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I've been reading about freeze drying and how it increases the shelf life of certain foods and is overall a good preserving technique.

My question is with regard to the actual process. I understand that freeze-drying involves freezing the actual material and then proceeding to dry it by placing it in a vacuum. This way the ice is turned into vapour without going into any liquid phase. My question is why is avoiding the liquid phase necessary and beneficial? When it comes to vaccines, foods, and polymer storage for example, why is it advantageous? I don't intuitively understand the reasoning behind why lyophilization is so favourable.

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I don't think the point of freeze drying is to avoid liquid water -- I mean, there's liquid water already in the substance to be dried -- but to avoid heat. Ordinarily, to dry something out to preserve it, you need to heat it to speed up the evaporation of water, certainly if you want to get rid of the water quickly (which you would want to do in any commercial setting, where time = money). But heating causes all kinds of changes to foods, and while some foods are fine with it, others suffer degradations of perceived quality, e.g. undesirable changes in color and texture. The point of freeze-drying is that you can get rid of the water at a low temperature.

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