Discussion
Which statement about freeze drying is TRUE?
Answer: C
Correct answer: Water leaves as vapour directly from ice
Freeze drying removes water by sublimation: the product is frozen and the pressure lowered below the triple point, so ice passes straight to vapour without ever becoming liquid. That is the defining feature and the source of every advantage the method has.
Because there is no liquid phase there is no capillary shrinkage, no migration of solutes to the surface and no thermal damage, so structure, colour and flavour are preserved better than by any other method. It runs at low temperature, is among the most expensive methods available, and is used for high-value products rather than bulk grain.
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