Grain Drying – Principles and Methods

Grain drying is the process to reduce the moisture from grain to a safe level of storage and handling following harvest. It is a critical step for maintaining grain quality and minimize the storage and processing losses. Moisture content is a critical factor in determining the harvest time.

Drying is need because the high moisture grain will cause heat build up (Hot spot) from respiration of microorganisms, low thermal diffusivity of grain ,increased the mold growth, reduced starch and sugar content.

Drying mechanism :

Two fundamental steps occur during drying

1. Water migration migration from grain interior to surface.2. Water evaporation from the surface to the surrounding air.

Source:www.fao.org/docrep/t1838e/t1838e0u

Moisture content decreases rapidly at first but slows down as grain dries.

Drying rate decreases with moisture content increases with air temperature.

Removal of water from grain during drying causes a reduction in grain quantity

Moisture content can be calculate :

Moisture shrink % = Initial moisturecontent−final moisture content

100−final moisturecontntX 100

Equilibrium moisture content:

During drying and storage , moisture content depends on temperature and relative humidity of surrounding air

When surrounding air is dry grain would loss moisture.

When surrounding air is humid grain would absorb moisture

Drying air Properties :

Vapor Pressure- Partial pressure exerted by the water vapor molecules in moist air.

Relative humidity- Ratio of the mole fraction of water vapor in the air to the mole fraction of water vapor in saturated air

Humidity ratio- Mass of water vapor per unit mass of dry air

Dry bulb temperature- air temperature indicated by an ordinary thermometer

Wet bulb temperature – air temperature indicated by thermometer whose bulb is covered with a wet wick

Dew point temperature-Temperature at which moisture condensation will occur as air is cooled

Enthalpy- Heat content of the moist air per unit mass of dry air above a certain reference temperature.

Specific volume-Volume per unit mass of dry air.

Specific Heat- Measurable physical quantity of heat energy required to change the temperature of air by a given amount.

Effect of Drying on grain qualities:

Low and uniform moisture content, Minimum broken and damage kernels,Low susceptibility to subsequent breakage retained nutritional value , high viability ,good visual and organoleptic properties.

After drying ,grain moisture should be low enough for storage, different grains have different safe storage moisture contents. It is also important to reduce the moisture variability of the grain.

Drying with high temperature air increase the drying rate ,but may also increase cracked kernels.(stress cracking)

Drying methods

Drying methods can be divided in Traditional sun drying and mechanical drying.

Sun drying : directly exposed to sun and wind . Field drying, Mat/pavement drying, crib drying( Maize )

Very cheap and workable but the limitation are dependant on weather, labor-intensive, poor milling quality

Mechanical Drying:

Heated air drying-Fixed bed drying , Re circulating batch dryer, Continuous flow drying

Low Temperature air drying-In Bin deep bed drying, Thin layer drying

More energy efficient

Recommendation for Mechanical drying:

Select a suitable drying system carefully considering the factors of like capacity, technical requirements and economic feasibility. Clean the grain before drying. Monitor the temperature of drying air . Monitor the moisture content of the grain during drying. Buy reputed make.