1. Food Dehydration (Drying)
Introduction to Food Engineering

2. Food Dehydration
Preservation
Transportation
Storage

3. Drying-Rate Curves

4. Drying-rate curves Rate-limiting Constant rate Falling rate
Mass transfer at surface
Falling rate
Moisture diffusion

5. Dehydration Systems Maximum Vapor-pressure gradient
Temperature gradient
Convective coefficient

6. Tray or Cabinet Dryers
Vacuum
Batch system
Non-uniform drying

9. Tunnel Dryers
Residence time
Concurrent flow
Counter-current flow

11. Puff Drying New process
Exposing small piece of product to high pressure & high temp -> short time
Product is then moved to atmospheric pressure
Flash evaporation
High porosity – rapid rehydration

12. Fluidized-Bed Drying Solid-particle foods Suspended in heated air
Equal drying from all surfaces
Small particles – low air velocities

14. Spray Drying Liquid foods – easy reconstitution
Liquid is sprayed into heated air
Product quality is protected by evaporative cooling

16. Freeze Drying Reduction of product temperature
Moisture is in solid state
Decreasing pressure around product
Sublimation of ice
Used when quality of product is important to consumer acceptance
Maintenace of product structure

17. Freeze Drying
But
Energy intensive
Freezing
Vacuum

18. Dehydration System Design
Mass and Energy Balance
Ma, Ta2,W2
Ta1,W1
Tp2,2
Mp, Tp1,1
Counter-current system

19. Overall moisture balance

20. Example
A cabinet dryer is used to dry a food product from 68 % moisture content (wet basis) to 5.5 %. Drying air enters system at 54 C and 10 % RH and leaves at 30 C, 70 % RH. The product temp is 25 C throughout drying. Compute the quantity of air required for drying 1 kg product solids.

21. w1 = 0.68/0.32 = 2.125 kg water/kg solids
From psychrometric chart
W1 = kg water/kg dry air
W2 = kg water/kg dry air

23. Drying-Time Prediction
For constant-rate drying period

24. For falling-rate drying

25. Total Drying time