Drying and Dehydration of Grains

Definitions:

1. Drying: “Drying is the universal method of conditioning grain by removing moisture to a moisture content level that is in equilibrium with normal atmospheric air to preserve its quality and nutritive value for food and feed and its viability for seed.”
2. Dehydration: “Dehydration means the removal of moisture to very low levels, usually to a bone-dry condition.”

Importance of Drying and Dehydration:

• Improved Storage Period: Drying grains to about 12% moisture improves storage life by inhibiting respiration and microbial growth.
• Prevents Microbial Growth: Lower moisture levels reduce free water availability, preventing microbial activity that could lead to qualitative and quantitative losses.
• Insect Attack Prevention: Drying minimizes the risk of insect infestation.
• Prevents Germination: Low moisture levels prevent grains from germinating during storage.
• Quality Preservation: Maintains the quality and nutritional value of grains.

Theory of Grain Drying:

• Definition: Drying refers to the removal of small amounts of moisture from a solid or nearly solid material through evaporation.
• Process: Involves both heat and mass transfer operations simultaneously. Convective Drying: The heat required for evaporation is supplied by an external drying medium, usually air.
• Types of Grain Drying: Thin Layer Drying: All grains are fully exposed to the drying air under constant conditions. Deep Bed Drying: Involves a heterogeneous drying process in deep layers, where drying is faster at the inlet end than at the exhaust end.

Methods of Drying Grains:

1. Conduction Drying: Heat is transferred directly through contact.
2. Convection Drying: Most common for grains, where hot air is blown over the grains.
3. Radiation Drying: Involves the use of infrared or microwave radiation.
4. Other Methods:
   • Dielectric Drying: Uses electromagnetic waves.
   • Chemical Drying: Involves chemical desiccants.
   • Vacuum Drying: Utilizes reduced pressure to accelerate drying.
   • Freeze Drying: Involves freezing and sublimation of moisture.

Equilibrium Moisture Content (EMC):

Definition: EMC is the moisture content of grains when they are in equilibrium with the surrounding air’s humidity.

Henderson’s Equation (1952):
1−RH=exp (−Me / Tn)
Where,

• • RH = Relative Humidity (decimal)
  • Me = Equilibrium Moisture Content (dry basis, %)
  • T = Temperature (Kelvin or Celsius)
  • n = Product constant (varies with material)

Classification of Grain Drying:

i) Thin Layer Drying:

• Definition: All grains are fully exposed to drying air under constant conditions.
• Features: Limited to 20 cm grain depth. Drying rate is independent of air velocity. Drying rate depends on the vapor pressure difference between grain moisture and drying air.
• Applications: Used in commercial flow dryers.

ii) Deep Bed Drying:

• Definition: Heterogeneous drying in deep layers (more than 20 cm), with varying drying rates.
• Process: The drying zone moves upward as drying progresses. Air picks up moisture in the drying zone and moves toward equilibrium. Drying ceases once equilibrium is reached between the product and air.
• Zones in Deep Bed Drying:
  • Dried Zone: Grains have lost moisture.
  • Drying Zone: Moisture is actively being removed.
  • Wet Zone: Grains are still moist and unprocessed.
• Drying Front: The interface where grains begin to lose moisture.
• Influencing Factors:
  • Temperature and humidity of entering air.
  • Moisture content of grains.
  • Air velocity.