Theory 27/Knudsen Number/Food drying
Drying Rate Is a Knudsen Effect
The claim
Vacuum drying of food is set by the Knudsen number in the pore network, so reducing pressure below a threshold stops improving drying rate.
01Mechanism
Below the transition, vapour transport becomes free-molecular and the effective diffusivity stops rising with lower pressure.
02Prediction
Drying rate should plateau below a Knudsen threshold.
03Refutation
Rate improving indefinitely with lower pressure would remove the transition.