What is cocoon drying?

Cocoon drying is also known as cocoon stifling. The main aim of cocoon drying is to protect cocoon quality, preserve condition of cocoons for reeling and prevent damage caused by long periods of storage. Cocoons excessive exposure to moisture results in putrification and moulds within the cocoon. Hence drying the pupa of silkworm, evaporated the excess moisture that might ruin the cocoons.

Developed countries like Japan which use advanced techniques of sericulture, the silkworm cocoons are dried using hot air generated by electricity or steam. But this modern technique is only suitable for bivoltine species. But for drying multivoltine cocoons which are predominantly found in tropical areas are dried by steam stifling.

There are many methods of drying or stifling the cocoons for commercial use:

• Sun drying
Sun drying process is no investment drying process, where the pupae are killed and cocoons are dried using the bright sunshine available. Since, bright sunlight is required, hence this method is only possible in tropical and subtropical zones. On thin layers on a mot or planks of wood fresh cocoons are spread and are exposed in the bright sunlight. Based on the strength of sunlight, the method takes around two to three days. Silk fibre is sensitive to ultraviolet rays because of which fibers strength and color are affected. This is the main disadvantage of this method. But, this methods is still followed by in tropical and subtropical areas because of low investment and limited facilities for quick marketing of cocoons.

• Steam stifling
In the steam stifling method, steam is used to dry the cocoons. Fresh cocoons are collected in baskets and are steamed in small reeling units. This is mainly done in many tropical and subtropical countries. For stifling a large amount of cocoons, chamber steaming is done, with a boiler. In chamber steaming, the cocoons are spread in thin layers. For drying multivoltine cocoons, using steam is the best option as they are soft and can be reeled easily without long periods in storage.
In India, after steaming, a common method of cooking these cocoons in an open pan is done. With thirty minutes, the rapid steam kills the papae. After the stifling process, the cocoons are immediately spread on spacious and well ventilated shelves. Prior to reeling they are left there for around three to four days for partial drying. To prevent the growth of mould on cocoons, they are turned from time to time. If proper care is not taken of the cocoons, the risk of mould might be pronounced even after frequent turning, especially in the rainy season.

• Hot air-drying
The hot air drying method of stifling is a very common method of sericulture, especially in advanced countries. This method is done for bivoltine types of cocoons. The hot air dryer essentially comprises of three parts:
• Drying chamber: This is the chamber where the fresh cocoons are placed in thin layers for drying.
• Fan: It is used to maintain constant and uniform air current throughout the layers. This is done to drive out the moisture avoiding the condensation inside the chamber and maintaining proper ventilation.
• Heater: The heater is used for heating the air inside the chamber which is driven by the fan. This is done to regulate and control the proper temperature inside the drying chamber.