freeze drying
http://en.wikipedia.org/wiki/Chuño
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After harvest, potatoes are selected for the production of chuño, typically small ones for ease of processing. These small potatoes are spread closely on flat ground, and allowed to freeze with low night temperatures, for approximately three nights.
Between the freezing nights, they are exposed to the sun, and they are trampled by foot. This eliminates what little water is still retained by the potatoes, and removes the skins, enabling subsequent freezing.
After this, they are exposed to the cold for two additional nights.
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