How does the food dry by freeze drying?
The food is heated to the point where the water within evaporates. The food items are then quickly frozen under vacuum conditions. The freeze dried food will retain its flavor and nutrients when it is rehydrated later.

Does freeze drying must be achieved in vacuum condition?
According to the physical principle, the sublimation of the ice below 0℃ can only happen if the pressure is below 610Pa. An atmospheric pressure is about 100000Pa, and it can be called as vacuum when the pressure is below 610Pa

The vacuum system is a combination network of a water-loop pump unit and oil-sealed pump unit. At the early stage of vacuum process, air is removed quickly by the high power oil-sealed pump unit. After that, the working vacuum condition is maintained by a low power Roots water-loop pump unit. This process has several advantages: firstly, energy consumption is reduced, secondly moisture can be removed by water-loop pump unit to maintain a working vacuum condition. Thirdly, unstable vacuum degree performance of oil-sealed pump unit can be avoided. The unstable vacuum degree performance is because the oil-sealed pump has a poor drainage ability, the pump oil is emulsified by the water.

• Keep freshness and preserve nutrition. Under low temperature and low pressure conditions, the decomposition or oxidation of some food components can be halted. The process also inhibits microbial growth, thus maintaining the freshness and nutrition of the food.
• Freeze dried food maintains its shape, color and flavor. During the sublimation process, the water is gradually removed from the surface all the way to the center of the food. Despite this, natural pigments and aromas are preserved. Re-hydrated food will taste nearly identical to the original.
• Good rehydration performance. During the sublimation process，sponge-like micropores are formed where the ice crystals previously formed. As a result, when freeze dried foods are immersed in the water, the food can quickly rehydrate be enjoyed as if they were fresh.

• Check the temperature, pressure, velocity of the cooling water system and ensures both instruments and water are clean.
• Check the all pipeline routes, including the cooling pipes, all the refrigerating pipes designed for evacuation and drying, hydraulic pipes, cooling pipes, and check for leakage in the convection circulation pipeline.
• Control system: the fuses, thermal protectors, relays, contactors and sensors should be functional, sensitive and reliable.
• Check the readings of the pressure gauge of the refrigeration system, determine that the oil level is within the correct range, as well as ensure color and luster of the refrigeration lubrication oil are correct. Add oil based on the instructions if need be.

• Vinegar deodorization method: If fish preserved in the cold storage room leaves behind a heavy, lingering smell, the room is not really suitable for storing other food. Use a heated vinegar solution to remove the odors, as well as sterilize the rooms.
• Ozone deodorization method: ozone has a strong oxidation ability, which not only eliminates odors, but it also inhibits the growth of microbes. Odor is removed from the cold storage room with the use of an ozone generator. It should be noted however, that if the storage room contains fatty foods, in order to avoid oxidation and souring of the fat, ozone removal should not be the method used.
• Formaldehyde deodorization method：Move products outside of the cold storage room, and remove odors using a 2% formaldehyde solution (i.e. formalin solution).