Table of Contents


Introduction

This note is mainly concerned with the handling, processing and storage of scallops and queens after capture. Advice is given on handling at sea, removal of the meats from the shells, dipping techniques, chilling, freezing and cold storage. Some figures for yields and composition, and brief comments on the problem of waste disposal, are also given.

Brief notes have also been included describing the species, history of the fishery, distribution and methods of capture.

All enquiries regarding the fishing grounds and catching methods should be sent to them or to the Fisheries Laboratory at Lowestoft. Both addresses are given at the end of this note.

Scientific names

Scallops belong to the family Pectenidae and several members of the family are found in the North Atlantic. The two species common to British coastal waters are the great scallop. Pecten maximus and the queen, a smaller member of the family, whose scientific name is Chlamys opercularis.

Common names

The scallop is sometimes called escallop or scollop, and the queen is sometimes referred to by the full name of queen scallop. Scottish fishermen often refer indiscriminately to both species as clams, although neither is a true clam.

Description of the scallop

The scallop is a bivalve shellfish, with the familiar ribbed, fan-shaped shell as shown in Figure 1. The upper valve of the shell is almost flat and reddish in colour, whereas the lower valve is convex and almost white. On the shell there are concentric rings, one of which is formed each year, the number of rings thus indicating the scallop’s age. Scallops grow to about 150 mm in diameter and are considered to be of reasonable commercial size from about 100 mm upwards.

Inside the shell, the soft parts shown in Figure 2 consist of white muscle meat and orange and white roe, surrounded by light brown gills and the frill with its numerous tentacles and eyes. The brown kidney and greenish-black liver are also indicated.

Fig. 1. Queen and scallop shells.

Fig. 2. Scallop with the flat shell valve removed.

The scallop is a hermaphrodite, that is, with the male and female organs in the one animal. The gonad or roe, which contains the orange-red female ovary and the creamy-white male testis, ripens during the winter months, then in the late spring the eggs and sperms are liberated into the sea within a short time of each other. Tiny free-swimming larvae develop from the fertilized eggs and after drifting about in the currents, sink to the bottom and attach themselves with sticky threads to seaweeds. After a time the tiny scallops leave the weed to lie in shallow depressions in the sea bed with their flat shell valve uppermost and just level with the surface. A second spawning takes place during late summer.

Scallops feed by filtering microscopic organisms from the sea water. Around the edges of the frill, which lines the inside of the shell, there are many eyes which warn the scallop to close its shell at the approach of a possible enemy. Scallops can move actively by expelling water from within the shell in order to swim away from their enemies, such as starfish, and also to recess themselves into the sea bed, to move from an unsuitable bottom and to right themselves if turned over, for instance, by a storm or by fishing gear.

Description of the queen

The queen is smaller than the scallop, with both valves rounded, thus making it readily distinguishable from the scallop; both are shown together in Figure 1. The shell is usually pink in colour, and shows the same annual growth rings. The queen rarely exceeds a size of 90 mm and the arrangement of the soft parts is much the same as that shown for the scallop in Figure 2, except that the roe is somewhat smaller in proportion to the muscle and is not normally used for commercial purposes.

The fishery

The scallop is common around the British coasts, and is found on sandy or muddy gravel bottoms below the tidal zone. It is commonest in depths of 20 to 40 m but is found both in deeper and shallower waters. It lies with the flat valve of the shell uppermost, and the rounded valve recessed in the sea bed.

The bulk of the British catch comes from Scottish grounds, ranging from the Ayrshire coast to the Hebrides. Orkney. Shetland and the Moray Firth. There is also a substantial fishery around the Isle of Man. Not all the resources are yet known, and new beds are continually being discovered and fished.

The queen occurs in much the same area as the scallop, but usually in somewhat deeper water, down to as much as 200 m or more. The fishery for queens on a commercial scale, for human consumption, is a fairly recent one. The Scottish scallop fishery commenced on the Clyde in the late 1930s, but before this there was a fishery for queens in the Firth of Forth, the catch being used as bait by line fishermen. Until the 1960s little exploitation of the fishery took place around the British coast. It was considered useful only as a seasonal source of income for the few boats employed and catches were usually sent live to the London market.

The practice of continuing fishing during the summer months has now greatly increased landings. Formerly fishing ceased in the spring, because this marked the time when spawning occurs and the roe empties, and because of the difficulty of keeping scallops alive and in good condition during transport, in warm weather, to the distant markets.

It is now known that the roes recover quickly after the spring spawning and rapidly become full in early summer, before again spawning in late summer. The establishment of processing plant fairly near the landing areas has helped to overcome the transport problem. Modern quick freezing and cold storage facilities also allow the end product to be distributed to a wide market both at home and abroad.

Fig. 3. Standard scallop dredge.

Fishing by dredge

Scallops and queens are usually caught by a dredge which is towed along the sea bed. A typical dredge is shown in Figure 3. It consists of a triangular iron frame, with toothed cross bar set at an acute angle to scrape the scallops and queens from the sea bottom into a bag, attached to the frame. The belly of the bag is made from steel rings, to withstand chafing, and the top part is of a sufficiently large mesh net to allow the small stones and gravel dug up with the scallops to pass through.

The dredge is thrown over the stern of the vessel, which travels at full speed until the correct amount of warp is paid out. The length of warp is approximately equal to three times the depth of water. The dredge is then towed for a period of 1-1½ hours before being hauled on board.

Handling on board

As soon as the catch comes aboard, the scallops and queens should be sorted from stones, sea urchins, starfish and other unwanted material. After sorting, the catch should be thoroughly washed and packed in sacks that have been soaked in sea water. The filled sacks are then stowed in the hold and kept cool and wet.

When stowed at chill temperatures, scallops and queens can survive out of water for several hours, and this period can be extended to a day or two if the storage sacks are kept well soaked or immersed in sea water. When catches are poor, boats sometimes remain on the grounds for up to 5 days, but the first pan of the catch is then dead on landing.

Freezing prior to processing

In times of glut, the landed catch is often frozen in the sacks prior to processing. This practice is highly unsatisfactory, because the rate of freezing attained is so slow that spoilage occurs, and the product is discoloured and tastes unpleasant.

Proper quick freezing of individual fish in shell gives an acceptable product. This is best done by laying the shells in single layers on trays and then freezing them in an air blast freezer. With an air speed of 3 m/s and an air temperature of-30°C, the average freezing time for scallops in shell is about 40 minutes and for queens in shell 25 minutes.

Thawing is usually done in still air, but where very large quantities are dealt with, water thawing has been used successfully. Water thawing also removed much of the silt and mud from the shells, before the shucking process.

Shucking of scallops

The method of removing the meat from the shell, that is shucking, depends on whether the shellfish are alive at the time of processing. A sharp thin-bladed curved knife is required to open the shell of the live scallop. The fish are held flat shell down, in the palm of the left hand with the hinge nearest the operator. The knife is inserted through a small opening between the shells, in the right hand side of the hinge, and the muscle is cut. If the knife is kept close to the flat shell during the cutting operation, a very clean cut is made, ensuring little or no waste of muscle. The fish is then turned over and the flat shell and viscera removed. This is best done by placing the left thumb on the cut muscle surface and then, by gripping the frill and gills between the right thumb and the knife held in the right hand, peeling off the viscera towards the operator, leaving the gonad and muscle attached to the convex shell. The muscle and roe are then cut free.

An alternative method of shucking the live scallops is to dip them them in boiling water for 10-15 seconds. This causes the shell to open and the contents can then easily be removed. After washing, the muscle and gonad are hand picked from the viscera. If the timing of the dipping process is not strictly controlled the surface of the muscle will become discoloured and tough. For this reason, many processors do not use this method of shucking.

Other methods of opening the shells of live scallops have been tried, but were found to be either less efficient than the two previously described, or they required such precise positioning of the shellfish in the test apparatus as to be impracticable. The experiments included the use of steam baths, steam jets played on the hinge, flame jets directed at the shell where the muscle is attached, electrical shock treatment, the action of ultrasonic vibrations, and immersion in sea water or salt solutions.

Scallops that have been frozen prior to processing will open when thawed, and dead unfrozen fish will open, and the meats can be extracted either by shaking or by cutting out with a knife. If they are only partly thawed, the muscle can be cut out whole, while still in a semi-frozen state. If pressure is applied with the thumb on the muscle of the flesh, the muscle block can be eased out of the frozen fish. This operation can be done very quickly and is particularly suitable for shucking fish that do not have large roes.

Shucking of queens

Queens can be shucked in the same way as scallops, but because of their small size a quicker method is desirable: the queens are scalded in boiling water for 40 - 50 seconds to open the shells, cooled by plunging into cold water and the meats, surrounded by the viscera, extracted either by shaking or pushing them out with the finger. It is then a simple matter to push the muscle out of the viscera.

Continuous processing lines comprising a hot water bath and a vibrating screen separator have been built.

Washing

After shucking either scallops or queens, it is essential to wash the muscle and roe thoroughly in cool running water, in order to remove all traces of sand and mud.

Soaking and dipping

The shucked meats often have a brown and slightly dried out appearance. The brown colour may be natural: for instance meats from fish more than eight years old generally tend to have a creamy or brown appearance. Discolouration can also be caused by a ruptured roe or kidney, or by spoilage. Spoilage is best controlled during processing by rapid handling at all stages and by keeping the fish chilled at all times.

At the expense of loss in flavour, some of the colour may be leached out by holding the meats in iced water overnight. Water containing a small amount of acetic, citric or ascorbic acid, 5 g acid to I kg water, is slightly more effective than water alone. Ascorbic acid is more expensive than either acetic or citric acid but is not correspondingly more effective. However, some users claim that ascorbic acid minimises rancidity changes in cold storage; this has not been confirmed.

Increase of acid concentration tends to bleach the fish but adversely affects the flavour and texture, and is not recommended. Soaking in water for 30 minutes increases the weight of the muscle by about 3 per cent, but much of this water is subsquently lost if the meats are kept at chill temperatures for a day or two.

On thawing frozen scallop meats, water is lost in the form of drip, and the weight of meat after freezing and thawing is less, by about 2 per cent, than the weight prior to freezing. When polyphosphates are added to the soaking bath, in the ratio of 100 g of a 1-to-1 mixture of sodium tripolyphosphate and sodium ‘glassy’ phosphate to 1 kg of water, the uptake of water by the meats is about 3-4 per cent after 30 minutes soaking.

Soaking of the meats for prolonged periods is not recommended, and may lead to problems with weights and measures regulations.

Chill storage

Scallops in shell slowly lose their flavour after 4-7 days in ice and develop off flavours after 9 days. Shucked scallops lose their flavour after 2 days in ice and become tasteless after 7 days but, unlike unshucked scallops, their storage life is more than 9 days in ice. Queens behave in a similar manner.

Freezing and cold storage

Scallops and queens should be quick frozen immediately after shucking and washing. This is best done in an air blast freezer, the meats being laid in single layers, on shallow trays. With an air speed of 3 m/s and an air temperature of -30°C, the average freezing time for scallop meats, 40 mm diameter and 23 mm thick, is about 20 minutes; queen meats, 20 mm diameter and 23 mm thick, take about 15 minutes.

After quick freezing, the meats may be glazed by dipping in clean fresh water, or packed in sealed bags, and then stored at -30°C. They will then keep in good condition for up to 6 months. After this period, loss of flavour in the roe becomes noticeable and some browning occurs in the muscle. Poor cold storage results in dehydration of the meats, causing flavour losses in the muscle and colour changes in the roe. Spoilage changes and dehydration in cold store can be further reduced by vacuum packaging.

Thawing

On thawing the frozen meats, the glaze and much of the water absorbed during soaking will drip away. Table 1 shows the percentage increase in weight of individually frozen scallops after various dipping treatments in water and polyphosphate solutions, and the increase in weight remaining after thawing.

TABLE 1
Weight increase after various dipping treatments

Dip Treatment

Dipping Time

Weight increase after dipping %

Weight increase after freezing and glazing %

Remaining weight gain after thawing %

Water...................................

30 minutes

3·0

7·3

2·0

Polyphosphate solution.........

1 minute

1·0

6·5

1·5

Polyphosphate solution.........

2 minutes

0·4

7·7

1·5

Polyphosphate solution.........

10 minutes

1·5

8·2

3·2

Polyphosphate solution.........

30 minutes

2·0

6·6

3·4

Yield of meat

The yield of edible flesh in scallops varies from 10 to 16 per cent of the weight in shell. Maximum yields are obtained just before spawning in the spring and late summer. The weight of the edible portion depends not only on the development of the roe, but also on the size of the shellfish and the location of the fishery. Thus the edible portion of large mature scallops may weigh up to 50 g whereas small fish, not necessarily young ones, that have just spawned may weigh as little as 23 g.

The weight of edible flesh from queens is variable, ranging from 4-5 g to 14 g per fish. The yield of edible flesh ranges from 12 to 15 per cent of the weight in shell.

Composition of meat

The approximate composition of fresh raw scallop meat is 73-79 per cent water, 19-5 per cent protein, 0-5-1-0 per cent fat. These values are for the total content of the shell, including gonad, gut and liver.

Waste disposal

There is a relatively small demand for scallop shells for catering and ornamental purposes, but the bulk of this waste material has to be dumped. The practice of dumping large quantities of shells in areas near the shore cannot be recommended, and can lead to an odour problem. Probably the most satisfactory solution at present is to ship the shells back to sea on outward voyages to the fishing grounds, and dump them in fairly deep waters.


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