Post Harvest Handling and Processing of Reeling Cocoons

In this article, you will get to know the Post-Harvest Handling and Processing of Reeling Cocoons, including their selection, transportation, weighment, storage, and stifling methods.

Table of Contents

Criteria for Selection of Cocoons

A lot that contains a higher percentage of defective co-coons, when reeled, will result in:

lower percentage of reliability;
higher percentage of silk waste and low yield of silk yarn;
increase in the renditta
variation in the denier, affecting the quality of silk such as evenness, neatness and cleanness and increase in the number of winding breaks, poor lustre and colour in the silk yarn.

How are Cocoons Purchased?

Regarding the method of purchase, at present the system followed for disposal of cocoons by the rearers is to sell the cocoons in the notified cocoon markets through open auctions. The reeler therefore, has to offer his price for the cocoon lot, keeping in view the quality of the cocoons of that lot as detailed above, the expected renditta and the selling price of the silk yarn.
Regarding the appropriate time for the purchase of cocoons, the reeler should study the silkworm rearing season, the arrival and demand of cocoons in the market as well as the marketing trend for the end produce i.e., the silk yarn.
While purchasing the cocoons, the reeler offers a price which should be properly commensurate with his assessment of the renditta, the likely cost of production and the expected selling price of the yarn. The reeler develops this acumen through sufficient experience and intelligence.

Transportation of Reeling Cocoons

The cocoon contains the live silkworm pupa which contains water to the extent of 60 to 70 per cent of its weight at the time of purchase. During transportation of the live cocoon, if care is not taken, the live pupa inside the cocoon will be deformed, thereby oozing out the body fluid inside the cocoon, damaging the silk fibre in the cocoon and discolouring that inner portion. The inner silk layer of the cocoon gets hardened, thereby making the cocoon unreelable and increasing the waste.
By not following the proper method of transportation, the live cocoons are often placed under pressure and the crushed cocoons get stained (generally making the cocoons black in colour) and stain the surrounding cocoons which are not crushed, thereby making many more cocoons unfit for reeling.
While transporting the live cocoons in a heap or in a closed bag or container, the pupae often suffocate, resulting in perspiration. The perspiration being alkaline in nature affects the sericin layer of the cocoon filament which results in poor reelability.
In the case of transportation of melted, flimsy and such poor quality cocoons along with good cocoons, the damage is much more as the pupae inside the cocoons are easily susceptible to melting (dead pupae) which will affect and stain the other good cocoons of the lot.
Improper transportation of live cocoons will result in low percentage of recovery of silk; increase in the percentage of recovery of waste, affecting the quality of silk; increase in the renditta which will have a bearing on the cost of production and lower the selling price besides affecting the reputation of the product.

What is the Safe Method of Transportation?

Cocoons can be safely transported by filling the cocoons in suitable containers. The container should be so designed to provide enough aeration, prevent jolting and crushing of cocoons, and prevent damage to the pupa inside the cocoon. Too many cocoons should not be filled in one container in order to minimize the pressure on the cocoons in the lower portion of each container. The container should generally be perforated for proper air circulation so that the live pupa inside the cocoons do not suffocate and die. This will also prevent absorption of moisture by the cocoons and desaturation of the sericin on the filament.

Weighment and Storing of Cocoons

Immediately after purchase, the cocoons should be weighed to know the quantity of the cocoons purchased. Even after transportation of cocoons to the reeling centre, they should be weighed to know the actual receipt of cocoons in the stores. This will help in ascertaining whether there is any pilferage or shortage in the purchase weight and receipt weight.
Immediately after making the receipt weight in the stores, the cocoons should be thinly spread in the trays and arranged in stands or racks to allow slow drying as the live cocoons will have suffocated and acquired moisture during transportation. This spreading will allow proper formation of the pupae inside the cocoons.

Stifling of Cocoons

The cocoon contains a live pupa inside. The pupa, on account of its natural metamorphosis, transforms into a moth between the 10th and 11th day of its formation from the larval stage (life cycle) and by piercing the cocoon emerges from the cocoon as a natural phenomenon. While emerging from the cocoons, the moth cuts the filaments in the silk layer, making the cocoon unfit for reeling.
The cocoon is a ball of silk filament having two ends, one beginning outside and the other ending inside. This pierced cocoon is fit for spinning and not for reeling. It is, therefore, necessary to kill the pupa inside the cocoon to prevent its emerging as a moth. This method of killing the pupa without affecting the breakage of the silk filament is called the process of stifling.

1 Steam stifling

Fresh cocoons on the 6th or 7th day after pupation are exposed to the direct action of hot, wet steam for a definite period. This is done in small containers or in specially designed chambers. The different methods of steam stifling are:

basket steaming;
barrel steaming; and
chamber steaming.

(a) Basket Steaming

In small charkha establishments where the consumption of cocoons for reeling is limited, this method of steaming is adopted. At a time, 10 to 15 kgs of fresh cocoons are filled in a strong, woven basket with sufficient perforations at the bottom. The cocoons are covered with a wet gunny cloth. The basket is placed over a cauldron containing boiling water. The cauldron is placed on an oven. Steam from the cauldron passes through the bottom of the basket. Approximately, half an hour is required for stifling the cocoons.

(b) Barrel Steaming

This procedure is similar to that of basket steaming except that in addition to a basket, a metal barrel is used. The barrel is permanently fixed on the oven with a platform inside the barrel to support the basket of cocoons. The water is filled in the barrel to two-third the height of the platform for the production of steam. The barrel is provided with a lid well secured to prevent escape of steam. The cocoons are stifled in fifteen to twenty minutes.

(c) Chamber Steaming

This method is followed in large reeling units where large quantities of cocoons have to be stifled daily. Steam chambers of the required dimension are constructed and steam pipes are provided inside the chamber. Perforated steam pipes inside the chamber allow the steam to fill the chamber. The pipes are connected to a steam boiler. The trays filled with cocoons fitted into a trolley are pushed inside the chamber and the chamber doors are securely closed. The steam is let in through a steam valve that is located outside. After the required period of time, the steam valve is closed and the trolley is pulled out. The next trolley load of cocoons is pushed inside the chamber and the steaming process is repeated.

Disadvantages of Steam Stifling

The disadvantages of steam stifling are:

It only kills the pupa inside the cocoon and does not dry it properly.
The large moisture content makes the pupa fragile and hence the cocoons cannot be stored for long periods.
The floor area of the store cannot be used very efficiently because the steamed cocoons are to be spread in very thin layers which limits the storing capacity.
Additional labour is required for turning the cocoons in the trays very often for quick drying.
Humidity in the store room increases due to natural evaporation of moisture from the pupae and as a result, conditions favourable for the development of fungus are created. This is a serious problem particularly in the rainy season.