Short note on cocoon processing
Answers
Cocoon cooking unwinds the cocoon filament spun by the silkworm. The sericin covering around the cocoon filament is agglutinated after silkworm spinning, then hardened through the cocoon drying process. In preparation for reeling, it should be softened.
Processing softens sericin by heat, water and steam. Ideally there will be uniform softening of the outer and inner cocoon shell.
Cocoon cooking methods
1. Pan cooking: Cocoons are put into a pan of boiling water. The groping ends of the cooked cocoon can be completed with the stirring rod in the pan.
The cocoons contained in the wire cage are placed into the boiling water and then boiled for a few minutes. After boiling, the cocoons with the wire cage are moved into a low temperature bath in another pan. If there is water permeation inside cocoon shell and swelling of the cocoon shell, this work can be repeated.
2. Machine cooking: Recently, machine cooking has become widely used in most silk reeling factories. In general, the machine cooking process is divided into six parts as follows:
a) Soaking part
The surface layer of the cocoon swells over at the soaking part. The first part is carried out by dipping the outer layer of the cocoons into a water bath at 55ºC. This step is comparable to fabric dipping for dyeing to ensure even results.
b) High temperature and low temperature permeation part
The uniform cooking of the outer and inner layers of the cocoon can be easily attained by replacing the air of the cocoon cavity with water since water transfers heat faster than air. Thus, the wet cocoons from the soaking part are exposed to steam at about 90-95ºC at an appropriate steam pressure. Next, the air inside the cocoon cavity is heated. When cocoons treated at high temperature permeation are moved into low temperature permeation around 65ºC, partial condensation occurs in the cocoon cavity. Then the cocoon sucks water in, evenly wetting all the layers of the shell.
The amount of water permeated into the cocoon cavity is controlled by the difference in temperature between high temperature and low temperature permeation parts, as well as the air permeability of the cocoon shell.
c) Steam cooking part
Now the cocoons treated in steps a) and b) are moved to the steam processing. This part causes the sericin to swell and soften the silk layers and the steam to fill up the cocoon cavity by diffusing the permeated water out of the cocoon. Steam heat is a highly efficient head conductor that can cause unwanted sericin loss. To minimize or stop this loss of sericin, infra-read rays have been attempted in this process, but proven unfeasible. For cocoons anticipated having poor reelability, the steam cooking part has to be prolonged.
Sudden variations in steam pressure can adversely affect the cooking process by producing over processed or insufficiently processed cocoons. These poorly cooked cocoons seriously deteriorate reeling efficiency by decreasing raw silk yield and quality and boosting cleanness defects during reeling. It is urged that greater vigilance be exercised to control steam cooking.
d) Cooking adjustment part
At this point the steam content of the cocoon cavity is replaced with water through gradual condensation of steam in the cocoon. This is effected by gradually cooling of the water from 98º to 65ºC. Sericin swollen by steam cooking becomes stable. As this step consumes large volumes of water for cocoon permeation, it needs to be longer in duration and requires more fresh water than other parts.
e) Low temperature-finishing part
Cocoons are finished in 50-60ºC water. Here, the cocoons, which were properly swollen by processing in the first five steps, become more stable and are prepared for the next stage in reeling.