CHAPTER 3
COCOON QUALITY AND CLASSIFICATION 

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3.1 Cocoon quality

A series of natural circumstances will produce variations in cocoon quality. Some of the most noteworthy include:

• differences in cocoon quality in the same batch;
• differences in cocoons produced in the same location by different farmers who have reared the same species;
• seasonal influences. In Japan for example, cocoons produced in the spring and late autumn are higher in quality than those in early autumn and summer;
• environmental conditions affect cocoon reelability such as temperature and humidity;
• processing technique in reeling will impact reeling efficiency as well as raw silk quality;
• bivoltine cocoons are superior quality compared to multivoltine silkworm species traditional farmed in tropical zones.

Recent silkworm cultivation now develops cross-breeds of multivoltine with bivoltine silkworms as a strategy to improve overall cocoon quality.

This section presents the measures to be taken during silkworm rearing and mounting to obtain a better quality of cocoons with higher silk content, longer filament, better reelability and lower percentage of defective cocoons.

Maintain temperatures at or near 25ºC and relative humidity around 65 percent for silkworms to spin good quality cocoons with a high reelability.

Although different mount practices are employed among producer countries, rotary mounting frames provide good ventilation. The result is improved reelability of cocoons.

Cocoons should be harvested only following complete pupation. In practice, the appropriate harvesting day would be the fifth day in tropical countries, and the seventh or eighth day in temperate countries, from the mounting date. If premature harvesting takes place, the silkworm will still be in its larval stage, weigh more, have fragile skin, and could likely be crushed, which would cause stains to the cocoon during handling and transportation.

After proper harvesting and removal of diseased or damaged cocoons, the fresh cocoons are taken to the market. For short distances, the farmer carries the cocoons in bamboo baskets or jut bags on his head or by bicycle. If the distance is longer, cocoons are transported in a van or a bus. Caution should be exercised when loading fresh cocoons on to the van to ensure that containers are loosely packed in tiers to avoid damage. Vibration and shock during long trips can spoil fresh cocoons. Cocoon quality is affected by steam produced while being transferred in a bag or basket. If there are defective cocoons (see section 3.3) fresh cocoon quality will be harmed.

Table 9 describes the impact of transportation on fresh cocoons. While it is advisable to avoid carrying cocoons over long distances, there are steps, which preserve silk reelability. First use of P.V.C. containers with 15 kgs capacity is recommended. Shock absorbers, such as sponge can prevent damage over long distances. To minimize the risk of heat deterioration, shipping should take place only during the night or early morning. Ideally, the fresh cocoons should arrive at the stifling unit within two to three days after harvest.

When cocoons are sold at the market, price is assessed on the basis of cocoon quality. This is judged by grading shell percent, filament length, reelability and the percentage of defective cocoons. If the percentage of defective cocoons is high, the price will be affected. The next section outlines the characteristics of defective cocoons.

A double cocoon is spun by two worms, producing a filament, which does not unwind smoothly and tangles easily. As these cannot be reeled along with normal cocoons, double cocoons are used for manufacture of a coarse, non-uniform, stubby yarn called "doupion". Double cocoons may be caused by crowded mounting conditions, high temperatures, high humidity and mutation of silk species.

Dead cocoons are also known as melted cocoons. In this case, the pupa is dead and sticks to the inside shell of the cocoon causing a stain. Melted cocoons are called mutes because they do not make a sound when shaken. These cocoons are difficult to process and will result in silk, which is dull in colour.

These are recognized by a rusty colour spot on the cocoon shell caused by absorption of intestinal fluid/urine of the mature worm formed during mounting. Reelability is very poor in this case.

This defect may happen due to improper mounting frames; these are also called scaffold pressed cocoons.

These are abnormally shaped cocoons, which may arise from species variation. This defect may be due to racial characteristics and breeding with mulberry leaves stained with agrochemicals.

Here, the shell is loosely spun in layers and has a low silk content. These cocoons are easily overcooked and produce waste.

One or both ends of the cocoon are very thin and risk bursting when processed. The cause of this defect may be attributed to species characteristics or improper temperature and humidity during rearing and mounting.

This happens when a moth has emerged, been eaten by beetles or in the case of the emergence of a parasite. Pierced cocoons are unfit for reeling and can be used only for hand spinning or as raw material of machine spun silk yarn.

In sericulturally advanced countries, cocoons are subjected to systematic testing and grading before sale. Prices are based on the quality of the cocoons. But in developing countries, there is no system for cocoon testing. Cocoons are sold on visual inspection and personal experience is relied upon in marketing of cocoons. No laws exist or compulsory testing and trading of cocoons. The result is that cocoons are simply auctioned or in certain instances, even sold at a price fixed by the Regional Departments of Sericulture. There is no direct correlation between price and quality of cocoons.

Major sericultural countries through the International Sericultural Commission (ISC) have studied an International Cocoon Classification System. However, diversion of silkworm species, techniques of breeding, silk reeling and other factors lead to the production of non-uniform cocoons. These parameters have challenged the effort to unify equipment and testing methods (see Table 10).

Cocoon testing and grading may be accomplished with a compact automatic reeling machine as well as a multi-ends reeling machine, which is typical equipment in major sericultural countries. Testing methods used by major sericulture countries are displayed in Table 10.

The quantity of fresh cocoons, which are taken out of a lot for testing purposes depends on the actual weight of the lot on offer.

The cocoons on offer are divided into three batches.
Batch weighing up to 1 000 kgs
Batch weighing up to 2 000 kgs
Batch weighing up to 4 000 kgs

The sample size of fresh cocoons taken out of each batch for testing is as follows:

1^st batch - 2.0 kgs
2^nd batch – 4.5 kgs
3^rd batch – 6.0 kgs

In the case of dry cocoons, the quantities taken out for testing from each batch are as follows:

1^st batch up to 400 kgs of dry weight – 0.8 kgs taken out for testing.
2^nd batch up to 800 kgs of dry weight – 1.8 kgs taken out for testing.
3^rd batch up to 1 600 and over - 2.4 kgs taken out for testing.

The cocoons received must be dried as soon as possible and to an acceptable degree. To do this, the moisture content of the fresh pupa and the cocoon shell percentage must be measured. The percentage of drying is calculated according to the formula.

Drying percent of cocoon = (0.0115 x mc of fresh pupa – 0.2104)
                                            x (percent of cocoon shell 1.15)
                                            x (mc of fresh cocoon + 115)
                                           (mc : moisture content of the cocoons)

Drying should be accomplished in one continuous process where the temperature is gradually decreased from 98ºC to 60ºC until the required ratio is obtained.

This evaluation usually happens on a table under natural light. If natural light is insufficient based on time of day or weather conditions, elimination takes place under artificial light of 500 lux. to calculate the percentage of eliminated cocoons according to terms and conditions defined by cocoon classification, the following types of cocoons must be removed: double cocoons, thin-end cocoons, scaffold marked cocoons, malformed cocoons, flimsy cocoons.

After elimination of the bad cocoons, the remaining cocoons are batched for the reeling test as follows:

A small sample of cocoons must be cooked to determine the correct cooking conditions for the specific batch. Once these parameters are established, actual cooking of the entire sample batch can be completed.

Reeling should be carried out under the following conditions:

This should be carried out on large reels with a circumference of 1.5 m and the standard re-reeling speed of 160 r.p.m.

The results of at least two or three reeling tests must be taken in order to calculate the classification. The resultant raw silk weight divided by the sample cocoon weight will indicate the raw silk percentage. The cocoon classification items are worked out by the following method:

Points to be observed for testing percentage of cocoon shell are as follows:

• The percentage of the cocoon shell can be calculated from the weight of the whole cocoon and cocoon shell obtained from 200 cocoons from which 100 cocoons are taken separately.
• If possible, equal amounts of female and male cocoons should be selected for the sample.
• No defective cocoons should be included in the sample.
• As atmospheric conditions make the moisture content of the cocoons too variable, the sample should be selected from a point 10 cm beneath the surface of the batch of cocoons.

The average value of the percentage of the cocoon shell obtained from repeating the test twice must be graded within a 0.3 percent deviation. But if the difference between both samples is + 0.3 percent another test should be taken.

The result should be expressed to one decimal place where the weight of sample cocoons is the sum of eliminated cocoons plus the weight of the good cocoons.

3.   Length of cocoon filament

Where, total reeled cocoon number

= Sample cocoon number – Converted carry over cocoon number

Length of raw silk is checked by the gauge. Sum of reeling cocoon number per thread is the total number of reeling cocoon verified 20 times during the reeling (this means once per unit work process). The length is based on the average of every reeling block. It is expressed by total number to the one decimal place. Converted unreelable cocoon number and carry over cocoons are calculated by converting them into length and expressed by full cocoon number to the one decimal place.

The cocoon number converted to full cocoon length

= 1.00P + 0.77H + 0.39M + 0.12L (1)

Where P : number of newly cooked cocoons but unreelable
           H : number of heavy shell cocoons but unreelable or carried over
           M : number of middle shell cocoons unreelable or carried over
           L : number of light shell cocoons unreelable or carried over

The reeling cocoon number per thread is estimated down to two decimal places. The reeling work is not completed to the last single cocoon, but up to about 50 cocoons. the remaining cocoons are "carry over cocoons". They are divided into three kings: (H)...cocoons where only the outside layer has been reeled, (M)...cocoons reeled up to middle layer, and (L)...cocoons reeled up to inner side layer. These cocoons can be reeled along with other cocoons. That is why they should be converted into full cocoon length or weight and then the converted number has to be deducted from the total sample of cocoons to obtain the exact raw silk percentage or actual sample.

The converted number of full cocoon number is called "carry over cocoon number" which is calculated by multiplying the number of cocoons with Heavy layer, Middle layer and Light layer by cocoon convert indices. Also, the unreelable cocoons during the reeling or after reeling, are converted into full cocoon number by applying the same indices as for carry over cocoon. 

For example:

                     Number of sample cocoons 340
                     Length of raw silk 55 150 m
                     Total cocoon number of snap check on
                     reeling cocoon number per thread 720
                     Total ends checked for reeling cocoon
                     number per thread 91 ends
                     Unreelable cocoons H(1), M(3)
                     Carry over cocoons H95), M(14), L(21)
                     Reeling cocoon number = 340-1-12 = 326

Where           ( 1 – Convert unreelable cocoon number)
                     (12 – Convert carry over cocoon number)

Ave. reeling cocoon number per thread = 720 + 91 = 7.91

This is the length of cocoon filament in (b) batch is 1 349.1, the average of (a) and (b) batch is 1 343.6 m. Thus, the length of cocoon filament = 1 344 m.

Where reeled cocoon number = number of sample cocoon
                                              - number of unreelable new cocoons
                                              + number of converted carry over cocoons

Number of ends feeding          = number of cocoons fed
                                              + number of carry over cocoons
                                              - number of converted carry over cocoons

Note: Reelability percentage means the average percentage of each reeling batch. It is estimated based on a total reduced to the decimal place. The converted carry over cocoons is shown by length level worked to one decimal place. Where, the reeling cocoon number and end feeding number are corrected by the conversion systems as follows:

                    Number of sample cocoon   340
                    Total of end feedings            521
                    Unreelable cocoon                H(1)
                    Carry over cocoons             H(5), M(14), L(21)

Reeled cocoon number = 340 – 1 – 12 = 327
\ End feeding number = 521 + 40 – 12 = 530
\ Reelability percetage = (327 ¸ 530) x 100 = 61.69 batch (a)

If reelability percent of batch (b) is 60.99 percent the average of batch (a) and (b) will be 61.3 percent.

Now, converted carry over cocoon silk (g)
= weight of bave (g) x number of carried over cocoons to be converted to full ones.

Where, number of reeled cocoons
            = number of sample cocoons
            - number of converted unreelable cocoons
            + number of converted carry over cocoons to full one

Note: Raw silk percentage is estimated to two decimal places. The weight of bave is estimated down to three decimal places. The converted unreelable cocoons and carry over cocoons are based on the weight system, and the number of converted cocoons is estimated on the total to one decimal place.

For example: The cocoon number converted to full cocoon weight
                     = 1.00P + 0.73H + 0.30M + 0.8L (2)

Where, P – number of newly cooked cocoons, but unreeled
            H – number of heavy shell cocoons, but unreelable or carried over
            M – number of middle shell cocoons, but unreelable or carried over
            L – number of light shell cocoons, but unreelable or carried over

Weight of cocoons sample – for example:   batch (a) : 300 g, batch (b) : 300 g
Conditioned silk weight:                              batch (a) : 131.05 g, (b) : 131.75
Sample cocoon number:                             batch (a) : 340, (b) : 340
Unreelable cocoons:                                   batch (a) : P(1), M(3)
                                                                  batch (b) : P(1), H(1), M(2), L(3)
Carry over cocoons:                                   batch (a) : H(5), M(14), L(21)
                                                                  batch (b) : H(3), M(11), L(27)

\ reeled cocoon number = 340 – 1 – 1 – 10 = 328 batch (a)
                                           340 – 1 – 1 - 8 = 330 batch (b)

Weight of bave = (131.05g ¸ 328) = 40.0(cg) batch (a)
                           (131.75g ¸ 330) = 39.9 (cg) batch (b)

Converted carry over cocoon silk yield = 0.400 g x 10 = 4 000 g batch (a)
                                                               0.299 g x 8 = 3 192 g batch (b)

The average of (a) and (b) batches is 19.26 percent.

In cocoon classification, the result for length of cocoon filament and result of reelability percent is shown in Table 11 (1), (2) added up to the grading result which is applied to the cocoon grading shown in Table 11 (3).

Cocoon classification is divided into 5 grades: A, B, C, D and E.

3.5 Cocoon exchange

The price of cocoons required for producing one kg of raw silk, (Cocoon Price Parameter [COO]), is calculated on the basis of 10 percent raw silk percentage cocoon to make 2A grade raw silk. The cocoon price parameter as a basis for estimating the cocoon price is obtained from the following formula:

(raw silk price + silk by-product income)

As the CPP by this method stands as the standard of C grade cocoons, if varied from it each 2.0% CPP per single grade is added to or deducted from the standard one.

Cocoon is computed from the following formula:

Cocoon price/kg     = CPP ± amended CPP) x raw silk percentage of cocoon
                               x (1-Discounting rate)

The resulting raw silk percentage is the raw silk test value multiplied by 0.9785, which equals fresh cocoon yield. The silk producer may have large stocks of cocoons in storage for reeling, which will result in the reduction of raw silk percentage. As the test sample does not match actual raw silk sold, consumed for testing in mills or in a "discounting rate" is used for raw silk during the reeling process of 2.15 percent (established in 1982).

Where no cocoon classification system exists, a derived arbitrary raw silk percentage is used based on visual inspection. A fictitious raw silk percentage is calculated using cocoon shell percentage, relative raw silk percentage from the previous year’s assumed value. For example, in Table 12 when the CPP is 29 and the fictitious raw silk percentage of cocoon is 18 percent, the commercial price per kg is 29 x 0.18 = US$5.22.

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