Discussion
Statement I: Freeze drying preserves structure and flavour better than hot air drying.
Statement II: Water is removed by sublimation of ice under vacuum without a liquid phase.
In the light of the above statements, choose the correct answer.
Answer: A
Correct answer: Both Statement I and Statement II are correct
Both statements are correct and the second explains the first. In freeze drying the product is frozen and the ice removed by sublimation under vacuum, so water passes directly from solid to vapour and no liquid phase ever exists within the product.
Because there is no liquid, there is no capillary shrinkage, no migration of dissolved solutes to the surface and no heat damage, so the porous structure, colour, flavour and volatile compounds are preserved far better than in hot air drying. The price is a long cycle and high capital and energy cost, which restricts it to high-value products.
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