Freeze-drying is a process used to preserve perishable materials, such as food products, by removing moisture from them under low pressure and low temperature. The technology of freeze-dried milk powder in milk freeze-drying machine involves several steps:

1. Pre-treatment: The milk is pre-treated by removing any impurities and standardizing the fat and protein content, if required.

2. Freezing: The pre-treated milk is then rapidly frozen to a temperature of around -40°C to -50°C. This process is known as nucleation.

3. Primary drying: The frozen milk is placed inside a vacuum chamber where the pressure is reduced to create a low-pressure environment. The temperature is raised to around -10°C to -20°C, which allows the water in the milk to sublimate (turn directly from solid to gas) and escape from the milk in the form of vapor.

4. Secondary drying: Once most of the water has been removed, the temperature is raised again to around 20°C to 30°C to remove any residual moisture from the milk.

5. Packaging: The freeze-dried milk powder is then placed in airtight containers to prevent moisture from re-entering the powder.

The milk freeze-drying machine used for this process is equipped with several components, such as a refrigeration system, a vacuum chamber, heating elements, and a control system. The refrigeration system ensures that the milk is frozen rapidly and efficiently, while the vacuum chamber allows the pressure to be reduced to the required level. The heating elements are used to raise the temperature of the milk during the primary and secondary drying stages. The control system ensures that the process is carried out within the specified parameters.