CHAPTER 5
COCOON COOKING

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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.

5.2 Cocoon cooking methods

1. Pan cooking

    1. 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 (see Appendix, Figure 14).
    2. 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 (see Appendix, Figure 15).

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 55C. 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 90-95C 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 65C, 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. 

Air expansion in cocoon cavity and partially replaced with steam at high temperature permeation part.

Figure 10. Air expansion in cocoon cavity and partially replaced with steam at high temperature permeation part.

 

Cocoon sucks in water at low temperature permeation part

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 98 to 65C. 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 50-60C 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.

Cocoon Cooking Machine 1.Cocoon hopper part, 2. Soaking part, 3. High temperature permeating part, 4. low temperature permeating part, 5. Cooking part, 6-7. Adjusting part, 8-9. Low temperature finishing part, 10. Cocoon outlet

Figure 12. Cocoon Cooking Machine
1.Cocoon hopper part, 2. Soaking part, 3. High temperature permeating part, 4. low temperature permeating part, 5. Cooking part, 6-7. Adjusting part, 8-9. Low temperature finishing part, 10. Cocoon outlet


5.3 Degree of cocoon cooking

Optimum degree of cocoon cooking can be gauged correctly by the reeling results, but it also can be evaluated as follows:

  1. Response to touch: In the case of optimum cooking, the cooked cocoon is velvety and the shell is neither soft nor hard when felt by the hand. In the case of over cooking, the cocoon is soft and collapses easily under finger pressure. If under or imperfectly cooked, it is hard.
  2. Groping and condition: In case if optimum cooking, the filament unwinds by itself at the moment the groped end is picked up and grasped by hand, with little resistance on the fingertip, and without breaking the end.
  3. Cocoon colour: In imperfect cooking, it is creamy and cooked partially. When over cooked, the cocoon is grey-yellow.
  4. Cooked cocoon shell weight: five to six times as heavy as the original shell weight is desirable; 4.5 times or less is defined as under or imperfect cooking, while the overcooked cocoon weighs six times or more than the original.
  5. Cooked cocoon weight: 10-11 times the weight of the original dried cocoon.
  6. Conditions of cooked cocoons in basket: The cooked cocoons should easily drop out of the basket, when the baskets are opened at the end of the cooking process.
  7. Crushed cocoon: There are no crushed cocoons in optimum cooking.

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
  • In pre-treatment (soaking, steaming, permeation) difference of temperature should be more, while steam pressure and temperature of steam cooking part should be lower
Thin shell cocoon with high reelability
  • Different of temperature part should be higher and steam pressure and temperature of steam cooking part lower
Thick shell cocoon with poor reelability
  • Difference of temperature in pre-treatment as well as steam pressure and temperature of steam cooking part should be more.
Insufficiently dried cocoons
  • Steam pressure and temperature of steam cooking part less
Over dried cocoons
  • Steam pressure and temperature of steam cooking part more

 

Table 18. Points for cooking operations

Phenomena

Counter measures

Crushed cocoons
  • Reduce the difference of temperature between high and low temperature permeation parts as well as steam cooking part and first cooking adjustment
Overcooking
  • Lower the temperature of steam cooking and high temperature permeation parts
  • Shorten the time for cooking
  • Modify the quality of reeling water
Insufficient cooking
  • Increase gradually the temperature of steam cooking part
  • Prolong the time for cooking
  • Use the soluble type water for cooking instead of hard type one
Floating cocoons
  • Make the temperature curve in cooking adjustment
  • Increase the difference eof temperature between high and low temperature permeation parts
  • Reduce the temperature of low temperature finishing part
Uneven cooking
  • Increase the temperature of soaking part
  • Check the steaming pipe in the high temperature permeation part
  • Strict assorting of inferior cocoons

 

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

 

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