CHAPTER 5
COCOON COOKING
5.1 Introduction
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.
1. Pan cooking
2. Machine cooking
Recently, machine cooking has become widely used in most silk reeling factories (Figure 12). 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.
Figure 10. Air expansion in cocoon cavity and partially replaced with steam at high temperature permeation part.
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Figure 11. Cocoon sucks in water at low temperature permeation part |
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.
Figure 12. Cocoon Cooking Machine |
Optimum degree of cocoon cooking can be gauged correctly by the reeling results, but it also can be evaluated as follows:
Table 16a. Criteria of insufficient and overcooking
Items |
Insufficient |
Overcooking |
Colour and touch of cooked cocoons |
Creamy and rough |
Grey-yellowish and soft |
Conditions of groping ends |
Hard |
Not difficult |
Number of dropping ends in reeling process |
Increase |
Decrease |
Increase of by-product silk |
Parchment layer |
Brushing waste |
Kinds of commonly occurring defects in raw silk |
Split ends and loop |
Slug |
Degumming ratio of raw silk |
High |
Low |
Tangle of cooked cocoons |
Slight |
Heavy |
5.4 Adjustments to cooking conditions
Cooking methods should be re-adjusted for better results depending on the different quality of cocoons as shown in Table 17.
During treatment, the temperature variations should be less for good reelable cocoons than for poor reelable ones, where steaming pressure and temperature of steam cooking should be increased. The temperature in cooking adjustment should be precisely maintained for good reelability cocoons. This level of precision is unnecessary for cocoons with poor reelability. Remedies for poor cooking results are shown in Table 18.
Table 17. Adjustment of cooking methods depending on variations in cocoon quality
Different cocoon qualities |
Re-adjustment of cooking methods |
Thick shell cocoon with high reelability |
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Thin shell cocoon with high reelability |
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Thick shell cocoon with poor reelability |
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Insufficiently dried cocoons |
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Over dried cocoons |
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Table 18. Points for cooking operations
Phenomena |
Counter measures |
Crushed cocoons |
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Overcooking |
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Insufficient cooking |
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Floating cocoons |
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Uneven cooking |
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5.5 Effects of cocoon cooking conditions on reeling result
The suitability of cocoon cooking has an important bearing on the reeling result. A high proportion of waste is caused whenever cocoons are over- or undercooked. For overcooked cocoons, this happens when the filament comes out in lumps, clogs the reeling button and increases the number of breaks during reeling. Each time a break is repaired, the silk filament must be pulled out and all of the silk removed becomes waste. The undercooked filament does not unravel easily and also causes breakages and waste. In each case reeling efficiency is diminished.
Table 19. Effects of cooking degrees on reeling results (Lee, et al. 1971)
Degrees of cooking |
Raw silk % of cocoon |
Reeling troubles per 10,000 m (times) |
Percentage of by-product silk |
Neatness (%) |
Cleanness (%) |
|
Under cooking | 16.00 |
1.47 |
1.28 |
1.23 |
92 |
91 |
Optimum cooking | 15.98 |
1.60 |
1.45 |
1.10 |
94 |
95 |
Over cooking | 15.94 |
1.67 |
1.54 |
1.03 |
92 |
93 |