Eli5: freeze drying. I just can’t wrap my head around how it gives different results even tho both take out water out of say fruit. And also how they evaporate in freezing environment

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Eli5: freeze drying. I just can’t wrap my head around how it gives different results even tho both take out water out of say fruit. And also how they evaporate in freezing environment

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It’s called sublimation. At different pressures and temperatures substances, like water, can exist as solid, liquid or gas. For water at atmospheric pressures raising thr temperature from below to above zero causes solid to liquid (melting). Rasing further above 100C causes liquid to gas (boiling). Lowering the atmospheric pressure below 611 pascal’s and the temperature below -50C forms a solid and heating to above that temperature causes a solid to gas change (sublimation).

If you let fruit dry at warm temperature it shrivels up when it loses the liquid water through evaporation.

if you freeze the fruit and put in in a vacuum the ice changes directly to water vapor gas in a process called sublimation. The fruit is frozen so it can’t shrivel up.

As others have said here, the basic principle is “sublimation”, which is where water changes from a solid to a gas (or vice-versa) without passing through the liquid phase.

Have you got ice cubes in your freezer? Ever notice that, if you leave them frozen for weeks and weeks, they get a bit smaller? That’s sublimation in action. Same thing if you get ice crystals growing inside a freezer.

At an industrial level, the main approach seems to be to freeze the food in a sealed container, and then lower the atmospheric pressure, such that the water molecules are drawn off. Then, raise back the pressure, but keep the food frozen, at least until you want to consume it.

That makes so much more sense, thank you guys

Lyophilization (the technical term for freeze-drying) involves cooling down something to the freezing point of water, then dropping the air pressure around it to make the boiling point match the freezing point (lowering air pressure lowers the boiling point of water). when this happens, the solid water sublimates or turns directly into a glass without becoming liquid first.

Freeze drying is quite a bit quicker and more thorough than simply waiting for water to evaporate. It also tends to preserve the shape of the drying thing and prevents chemical reactions with the air (that do things like discolor fruit), or the growth of microbes (which are dried out and destroyed by the process).