1. Pre-freezing speed
Most freeze dryers cannot control the pre-freezing speed, so the pre-freezing rate can only be determined by the pre-freezing temperature and the packing time. If the pre-freezing rate is fast, the freeze-drying chamber should be lowered to a low temperature before the product is put into the box; if the pre-freezing rate is slow, the product should be cooled down after entering the box.
2. The minimum temperature for pre-freezing
This temperature depends on the eutectic temperature of the product, and the minimum pre-freezing temperature should be lower than the eutectic temperature of the product.
3. Pre-freezing time
Generally, the pre-freezing time is set according to factors such as the packaging material, the properties of the freeze-dried sample, and the performance of the freeze-drying machine. Generally, it is required to keep the temperature of the sample for 1-2 hours after the temperature reaches the predetermined minimum temperature.
4. Condenser cooling time
The condenser is required to cool down at the end of the pre-freezing period, before the pre-freezing has ended, and before vacuuming. How much time before depends on the cooling performance of the condenser machine. It is required that the temperature of the condenser should reach about -40 ℃ when the vacuum is drawn after the pre-freezing. A good machine generally starts to cool down half an hour before. The cooling of the condenser is usually continued from the beginning until the freeze-drying is completed. The temperature should always be below -40°C.
5. Vacuum time
The end of pre-freezing is the time to start vacuuming, and it is required that the vacuum degree can meet the requirements in about half an hour. At the same time as vacuuming, the vacuum valve between the freeze-drying chamber and the condenser is opened, and the vacuum pump and the vacuum valve are also opened until the freeze-drying is completed.
6. Pre-freezing end time
After the pre-freezing is over, the freezer starts to operate, and usually stops the operation of the freezer at the same time as the vacuum is pumped or when the vacuum is pumped to the specified requirements.
7. Start heating time
It is generally believed that the time to start heating is after the vacuum degree reaches the requirement. Some freeze dryers use the vacuum relay to automatically turn on the heating, and the heating can start automatically; some freeze dryers start to heat half an hour after vacuuming. At this time, the vacuum degree The requirement has been met.
8. Working time of vacuum alarm
Since the degree of vacuum is very important for sublimation, the freeze dryer should be equipped with a vacuum alarm device. The working time of the vacuum alarm device is from the start of heating to the use of calibrated leaks, or from the beginning to the end of freeze-drying.
Once the vacuum degree drops during the sublimation process and a vacuum alarm occurs, on the one hand, an alarm signal is issued, and on the other hand, the heating of the freeze-drying chamber is automatically cut off. At the same time, the freezer in the freeze-drying bin is also activated to cool the product to protect the product from melting.
9. Correction leak working time
The purpose of correcting the leak is to improve the heat transfer in the freeze-drying chamber, and it is usually used during the second stage of freeze-drying work, continuing to restore the high vacuum state. The length of use time is determined by the type of product, the amount of filling and the value of the set vacuum.
10. The maximum allowable temperature for product heating
The maximum permissible temperature of the board heating is determined according to the product. During the sublimation, the heating temperature of the board can exceed the maximum permissible temperature of the product because the product is still in the low temperature stage. Increasing the temperature of the board can promote the sublimation;
The layer temperature needs to drop to match the maximum allowable temperature of the product. Due to the temperature difference of heat transfer, the temperature of the sheet layer can be slightly higher than the maximum allowable temperature of the product.
11. Total freeze-drying time
The total lyophilization time is the pre-freezing time, plus the sublimation time and the second stage work time. The total time is determined, and the lyophilization end time is also determined.
This time depends on the type of product, different packaging materials, packing method, loading capacity, performance of freeze-drying machine and other factors. Generally, the freeze-drying time is longer, about 18-24 hours, and some special products even take a few days. time.
Application of freeze-drying
Used in food industry, french fries, banana chips, french fries and some vegetables, snacks, etc.