Effect Of Pre-Freezing On Sequent Drying Process Of The Potato

The process of vacuum freeze-drying technology is freezing the moisture materials into a solid state at low temperatures, and then making the moisture sublimate directly into gaseous in a vacuum to dehydrate the materials. As the ability to maintain the original chemical composition and the physical properties, such as porous structure, colloidal properties and so on, the vacuum freeze-drying technology is widely used in food, biological, pharmaceutical and other industries in our country.The process of vacuum freeze drying consists of pre-freezing, follow-up primary drying and secondary drying phases, the position of drying zone, freezing zone and the sublimation front zone changes with change of the drying time during the drying process. The frozen zone changed into the drying zone when the ice crystals sublimated. The heat to sublimate the ice crystals is conducted into the drying zone from outside, and then water vapor diffused to the external environment through the dry area. The internal pore structure of the dry zone materials affect the heat and mass transfer characteristics, thereby affect the drying process,but the internal pore structure of the dry zone material is influenced by the morphology of ice crystals formed in the pre-freezing. Therefore the pre-freezing produce an important effect on the subsequent drying process of the material, but there is a lack of relevant knowledge.This paper studies how pre-freezing influence the material freeze drying process based on homemade freeze drying experiment station. The potato is the experimental material. The main content of the article includes the following four aspects:The pre-freezing methods affect the internal pore structure of materials. Using pre-freezing slowly rapidly and frozen in liquid nitrogen to freeze cubes of potato samples to see the internal pore structure image by electron microscopy (SEM). SEM image processing method and mercury intrusion measurement measure the internal porosity and pore size distribution. The experiments show that the ice crystals nucleus, internal porosity and the pore size distribution formed by the slow pre-freezing is larger.The pre-freezing methods affect the material thermal conductivity. Using the laser flash method to measure the potatoes dry products’thermal conductivity parameters such as thermal diffusivity, specific heat, thermal conductivity in different pre-freezing cases. The results show that the pre-freezing has little influence on the sample thermal diffusivity but great on the sample thermal conductivity.The pre-freezing methods affect the mass transfer property. Using data fitting method to analyses the freezing drying curve of the potato to obtain the effective diffusion coefficient of the dry product quality. The results showed that the materials have bigger porosities and effective diffusion coefficient by using slowly pre-freezing method. And the water vapor is easier to diffuse.The pre-freezing methods affect the freeze drying process of the materials. We established theoretical models of potato freeze drying process to study the impact to the parameters of the different pre-freezing mode based on Luikov diffusion theory. And we found the correction relationship between diffusion constant and freezing temperature in slowly pre-freezing mode.