Freeze-drying produces products that can store on shelves for more than a decade if correctly done. However, people sometimes get products that do not freeze-dry adequately at the end of the lengthy procedure. This article has gathered some of the common freeze-drying problems and their solutions.
1. There is a hard shell on the surface of the product
Causes
During the pre-freezing, glass-like structures that do not allow air to move through build on the product’s surface. When the product starts heating, only some parts of the product melt and shrink, and it sinks to become the lower layer. This lower layer sublimes as it should. However, the remaining amount that does not melt becomes the hard shell on the surface.
Solution
To prevent this, the products should be heat-treated during pre-freezing.
2. Uneven coloration and layered product
Causes
The slow pre-freezing rate commonly causes both situations.
When the pre-freezing rate is slow, the upper part has frizzy, crystalline patterns, while the lower part is delicate and properly freeze-dried. This happens because slow pre-freezing causes ice to form initially in the bottom, and the solute gathers in the upper part.
Another possibility is a very high concentration of the substance in the solution. When the solution has a high concentration, it forms a precipitate after a long time due to temperature and pH changes.
Solutions
To solve the problem of slow pre-freezing, you can use a repeated freezing method or increase the pre-freezing speed to get well freeze-dried products.
Reducing the solution’s concentration or reducing the solution’s resting time can be adopted to solve the situation in the second instance.
3. Freeze-dried products have voids, debris and shrink once it is out of the chamber
Causes
This happens when the product has residual ice crystals and does not dry completely.
Solutions
To solve this, the drying sublimation temperature should be increased or the time extended.
4. Products shrink and have bubbles
Causes
When freeze-dried products have bubbles and shrink, it is usually because the product was overheated in the sublimation stage. Alternatively, it could be because the product entered the analysis stage earlier than it should have. The sublimation stage had not yet ended, and the product was already in the analysis stage.
Temperatures that are too high can also result in shrinkage because it causes local melting, making the product change state -from solid to liquid or from liquid to gas. This melted portion evaporates, which leads to shrinkage of bubbling in severe cases.
Solutions
Reducing the temperature of the sublimation stage, extending the time, and increasing the vacuum of the freeze-drying chamber are simple solutions to this problem.
Most common freeze-drying problems can be solved by paying proper attention to the techniques of the method.
5. The freeze-dried product doesn’t store for long.
Causes
As stated above, freeze-dried products can stay for more than a decade when the procedure is duly followed. However, sometimes even with a proper freeze-drying process, the product refuses to store well. This might be because of exposure to humidity from the atmosphere.
Solution
Store in airtight bags and containers.