Study On The Optimization Of Vacuum Freeze-drying Process And Moisture Diffusion Of Lanzhou Lily

With the continuous improvement of people’s living standards and the change of consumption concept,plant food products with high nutritional value are widely loved.Lanzhou lily,as the only dual-use sweet lily in China,has high nutritional and medicinal value.In the process of making dried lily slices,hot air drying and microwave drying are frequently used.These two methods are easy to cause the loss of nutrients and serious shrinkage deformation.Vacuum freeze drying as a low-heat drying technology under vacuum conditions,could retain the nutrients and maintain the original form.However,the process is complicated,and different parameters have different effects on vacuum freeze drying.In this paper,we investigate the effects on the freeze-drying rate and product quality to optimize the vacuum freeze-drying process of lily,including different pre-freezing rate,the heating rate in the sublimation drying phase and the heating rate in the resolution stage.And the diffusion process of water in nanoporous media was analyzed from a microscopic perspective by molecular dynamics method.The main research of this paper is as follows:(1)The experiment was set up with different parameters of pre-freezing rate(-10 constant temperature and 5℃ step heating),the heating rate in the sublimation drying phase(-10 constant temperature and 5℃ step temperature rise)and the heating rate in the resolution stage(60℃ constant temperature and 5℃ temperature rise).And the effects of different parameters on the freeze-drying rate and product quality were compared and analyzed by measuring the freeze-drying curve and other parameters,such as the weight loss,water content,color difference,rehydration ratio and texture of the dried lily slices.(2)In the pre-freezing stage,slow freezing at-20℃/h and-10℃/h will increase the freeze-drying cycle,but it is beneficial to the formation of porous structure,which makes the dried lily with lower water content and better rehydration performance.It has a better effect on the rate of vacuum freeze drying and product quality by the slow freezing rate of-20℃/h.However,the freezing rate needs to be reduced.(3)In the sublimation drying stage,the stepwise heating mode is beneficial to water diffusion,and the dry product has good rehydration performance.But it is not conducive to the maintenance of lily color.A step temperature rise of 5℃ can provide sufficient thermal power for ice sublimation and diffusion.(4)In the resolution stage,the heating rate is too high to decrease the brightness and increase of hardness.The step heating rate of 5℃/h could not provide enough thermodynamic force for the desorption and diffusion of bound water in the cell.The heating rate should be further increased.(5)In the molecular dynamics simulation,the water molecules were modeled by TIP4 P and the walls were composed of FCC face-centered cubic atoms.When the interaction potential energy between water and the wall is greater than 0.3 kcal/mol,the wettability is expressed as immersion.And the model can reflect the Kelvin effect.(6)It has a greater effect on the moisture diffusion coefficient by the increase of temperature and the change in diameter of the small porous media(< 4.5nm).Temperature affects the rate of hydrogen bond formation and breakage by changing the thermal motion of water molecules,which in turn affects the diffusion coefficient.Meanwhile,the size of porous media affects the diffusion number by influencing the radial distribution function of water molecules,which is the density distribution of other water molecules that appear around the water molecule.