Discussion
Which of the following best explains why the last few points of moisture are the most expensive to remove?
Answer: D
Correct answer: Drying decays exponentially with time
In the falling rate period the moisture ratio follows an exponential decay, \( MR = \exp(-kt) \), so each further reduction takes progressively longer. With \( k = 0.25 \) per hour, four hours reaches a moisture ratio of \( 0.368 \) but reaching \( 0.10 \) takes \( 9.2 \) hours - more than twice as long again for a much smaller quantity of water.
The underlying reason is physical: the remaining moisture is more firmly held and has further to travel to the surface. The latent heat does not rise sharply, the product's specific heat in fact falls as it dries, and increasing the air flow does not help once the rate is internally controlled.
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