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Coffee Processing (Practical Action Technical Brief)
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=== Freeze drying === Freeze drying takes place by sublimation (ie a solid material transforming directly to vapour without passing through a liquid stage). The coffee solution is slowly frozen in conventional freezing equipment. The frozen material is put under reduced pressure and heated. As long as the pressure in the freeze drier is below the vapour pressure at the surface of the material, it will sublime directly to vapour without melting. The vapour needs to be removed by a vacuum pump and is condensed on refrigeration coils. The final drying stage involves evaporative drying (desorption). This is achieved by raising the temperature to near ambient whilst retaining the low pressure. Coffee needs to be frozen as a foam (by gas inclusion) to prevent formation of a glassy vitreous frozen material.
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