Processing of Mulberry Cocoons
I. Bivoltine Silk Reeling Technology Package for Reeling International Grade Raw Silk from CSR Hybrids
Quality of raw material, reeling machinery, reeling process parameters, human skill involved in manual and mechanical operations and quality of water are the major factors that have direct bearing on the productivity and quality of raw silk. Among these, quality of cocoons plays a major role on raw silk yield and productivity. In order to achieve better reeling performance and quality raw silk the role of reeling technology is significant.
In order to develop Bivoltine sericulture technology to produce gradable quality raw silk in India, Central Silk Board in technical collaboration with Japan International Co-operation Agency (JICA), Japan has taken up the project on “Bivoltine Sericulture Technology Development” in the year 1991. In order to improve the quality of the raw material, scientists at Central sericulture research and training institute (CSR&TI), Mysore and JICA experts have developed elite bivoltine hybrid (CSR Race) cocoons. Simultaneously scientists at Central Silk Technological Research Institute (CSTRI), Bangalore and JICA short-term experts have developed bivoltine silk reeling technology package adopting multiend reeling machinery package to produce International superior grade raw silk from CSR race bivoltine cocoons. This package has been developed during 1992 ~ 1997 under JICA assisted project at CSTRI, CSB, Bangalore. The package has been explained briefly below.
The CSR race bivoltine hybrid cocoons have number of promising features associated with integrated package for the production of bivoltine cocoons. Assessment of these cocoon quality have brought out that they possess good shell ratio, longer filament length and better reelability. Further the raw silk produced from these cocoons have very good Neatness percentage and less degumming loss %.
1. Drying of Cocoons
Cocoons should be hot air dried to the optimum level to achieve better quality silk. This method achieves the major objective of hardening the sericin uniformly in all the layers of cocoon shell apart from killing pupa and removing the moisture. Hot air drying can be done by batch type hot air driers or in Ushnakoti.
The temperature and time profile recommended for hot air drying of bivoltine cocoons is as follows:
The optimum degree of drying is decided based on the shell ratio of the sample cocoons using a simple formula.
100 – Shell ratio (%)
Optimum degree of drying = ————————– + Shell ratio (%) (4 & 5)
In case the cocoons are not dried to the optimum level then may be once again dried at the lower temperature viz., 70 °C and 60 °C depending upon the requirement.
2. COCOON STORAGE
The cocoons dried to the optimum level may be conditioned for a minimum period of 7 – 10 days before taking up for reeling in order to achieve better reeling performance. The cocoons should be stored in an appropriate storage room for long duration storage. The cocoon storage should have 20 °C and below temperature in the center of the room and the air inside the room should have 55 % and below relative humidity so that fungus will not attack the cocoons (6).
In order to achieve better results in reeling, the defective cocoons namely., double cocoons, flimsy cocoons, melted cocoons, uzi infected cocoons, urinated cocoons and malformed cocoons which are unsuitable for reeling quality silk should be sorted out using CSTRI cocoon sorting table (6).
4. COCOON MIXING
Mixing of cocoons should be done based on the single cocoon filament denier and reelability of the cocoon lots. Among the cocoon lots taken for mixing, cocoon lots having similar single cocoon filament denier and reelability should be taken.The cocoons lots having wide variation in single cocoon filament denier and reelability should not be mixed together (6).
5. COCOON COOKING
The ideal cocoon cooking for bivoltine cocoons by employing two/ three pan cooking and circular type pressurised cocoon-cooking machine (5,7,8 & 9).
5.1. Three pan cooking using cocoon cooking cage
In order to cook the hot air dried bivoltine cocoons effectively three-pan cooking can be adopted. The procedure while three pan cooking (with water as media) is as follows.
- The cage with cocoons is immersed in first cooking pan at 50 -55 °C for about 45 – 60 seconds.
- The cage is transferred to second cooking pan, which is at 90 – 93°C for about 90 – 120 seconds.
- The cage is then dipped in third pan at 60 – 65 °C for about 45 – 60 seconds.
- The cage is then returned to the second pan at a boiling temperature of about 95-96 °C for 90-120 seconds.
- Steam supply is stopped and cold water is sprinkled on the cage for about 10 seconds.
- The cage is opened and the cocoons are transferred to the cooking pan. The floating cocoons are then pressed using perforated pressing plate for 30 seconds without steam and 30 – 45 seconds with steam.
- Steam supply to the pan is stopped and cold water is sprinkled gradually on the cocoons to reduce the temperature to 70 – 80 °C gradually.
- The cocoons are then brushed at 75 – 80 °C and transferred to reeling basin.
For large scale reeling establishments it is advisable to adopt circular pressurised cooking technique for hot air dried bivoltine cocoons to achieve better productivity and quality raw silk. The methodology to be followed is as follows.
- The cooking vessel shall be filled with water up to half level mark.
- Dry cocoons are then filled up in the baskets.
- The water temperature is raised to 70 to 75 °C by passing steam.
- The cocoon baskets are immersed at 70 -75 °C for about 45 – 60 seconds.
- The baskets are raised above the water level and steamed by passing steam at 90 – 93°C for about 90 – 120 seconds.
- The baskets along with cocoons are immersed in water at 70 – 75 °C for about 60 – 90 seconds for low temperature permeation.
- After low permeation, the cocoons are raised above the water level and kept at about 80 °C. During the same period the water temperature will be raised to 97 °C.
- The cocoons are again steamed above water level at a boiling temperature of about 97 – 98 °C for 90 – 120 seconds to cook the cocoons.
- After cooking treatment the cocoons are immersed at 97 °C water and allowed to stand for 60 – 90 seconds.
- Then the steam valve will be closed and the lid will be opened.
- Cold water is sprinkled (Initially very gradually and later rapidly) to reduce the water temperature from 97 °C to 75 °C in about 4 – 6 minutes.
- The cocoons are taken from the baskets and stored at 40 °C water.
- The cooked cocoons are brushed at about 80 °C and transferred to reeling basin.
Cooked cocoons are then brushed in a pan at 80 °C using paddy straw brush
It is recommended to reel bivoltine cocoons on Multiend reeling machine in order to produce superior grade raw silk with following reeling parameters (6 & 7).
1. Reel speed : 100 – 120 meters/min.,
2. Croissure length : 8 cms for 20/22 denier.
3. Reeling basin water temperature : 40 °C
4. Good quality reeling button with appropriate hole size should be used.
8. WATER QUALITY
The quality standards for the water used in the reeling industry: (5)
- Water should be colourless, limpid and odourless.
- pH of the water should be 7.0 at ambient temperature
- Total hardness should be less than 75 ppm.
- Total alkalinity should be less than 50 ppm.
9. REEL PERMEATION
The silk reeled on small reels should be soaked in CSRTI reel permeation chamber kept at low pressure (vacuum up to 400 mm Hg).The emulsion medium is water with wetting agents. The permeation of the liquor is effected three times to facilitate easy unwinding of silk from the small reels (6).
Silk reeled on the small reels are converted in to standard size skeins after permeation. Re-reeling may be done at a reel speed of 150 – 180 mtrs/min. Care should be taken to dry the silk during re-reeling to avoid gum spots either by heat radiation through steam pipes. The broken ends should be knotted properly to improve the performance of silk in winding.
For producing superior grade raw silk closed type Re-reeling with drying of silk using steam pipe is preferable. Recommended temperature of drying inside the re-reeling machine is 35 °C – 40 °C and relative humidity of 40 – 45 % (6 & 7).
11. FINISHING OF SILK
11.1. Lacing and skeinning
The silk skeins should be laced at six places using cotton thread so that the silk threads will be retained without entanglement.
The re-reeled silk should be cleaned for gum spots and extraneous impurities gently by hand without damaging the silk threads. During lacing, cleaning, skeinning and book making extra care should be taken to avoid end breakages in the skeins. The laced silk in the form of hanks are made in to skeins on a skeinning machine. The silk is folded and twisted to have five spirals. Care should be taken to avoid any break of silk threads during skeinning.
11.2 Long skeinning and book making
In order to avoid mishandling of raw silk and to keep the silk intact to avoid entanglements, it is recommended to practice long skeinning and long skein book making (5 kgs books).
CSR race bivoltine hybrid cocoons in association with bivoltine reeling technology package developed by CSTRI are having a high potential to produce international (superior) grade raw silk. Techno-economically reelers can have better advantages by reeling CSR hybrid cocoons using bivoltine silk reeling technology package, because of superior cocoon quality, reduced waste percentage and hence better raw silk recovery and production of international grade raw silk. CSR race raw silk could be effectively used as a warp for Indian powerloom industry for the production of high quality fabric.
CSB in association with JICA experts and Department of sericulture of respective states is putting consistent efforts for the mass production of CSR hybrid bivoltine cocoons and adoption of bivoltine silk reeling technology by more number of filatures for the production of required quantity of warp quality raw silk. Much number of reeling units have already adopted this technology package in traditional sericultural states and producing superior grade raw silk.
Central Silk Technological Research Institute, Central Silk Board, Bangalore
II. PROCESSING OF MULTIVOLTINE MULBERRY COCOONS
Multibivoltine cocoons are generally loose in structure, more flossy in nature and yield less shell and filament length as compared to bivoltine hybrid cocoons. Quality of raw material, reeling machinery, reeling process parameters, human skill involved in manual and mechanical operations and quality of water are the major factors that have direct bearing on the productivity and quality of raw silk. Among these, quality of cocoons plays a major role on raw silk yield and productivity. In order to achieve better reeling performance and quality raw silk the role of reeling technology is significant.
In this regard Central silk technological research institute (CSTRI), Central silk board, has been striving hard to evolve better techniques and practices on scientific basis to achieve better reeling performance and quality of the raw silk. Multiend reeling machinery package is one of the important developments of CSTRI, which is capable of achieving better reeling performance and producing International grade raw silk from Multibivoltine cocoons..
Some of the important factors that have a direct bearing on productivity and quality of raw silk are as follows:
|a)Quality of raw material||
1. Cocoon characteristics such as race,
2. reelability, filament length, size, build, weight.
3. filament neatness etc.,
1. Stifling, cooking, reeling, re-reeling and silk
|c) Machinery||Drying, cooking, reeling machineries|
|d) Human skill||
1. Human skill involved in manual and mechanical
|e) Quality of water||
1. Source of water, physical and chemical
2. properties of water and the seasonal variations.
In order to arrive at optimum process parameters in the reeling process to achieve better reeling performance and quality of raw silk of International grade, a research project entitled “Influence of reeling process parameters on reeling performance and quality of raw silk of Multibivoltine cocoons” was taken up from 1991 to 1996 at CSTRI, Bangalore. Necessary machineries / equipments/ gadgets have been developed and process parameters have been standardized at each stage of the reeling process by conducting detailed experiments. From the research findings process parameters have been arrived at for achieving better reeling performance and quality of raw silk, and the recommendations are as follows:
1. COCOON QUALITY:
The multibivoltine cocoons preferably having shell ratio of 16% and above and reelability of 70% and above should be used for producing quality silk. Further cocoons, which are having wide variation in shape and size, should not be mixed together (Fig.1).
2. DRYING OF COCOONS:
2.1 For long periods of storage prior to reeling:
Cocoons that are to be stored for long periods prior to reeling need to be dried to the optimum levels in order to achieve better quality of raw silk. This method achieves the major objective of hardening of the sericin uniformly in all layers of the cocoon shell, besides killing of the pupa and removal of the moisture. Further, optimal drying prevents fungus attack. These objectives cannot be achieved by the traditional steam stifling. Hot air drying can be achieved by employing a batch type hot air drier or a ushnakoti. Fig. –2 & 3.
The procedure to be adopted for drying of cocoons in a batch type hot air drier is as follows:
The optimum degree of drying is decided based on the shell ratio of the sample cocoons using a simple formula.
100 – Shell ratio (%)
Optimum degree of drying (%) = ————————– + Shell ratio (%)
In case the cocoons are not dried to the optimum level then may be once again dried at a lower temperature viz. 70 and 60 °C for 1 to 2 hours depending upon the requirement.
The cocoons dried to the optimum level may be conditioned for a minimum period of 7-10 days before reeling in order to achieve better reeling performance.
2.2. Drying of cocoons for reeling without storage:
Even if the cocoons are to be reeled within a week’s time, then also partial hot air drying of cocoons is recommended in place of steam stifling. The procedure followed for partial drying of cocoons is as follows:
The dried cocoons are to be conditioned for one or two days and taken up for reeling. The quality of raw silk obtained by this method is not as superior as that obtained from optimum drying of cocoons but much superior to steam stifled cocoons.
3. COCOON STORAGE:
Storage of cocoons for long periods becomes necessary when availability of cocoons is not continuous (e.g. Jammu and Kashmir, Uttar Pradesh, West Bengal, etc.). Storage of cocoons may also be warranted when reeling units make opportunity purchases in bulk quantity. Besides, storage of dried cocoons for a minimum period prior to reeling, so as to condition the dried cocoons is necessary for quality raw silk reeling.
From the experiments conducted at CSTRI it is observed that cocoons dried to the optimum level need to be conditioned for a minimum period of 2 to 7 days in atmospheric conditions, before reeling in order to achieve better reeling performance and produce better quality raw silk. From the experimental results it is also observed that, reeling performance results are better when hot air dried cocoons are conditioned for a period of 7 to 10 days.
When long periods of storage are not necessary partially dried cocoons may be stored in thin layers in trays kept on wire mesh racks. These racks may be placed in a well-ventilated room so that the cocoons are not affected by fungus due to presence of moisture (Fig.4).
For long periods of storage, cocoons need to be hot air dried to the optimum level in order to remove the water content from the pupa. The cocoon storage room shall be double walled of brick construction. Thermocol shall be filled in between the two walls and the walls shall be plastered with cement. This is to provide maximum insulation. The inside wall shall be covered with wire mesh. Double doors shall be provided at the entrance, so that leakage of outside air in to the storage room is prevented. The room shall be disinfected using chloropicrin to prevent beetle attacks. The building shall be provided with slanting wall at plinth level projecting outside the building so that rats do not enter the storage room.
The cocoon storage room shall be maintained at a temperature and relative humidity not exceeding 25°C and 55% respectively so that fungus attack on the cocoon is prevented. In a storage room if the temperature is above 25°C, then both at the bottom and top of the room moisture will condense into droplets affecting cocoon quality. At the same time if the relative humidity of the air inside the cocoon goes beyond 60% then there will be difference in moisture level in pupae and cocoon shell leading to fungus attack.
4. COCOON SORTING:
Defective cocoons affect the reeling performance and quality of raw silk significantly. In order to achieve the optimum results in reeling the defective cocoons are to be sorted out. Cocoon sorting is to be carried out in a dark room, using a cocoon-sorting table so that the inner stained cocoons are easily visible and sorting facilitated. In order to achieve better results in reeling, the defective cocoons viz., double cocoons, flimsily cocoons, melted cocoons, uzi infected cocoons, urinated cocoons and malformed cocoons should be sorted out (Fig. 5).
Defective cocoons can be sorted out effectively using a cocoon-sorting table developed by CSTRI, which is of low cost and affordable. The rectangular shape sorting table consists of a hopper for feeding the cocoons on to the semi translucent glass top below which fluorescent tube lights are fixed. Defective cocoons are sorted out manually and good cocoons collected separately. (Fig.6)
5. COCOON MIXING:
Mixing of cocoons should be done based on the single cocoon filament denier and reelability of the cocoon lots. Among the cocoon lots taken for mixing, cocoon lots having similar single cocoon filament denier and reelability should be taken. The cocoon lots having wide variation in single cocoon filament denier and reelability should not be mixed together. Single cocoon filament denier could be determined by the reeler himself using a hand operated epprouvette and a balance.
6. COCOON COOKING:
Cocoon cooking is a process in which water is made to penetrate from outside to inside and inside to outside the cocoon shell, so as to soften the sericin uniformly throughout the cocoon shell and facilitate unwinding cocoon filament smoothly without breaks.
In the sericulturally advanced countries, cocoon cooking is carried out by treating the cocoons at various temperatures and duration, to soften the sericin in the cocoon layers. In India cocoon cooking is carried out in open pan system by simply treating the cocoons at boiling temperature. In this method of cocoon cooking the principle of cocoon cooking cannot be adopted and cooking will not take place to the required level, resulting in more waste percentage and quality of silk will not be up to the mark. Particularly hot air dried cocoons require better method of cooking in order to achieve better reeling performance and quality of raw silk.
6.1. Use of pressing disc for open pan cooking:
In order to improve the cooking efficiency of the cocoons in open pan method of cooking, a small gadget namely the pressing disc has been developed by CSTRI and recommended (Fig. 7). The methodology followed is:
- Put the cocoons in the open pan cooking vessel at 85-90 °C and press them slightly using the pressing disc so that the cocoons are immersed in the water for about 60 seconds.
- Then press the cocoon to the bottom of cooking vessel and the cocoons are treated at 93-96 °C for about 60 to 120 seconds.
- Again the cocoons are brought back to the top surface of the open pan and cold water is sprinkled to reduce the temperature from 93 to 96 °C to 85-90 °C in about 60 seconds. Then the cocoons are brushed and taken for reeling.
CSTRI model multifuel economic oven is recommended for the reelers who do not have boilers for cooking purpose (Fig. 8).
6.2 Two pan cooking using CSTRI cocoon cooking cage:
The cocoons are cooked in two pan using CSTRI cocoon cooking cage following low-high-low temperature principle for about 5-7 minutes and the ends are groped. The temperature and duration profile is as follows:
|Ist pan||2nd pan|
|1. Soaking at 55 °C for 45 seconds||2. High temperature for permeation at 90° C for 60 -90 seconds.|
|3. Low permeation temperature 65-70 °C for 45-60 seconds||4. Cooking at 95-96 °C for 60-90 seconds|
|5. Stop heating and allow the cocoons to remain for 60 seconds.|
|6. Sprinkle cold water to reduce temperature to 80 C in 75-90 seconds.|
|7. Brush the cocoons at 80 °C|
6.3 Three pan cooking using CSTRI cocoon cooking cage:
In order to cook the hot air dried multibivoltine cocoons effectively; three-pan cooking can be adopted by reeling units with steam boilers (Fig. 9).
- The cage with cocoons is immersed in first cooking pan at 55-60 °C for about 45 seconds.
- The cage is transferred to second cooking pan, which is at 90 °C for about 60 – 90 seconds.
- The cage is then dipped in third pan at 60 -65 °C for about 30 -45 seconds.
- The cage is then returned to the second pan at about 95 – 96 °C for 60 -90 seconds.
- Steam supply is stopped and cocoons are kept in the same pan for 30-60 seconds.
- Then cold water is sprinkled gradually on the cage to reduce the temperature to 80 °C in 60-90 seconds. The cage is opened and the cocoons are transferred to the cooking pan.
- The cocoons are then brushed at 75-80 °C and transferred to reeling basins.
Silk reeling in India is carried out with traditional reeling devices viz., Charka, cottage basin/domestic basins and CSTRI multiend reeling machine. CSTRI has designed and developed improved multiend reeling machine with improved mechanisms, suitable to Indian conditions (3), (4),(5).
7.1 Multiend Reeling:
It is recommended to reel multibivoltine cocoons on CSTRI multiend reeling machine (Fig.10) in order to produce superior grade raw silk with following parameters.
|1. Reel speed||100 – 120 meters/min.|
|2. Croissure length||8 – 10 cms.|
|3. Reeling basin water temperature||40 °C|
|4. Good quality reeling button with appropriate hole size developed by CSTRI.|
7.2 Mixed cocoon reeling principle:
CSTRI has developed a mixed cocoon principle of reeling to improve the uniformity character of raw silk (in the absence of denier indicator).
During reeling the reeler shall maintain mixed cocoons (reeling cocoons shall consist of mixture of full cocoons, half reeled cocoons and inner layer cocoons) in order to produce raw silk with better uniformity.
Particularly, when the single cocoon filament denier variation from outer layer to inner layer is high, mixed cocoons principle of reeling is very much essential to produce uniform yarn.
8. WATER QUALITY:
Water characteristics such as hardness, M-alkalinity, pH, conductivity etc., influence the quality and productivity of raw silk. Therefore water should be treated suitably in order to bring water to the standard quality required for reeling.
From the studies conducted at CSTRI, standards of different characteristics of water have been developed for better reeling performance along with quality raw silk production. The details are given below:
|Characteristics of water||Standard||Range|
|pH: At room temperature||7||6.6 – 7.6|
|pH: At boiling||8.6||8.4 – 9.2|
|Acidity (ppm)||1||0 – 10|
|M -Alkalinity (ppm)||27||15 – 80|
|Total hardness (ppm)||80||70 – 90|
|Electric conductivity (uW /cm)||70||35 – 350|
Water quality can be got tested at CSTRI and its Sub-units by bringing 1 litre of water in a clean plastic can. CSTRI water softening chemical (Oxipon – WSC) can be used if necessary on the basis of recommendation given by CSTRI. As per the recommendation of CSTRI, reeler can adopt any one of the softening methods i.e., use of water softening chemical (oxipon-WSC) or use of water softening plant, to soften the water to make it suitable for reeling.
9. REEL PERMEATION:
Wetting of raw silk on small reels before re-reeling is very essential for better re-reeling performance. This process softens the gum spots between the silk threads on the small reel and facilitates smooth unwinding. CSTRI has developed a small reel vacuum permeation chamber with capacity of 20 reels to achieve this objective (Fig. 11).
In the reel permeation chamber small reels are soaked in water with wetting agent using low pressure (vacuum up to 300 mm Hg to 400 mm Hg). Permeation is achieved by creating vacuum condition for 1-2 minutes and released. This cycle is repeated three times to achieve better reel permeation.
Silk reeled on the small reels are converted into standard size skeins in re-reeling machine. Re-reeling machine and its mechanical condition and re-reeling process parameters play a significant role on the production of quality silk, particularly winding performance of silk skeins.
CSTRI has standardised the specifications of closed type of re-reeling machine for production of quality silk. From the studies conducted at CSTRI following re-reeling process parameters are recommended (Fig.12).
Re-reeling speeds for different raw silk deniers as follows:
|Size||Speed of normal re-reeling machine|
|14 d||150 ± 10 rpm|
|21 d||160 ± 10 rpm|
|27 d||150 ± 10 rpm|
|42 d||130 ± 10 rpm|
Temperature and humidity to be maintained in the re-reeling section and re-reeling machine are as follows:
|In the re-reeling section||20 – 30° C||
65 ± 5%
|In the re-reeling machine||35 – 40° C||
40 ± 5 %
11. SILK FINISHING:
Re-reeled raw silk needs to be bundled and packed to facilitate transportation as a commodity, for raw silk users convenience.
The silk skeins should be laced at six places using cotton thread in the form of figure “8” so that the coils of silk will be retained without entanglement (Fig.13).
In order to facilitate finding of ends of skeins during winding process it is essential to join and tie the top end and tail end together by extra lacing thread, which is inserted in the middle of the width of the skeins.
The re-reeled silk should be cleaned for gum spots and extraneous impurities gently by hand without damaging the silk threads. During lacing, cleaning, skeining and book making extra care should be taken to avoid end breakages in the skeins (Fig.14).
11.2 Skeining and book making:
The laced silk in the form of hanks is made in to skeins. Care should be taken to avoid any break of silk threads during skeining. In order to avoid mishandling of raw silk and to keep the silk intact (without entanglements) it is recommended to practice long skeining and long skein book making.
In order to popularise long skeining and long skein book making amongst reelers, CSTRI has developed a hand operated economical model long skeining and long skein book making equipment for Indian silk reeling units. In this method long skeins are made into 5 Kg. books (Fig.15).
In order to produce gradable quality raw silk in the country, using above multiend reeling technology package, CSB in co-ordination with the Departments of sericulture of state Governments have established 500 multiend reeling units in the country through centrally sponsored CDP schemes under 9th, 10th and 11th plan schemes. These Multiend reeling units are running successfully and achieving better performance and producing international grade raw silk from Multibivoltine cocoons.
Central Silk Technological Research Institute, Central Silk Board, Bangalore