Seed Production of Vegetable, Tuber and Spice Crops

3. Use of seed desiccants (Chemical drying)

• In this method silica gel or fused calcium chloride (CaCl₂) is used to absorb the moisture from the seed and its surrounding environment.
• Silica gel is of two types, as
i) Indicator type
ii) Non-indicator type

• Active ingredient in Silica gel is Lithium chloride, which is responsible for drying process. Silica gel can absorb moisture upto 15 per cent of its weight. So to get very low moisture content we can use this, which is not possible in mechanical driers.
• Indicator type will be blue in colour and on absorbing moisture, this turns to pink colour. So we can remove this and reuse after dehydration. Non – indicator type will be white in colour and remains same (white) even after absorption of moisture content. So there is no indication in this type. But this can also be reused after dehydration.
• Calcium chloride is used for most of the vegetable and flower seeds of breeding material. Here the quantity needed is more. It can absorb 10 per cent of its weight.
• The method is suitable for drying small quantities of seeds only. It is a sophisticated and costlier method.
Advantages

• Less time consuming
• Drying rate is uniform
Disadvantages

• It cannot be used in large scale
• A skilled person is required to monitor the operation
The rate of drying the seed depends upon the following factors:

• The moisture content of the seed
• The existing relative humidity and temperature of the environment
• Depth of spread of seeds
• Rate of air blow
• Drying temperature
• Size and capacity of the drier and
• Kind of seeds
Tempering

• When the heated air is used for drying, moisture content in the surface layer of the seed is removed at a faster rate, while the moisture present inside tends to reach outside slowly to maintain the equilibrium.
  
• On continuous drying a pressure gradient is developed inside the seed due to difference in moisture content between the dried outer layer and drying inner layer of the seed.
  
• This results in the damage of seeds by formation of hair like cracks in the seed.
  
• Hence tempering is to be followed.
  
• It refers to the discontinuation of drying operation for a specified period to allow the moisture present in the interior of the seed to migrate all over the exterior portion uniformly.