The faster the pre-cooling speed of the production freeze dryer is the better?

The faster the pre-cooling speed of the production freeze dryer is, the better?
When producing a product using a production freeze dryer, the material needs to be pre-frozen. The purpose of pre-freezing is to sublimate the product under vacuum. If not frozen. When a vacuum is applied, the product will come out of the bottle and have no shape; if it is too cold, it will not only waste energy and time, but also reduce the survival rate of some products.
The pre-freezing process is the speed of the drying process and the quality of the lyophilized product in a large program. So, when pre-cooling, is the cooling rate as fast as possible?
Books introducing freeze-drying theory often mention that the greater the cooling rate, the greater the degree of subcooling and supersaturation of the solution, the smaller the particle size of the critical crystal, and the faster the nucleation rate, the easier it is to form. Smaller particles. Exquisite crystal. Therefore, after the ice crystals sublimate, the pores formed in the material are small, the drying rate is low, but the rehydration after drying is good; on the contrary, the slow freezing tends to form large particles of ice crystals, and the water vapor escape passage formed by the sublimation of the ice crystals The size is larger. It is beneficial to increase the drying speed, but the rehydration after drying is poor. In theory, this is true, but this theory is obtained on the premise that all parts of the lyophilized material are uniformly heated. However, in actual operation, uniform heating of the material does not meet the theoretical requirements.
Based on the actual operation of the production freeze dryer, we have summarized several types of fast freezing and slow freezing:
1. The fastest way: immerse the product in an ultra-low temperature environment (such as liquid nitrogen), instantly forming the entire crystal, forming a very fine crystal (or amorphous), which is truly fast freezing.
2. The temperature of the board is slow, and the temperature difference between the board temperature and the product temperature is not large. The entire product is uniformly cooled and formed too cold. When the energy accumulation is sufficient, all the crystals are immediately crystallized and a quick freezing effect is obtained.
3, the plate temperature drops very slowly, and at a suitable temperature below the eutectic point (or slow cooling), the product forms a small degree of supercooling, a small amount of crystals appear in the liquid, and continues to cool the crystal growth until large crystals are obtained. Real slow freezing.
4, the plate temperature drops faster, the plate temperature is much lower than the product temperature, the bottom of the product freezes first to produce crystals, but the upper liquid is still very hot, so it will not crystallize immediately, the crystal will grow slowly, it will slowly freeze. influences.
In fact, the speed of pre-cooling should be based on the product to choose a good freezing rate to make the perfect freeze-dried product.