Many people think the core of freeze‑drying lies in vacuum sublimation stage. In fact, pre‑freezing determines the internal ice crystal structure inside materials, and largely lays the foundation for final product texture, appearance and rehydration performance. If pre‑freezing parameters are not matched with material characteristics, subsequent sublimation cannot make up for the defects.
When pre‑freezing temperature is not low enough, internal material cannot be completely frozen into solid ice. Partial unfrozen liquid will produce melting phenomenon under vacuum condition. The finished product will shrink, collapse, surface hardening occur, with poor loose porous structure. For high‑sugar materials such as fruits, super‑cooling phenomenon is easy to happen. Materials look solid on surface, while inner part still keeps viscous liquid state, which is hard to identify by naked eyes.
Too fast freezing speed will form tiny ice crystals inside raw materials. Tiny ice crystals will not form enough pore channels after sublimation, resulting in slow rehydration speed of finished products. Conversely, properly controlled slow freezing generates larger ice crystals. After sublimation, more uniform pore passages are left inside products, and rehydration performance will be significantly improved. But excessive slow freezing may cause cell damage and material juice loss.
Different categories of materials require differentiated pre‑freezing strategies. Meat raw materials need thorough deep pre‑freezing to avoid partial melting. Fruits with high sugar content need to avoid super‑cooling effect. Vegetables after blanching should match suitable cooling rate. Operators need to set temperature gradient and holding time according to material moisture, sugar content and thickness.
Enterprises should complete small‑scale process test before formal mass production. Verify pre‑freezing parameters combined with target product effect. Reasonable pre‑freezing process can reduce collapse, shrinkage and discoloration problems, improving appearance, taste and rehydration property of freeze‑dried food.
