Crystallization and matrix formation - Freeze drying
7 important questions on Crystallization and matrix formation - Freeze drying
What is freeze drying?
You have a table with temperature below 0 and saturated vapour pressure above ice (Pa), at what temperature can you freeze dry?
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?
What happens in freeze drying?
Freeze drying gives a very different morphology than hot air drying, how?
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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