Food Dehydration (Drying) Introduction to Food Engineering

Food Dehydration Preservation Transportation Storage

Drying-Rate Curves

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

Dehydration Systems Maximum Vapor-pressure gradient Temperature gradient Convective coefficient

Tray or Cabinet Dryers Vacuum Batch system Non-uniform drying

Tunnel Dryers Residence time Concurrent flow Counter-current flow

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

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

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

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

Freeze Drying But Energy intensive Freezing Vacuum

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

Overall moisture balance

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.

w1 = 0.68/0.32 = 2.125 kg water/kg solids From psychrometric chart W1 = 0.0186 kg water/kg dry air W2 = 0.0094 kg water/kg dry air

Drying-Time Prediction For constant-rate drying period

For falling-rate drying

Total Drying time