What is the difference between freeze-drying technology and ordinary drying?

Freeze drying is the sublimation of water, and drying is the evaporation of water. The former is that the water in the food material changes directly from solid state to gaseous state, so the physical structure of the food material is maintained to the greatest extent in the drying process, while the latter changes the water from liquid state to gaseous state.
So far, the best method is vacuum freeze-drying. Because the physical structure of food materials is basically not affected under low temperature and low pressure. It will not penetrate due to the humidity gradient inside and outside the cell wall, resulting in surface pore shrinkage or internal water loss, and the sublimation point will be increased under low pressure and vacuum, so the drying efficiency is higher.
Drying, also known as thermal drying, is a drying process completed by the mutual cooperation of heat carrier and wet carrier. Usually, hot air is used as both heat and wet carrier, that is, heating the air and then allowing the air to heat the food materials, and the evaporated water of the food materials is taken away and discharged by the air.
Because the effect of thermal drying on food materials is that the temperature is from the outside to the inside, and the humidity is from the inside to the outside (on the contrary), and the speed has a limit, that is, the maximum speed of water diffusion from the inside to the outside when the food materials can withstand the limit temperature. Too high speed will cause skin shrinkage, reduce the drying speed, and too slow will affect the efficiency. At the same time, too high temperature will cause too fast evaporation of internal water, It is too late to discharge through the capillary pipe and expand, causing cell rupture and certain nutrient loss.