Vacuum freeze-drying technology has made great strides in the last two decades. After entering the 21st century, with the further enhancement of people's awareness of environmental protection and health, they will place higher demands on the quality of consumer goods. The pace of life is accelerating, and the demand for convenient fast food will also grow stronger. All of these will strongly promote the further development of vacuum freeze-drying technology, expanding its application scale and expanding its application fields. Therefore, vacuum freeze drying will become an important application technology in the 21st century.

 The basic principle of vacuum freeze drying

Vacuum freeze-drying is a process in which the aqueous material is frozen and heated in a vacuum environment to directly remove the moisture in the material, thereby dehydrating the material to obtain a freeze-dried product. Vacuum freeze-drying is a physical dehydration, knowing that it should start with a three-phase diagram of the water.

The phase transition of water at low pressure is generally similar to that at atmospheric pressure, but the specific temperature at the time of phase change is different. For example, at a pressure of 103 Pa, the temperature at which solid ice is converted into liquid water is slightly higher than 0 ° C, and the temperature at which liquid water is converted into steam is 6.3 ° C. It can be seen that the freezing point does not change much after the pressure is lowered, and the boiling point is greatly reduced. It is conceivable that when the pressure is reduced to a certain value, the boiling point coincides with the freezing point, and the solid ice can be directly changed to the gaseous state without the liquid state. The pressure at this time is called the triple point pressure, and the corresponding temperature is called three-phase. Point temperature.

The three aggregate states of water vary with temperature and pressure, with the pressure as the ordinate and the temperature as the abscissa indicating the aggregate state of water, called the phase equilibrium diagram of water. It can be seen from the figure that the three curves of OS, OL and OK divide the phase diagram into three regions, namely gas phase, liquid phase and solid phase. The OS curve is a state in which the gas-solid two-phase equilibrium coexists. At this time, the water vapor pressure is the saturated vapor pressure of water, the OL curve is the state in which the liquid-solid two-phase equilibrium coexists, and the OK curve is the state in which the gas-liquid two-phase equilibrium coexists, K It is the critical point of water, the temperature at point K is 374 ° C, and the pressure is 2.11 × 107 Pa, at which point liquid water does not exist. The point O is the intersection of the three curves, that is, the three-point point, the temperature of the triple point is 0.01 ° C, and the pressure of the triple point is 610.5 Pa, which is a state in which the three-phase balance of water coexists. Vacuum freeze drying is performed below the triple point.