Crystallization and matrix formation - Freeze drying

7 important questions on Crystallization and matrix formation - Freeze drying

What is freeze drying?

It consists of the separation of liquid water from a wet solid product (or from a solution or dispersion) in the form of a solid phase, ice, and its subsequent removal by vacuum sublimation, leaving the product in its anhydrous, or almost anhydrous state.

You have a table with temperature below 0 and saturated vapour pressure above ice (Pa), at what temperature can you freeze dry?

When the temperature of the ice is ...C and you subject the frozen product to a pressure below the Pa amount, the ice will start to sublimate into water vapour. This will be taken away by the vacuum.

What happens if you increase the temperature too fast during freeze drying?

  • Increasing too fast, the product will be too much above the glass transition. The moisture diffusion will be faster, but the whole product matrix will also become rubbery. The porous matrix (due to sublimation of ice crystals) can collapse: due to capillary pressure, the pores will close, the product will shrink and the product will lose its porosity.
  • It is in the WLF area, other degradation processes may take place, such as chemical or enzymatic processes.
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What do you have to do after freeze drying?

Heat up to temperatures that are significantly above room temperature. You  need to lose so much moisture, that at the end, when you cool the product down again, it will be firmly inside the glassy area.

What happens in freeze drying?

The product is first frozen. Then milled into small particles and spread over large plates (while the product remains frozen). These please are positioned into a vacuum system. The system is closed and evacuated. When the water vapour pressure above the product becomes lower than that of the ice, then the ice will start to sublimate.

Freeze drying gives a very different morphology than hot air drying, how?

With freeze drying the product is similar to the original product for the consumers eye. But the product is very porous and quickly takes up water due to capillary action. It will always have the texture of a micro sponge.
With hot air drying, the product shrinks. They will rehydrate slowly (no pores). However, when they are rehydrated, the texture is much more similar to that of the original product.

How can you improve the porosity of freeze dried products?

  • If you freeze quickly, you get small ice crystals, resulting in small pores. There is a limitation in the size of the products, as large products take a long time to freeze.
  • Another way is to adapt the formulation such, that the glass transition will shift. If you add mixtures of sugars, you can shift the glass transition Tg' and will get different crystal sizes. For example with the high Mw maltodextrin. A much larger part is not ice crystals now, but remains unfrozen in the glassy matrix. Hence, the crystals remain smaller.

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