropriate concentrations of foaming agent and stabilizer were determined to obtain stable foams with high overrun.Unfoamed and foamed frozen samples were prepared.Quartz and silicon carbide were used as the supporting pad materials.The conventional freeze-drying and microwave freeze-drying experiments were conducted to explore the effects of foaming treatment and microwave heating assisted by wave-absorbing material on the freeze-drying process.The moisture content,color,anthocyanin and total phenolic content of freeze-dried products were determined to evaluate the product qualities.SEM morphology was used to characterize the freeze-dried products from unfoamed and foamed materials,and the enhancement mechanism of foaming treatment was analyzed from the microscopic point of view.At 30? of radiation temperature and 15 Pa of chamber pressure with quartz as the sample pad,the foamed sample saved 39.1% of drying time compared with the unfoamed one.When the temperature was increased to 40?,the unfoamed and foamed samples can reduce15.2% and 17.9% of drying times,respectively.At 30?,15 Pa and 2 W with silicon carbide as the pad instead of quartz,microwave freeze-drying of the foamed sample decreased by14.3% and 47.8% of drying times,respectively,compared with conventional freeze-drying(0W)of foamed and unfoamed samples.When the microwave power was increased to 4 W,the drying time was shortened by 25.0% and 54.3%,respectively.There were no significant differences in color parameters,total monomeric anthocyanin and total phenolic content between microwave freeze-dried products and conventional freeze-dried products(P>0.05).The retentions of total monomeric anthocyanin and total phenolic content in conventional freeze-dried products and microwave freeze-dried products were great,which were more than80% and 75%,respectively.SEM images of dried products revealed that the structure of the freeze-dried product from unfoamed material was compact.However,the freeze-dried product from foamed material had a loose and porous structure with thin pore wall,which reduced the resistance to the migration of water vapor.Wave-absorbing material assisted microwave freeze-drying of foamed material achieved the simultaneous enhancement of mass and heat transfer in freeze-drying,which greatly reduced the drying time and improve the process economy,the product quality was comparable to that obtained by conventional freeze-drying.The combination of low energy consumption in the process and high product quality was realized by this drying method.