This note describes the species of oyster marketed in the UK, briefly describes cultivation, harvesting and cleansing, and then discusses in more detail the handling and distribution of live oysters, the freezing and cold storage of whole oysters and oyster meats, and the preparation of oyster products. Some information is also given on grading, yield and composition, and on utilization of oyster shells.
Three species of oyster are met with in the UK trade. The native oyster, Ostrea edulis, and the Pacific oyster, Crassostrea gigas, are cultivated in the UK. The Portuguese oyster, Crassostrea angulata, is no longer grown here, imports of seed having ceased in 1970. All three species may be offered for retail sale simply as oyster, but use of the name native, Pacific or Portuguese oyster is restricted to the appropriate species. The native oyster is sometimes called the flat oyster, and the Pacific oyster is sometimes called the Japanese oyster, but these names are not permitted in UK retail trade.
The native oyster is roughly fan shaped, almost circular, with one half of the shell almost flat and the other half cupped. The shell of the Pacific oyster is more deeply cupped, rougher and more elongated than that of the native, while the shell of the Portuguese oyster usually has even sharper ridges, but Pacific and Portuguese oysters are not always readily distinguishable from each other. All show a wide variation in shape, colour and texture according to their origin.
Native oysters are marketed when they reach a size of 62-64 mm, measured across the shell, depending on local bylaws, and Pacific oysters are marketed from 60 mm upwards. All species can grow to a size of about 100 mm.
The name oyster is written in other languages as follows:
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Danish |
østers |
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Dutch |
oester |
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French |
huitre |
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German |
Auster |
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Greek |
ostrea |
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Icelandic |
ostra |
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Italian |
ostrica |
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Japanese |
kaki |
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Norwegian |
østers |
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Portuguese |
ostra |
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Spanish |
ostra, ostion |
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Swedish |
ostron |
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Turkish |
istiridye |
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Yugoslavian |
kamenica |
native oyster (Ostrea edulis)
Pacific oyster (Crassostrea gigas)
Portuguese oyster (Crassostrea angulata)
Native oysters have been fished around the coasts of the UK at least since Roman times, and were for long an important part of the nation's diet. Overfishing, pollution, pest infestation, unfavourable summers and severe winters have all taken their toll over the years, and stocks are only now beginning to recover. The three most important fishing areas in the UK at the present time (1980) are on the south east coast between the river Blythe in Suffolk and Whitstable in Kent, on the south coast between Chichester and Poole, and on the south west coast between the rivers Dart and Helford. A Welsh fishery that previously flourished around Swansea and Milford Haven is no longer active, although there are some recent signs of a revival, and previously active Scottish fisheries, in the Firth of Forth, around Shetland, and on the west coast, with the exception of Loch Ryan, are no longer important commercially. Attempts are being made at the present time, with some success, to grow and harvest Pacific oysters in many coastal areas around the UK, and small numbers of natives are harvested in a few other places, for example in the Foyle estuary in Northern Ireland.
Stocks of the native oyster are replenished by natural reproduction, by the relaying of natural stock, and by the planting of artificially reared seed oysters. The free-swimming larvae which are produced by mature oysters eventually settle on the bottom of the sea, a stage known as the spatfall. Young oysters may subsequently be lifted from a breeding ground and relaid in a more suitable growing or fattening area. Scotland is near the northern limit for this species, and successful settlement is uncommon. The sale of native oysters is prohibited between 14 May and 4 August, the main period of reproduction.
Pacific and Portuguese oysters do not normally breed naturally in UK waters because the temperature is too low; they are cultivated by transplanting seed from hatcheries. Both species can be harvested throughout the year since there is no close season.
Native oysters take up to 4 years to grow to a marketable size; Pacific and Portuguese oysters grow much faster, and can reach marketable size in one or two growing seasons, depending on conditions. Oysters can be cultivated in three ways, growing directly on the sea bed, on trays raised off the bottom, and on trays or ropes suspended from rafts down to depths of 10 m. Oysters growing naturally on the sea bed are harvested by dredging; dredges range in size from small ones operated by hand from rowing boats to ones with a mouth opening of about 2 m towed by powered vessels.
In England and Wales, some areas of tidal water constitute a public fishery for oysters, some are in private ownership, and some are under direct or delegated government control.
Some areas are still held privately under ancient charter from the Crown, the current owner having the sole right to use, let or sell the fishery as he pleases. In other areas the oyster fishery is controlled by a responsible body, for example a harbour board or local authority, using powers delegated to it by the Ministry of Agriculture, Fisheries and Food under a Regulating Order; the controlling body issues fishing licences to the public, and requires licensees to observe any bylaws it makes. The Ministry may also grant directly, by means of a Several Order, the sole rights to an oyster fishery for a fixed number of years, provided that the individual, company or other body given those rights subsequently shows that the area is being fully and beneficially cultivated. Waters that remain public can be fished for oysters by any individual, provided there is no contravention of local bylaws made by the appropriate Sea Fisheries Committee. Similar arrangements exist in Northern Ireland. In Scotland the sea bed in tidal waters is the property of the Crown, and leases for the purpose of oyster growing are usually made for periods of five years.
Most oysters produced in the UK are distributed live, and are frequently eaten raw or sometimes lightly cooked. Inshore waters are subject to pollution by sewage to varying degrees, and some microscopic algae on which the mussels feed may produce toxins. Consequently, oysters are regarded as a potential health hazard. Consumption of oysters contaminated with pathogenic bacteria, viruses or algal toxins is a significant cause of food poisoning. Therefore, harvesting, cleansing, handling and heat treatment are covered by specific regulations in the food safety legislation.
Live oysters are usually graded into four sizes for sale in the UK, grade 1 being the largest; size is determined by measuring the length of the longest axis of the shell. Grading is usually by weight for sale in Continental markets, the weight range in each of five grades being fixed annually by the French; weight limits are generally set higher following a good growing season than after a poor one, but grade 1 oysters are typically about 100 g, and grade 5 are 35-40 g.
Length and weight of the whole oyster are not necessarily directly related to quality as determined by the amount and appearance of the meat; variation with season and from ground to ground can sometimes result in a large shell containing a small amount of meat of poor appearance or a small shell comparatively well filled with firm meat. Selling price is usually determined within a species by assessment of size, yield and quality. The native oyster is more highly prized than other species and in consequence generally commands a higher price.
Oysters can survive out of water for weeks if carefully handled and kept moist and cool but, since they rapidly lose liquor from within, particularly if the edge of the shell is damaged, they should reach the inland wholesaler within 3 days of harvesting to be in prime condition. They should be carried and stored with the cupped half of the shell downwards, and kept moist by covering with a damp cloth. They should be packed in a manner that protects them from mechanical damage, and should be kept at all times at a temperature between 1 and 10°C. Commercial packaging ranges from a simple barrel, box or sack to a specially designed container with separate compartments for individual oysters, the degree of sophistication depending on the value of the product, the journey time, and the market for which it is destined.
Small holding tanks using artificial sea water can be used to keep live oysters at a wholesaler's premises for a few days. Live oysters stored in a shop or restaurant should be inspected daily, and any that are dead or moribund should be removed. A healthy live oyster feels heavy, sounds solid when tapped, and either remains tightly closed or closes quickly when it is handled. Supplies should be regulated so that oysters are not kept on the premises for more than 3 days.
Shucking, that is removal of the meat from the shell or detachment of the meat so that it can be readily removed by the consumer, is usually done by hand in the UK. A righthanded person holds the oyster in a cloth in the left hand, with the flat side uppermost and the hinge towards him. A shortbladed oyster knife is inserted between the two halves of the shell, close to the hinge, and twisted to break the hinge and lever the oyster open. The knife is then used to sever the adductor muscle at its point of attachment to first the flat half of the shell, then the cupped half, and the meat removed. If the meat is to be served on the half shell, the detached meat is turned over, cleared of any shell fragments, and left in the cupped half along with the natural liquor.
Many devices have been tried to reduce the labour of hand shucking. These include a means of holding the shell while it is prised open by hand, the shearing off of the hinge by guillotine, and a wide range of treatments that cause the shell to spring open, including the use of chemicals, heat, cold, vacuum, microwaves and lasers. Plants have been built elsewhere that can shuck automatically several tonnes of oysters a day, but on the more modest scale of the UK industry the use of freezing appears to be the most practicable treatment. Microwave treatment is difficult to control; a live oyster exposed to microwaves in an oven will usually open in 30-40 seconds, but it is not always possible to avoid the meat being cooked.
There is at present no commercial traffic of chilled meats in the UK. In the USA newly shucked meats are washed, graded and packed in cans fitted with snap-on lids; when the cans are chilled by surrounding them with crushed ice, the meats are claimed to keep in good condition for up to 12 days during inland transit and storage.
Oyster shells vary considerably in thickness according to origin; consequently the percentage yield by weight of meats is also very variable. The yield of meats from native oysters typically ranges from 6 to 18 per cent, and from Pacific oysters 5 to 14 per cent.
The composition of the meat of the native oyster is 77-83 per cent water, 1-3 per cent fat and 9-13 per cent protein. The energy value of the meat is 3-4 kJ/g.
Oyster meats generally contain 1-5 per cent of the carbohydrate glycogen, but at times the proportion can be as high as 10 per cent. The amount of glycogen is usually least during the breeding season.
In shell
Only live oysters should be selected for freezing. Whole oysters can be frozen satisfactorily spread in single layers in an air blast freezer. Freezing causes the shells to open, and makes subsequent shucking easier.
Oysters can also be frozen in the half shell; they should be laid in single layers on trays in an air blast freezer, with plastics film stretched over each tray to protect the open surfaces of the oysters.
Oysters should never be put into a cold store to freeze. Cold stores are designed only to hold already frozen products at the required low temperature; they do not have the refrigeration capacity to handle large amounts of unfrozen material.
Frozen whole oysters packed in polyethylene bags can be kept in good condition for 6 months in a cold store at -30°C. The liquor within the shell acts as a glaze to protect the meat from dehydration. The meats of whole oysters held at -15°C begin to develop slightly rancid off flavours and start to acquire a greenish yellow discoloration after about 3 months. Meats of frozen whole oysters are admirably suited for use in cooked dishes.
Freezing oyster meats
Unless there is a market for frozen whole oysters, or freezing is to be used as means of opening the shell, it is more economical to freeze the shucked meats. Misshapen whole oysters that are unacceptable for the live trade can also be utilized in this way. Meats frozen either individually or in blocks can yield an excellent thawed product.
The freezing of oyster meats is a well established process in the United States and Japan, but as yet has found no favour in the UK. The shucked meats are thoroughly washed to remove sand, grit and other shell debris, inspected to remove any ragged or broken meats, drained to remove excess water, and frozen individually in an air blast freezer before packing. Good packaging is essential to prevent rancidity and dehydration in cold store. Typically the frozen meats are glazed and packed under vacuum in bags made of a flexible film that has a high resistance to the passage of water vapour and oxygen. An alternative procedure is to pack the meats under vacuum in a rigid container before freezing them; containers of metal, plastics or waxed cardboard with inner liners have been used, sometimes with shaped pockets for individual meats. The strained shell liquor is also sometimes added to the packed meats before freezing them; this has the added advantage that any remaining air spaces in the pack are filled.
Frozen oyster meats in suitable packaging will keep for 6 months at -30°C. Storage temperatures above - 12°C are unsatisfactory.
Experimental cultivation of Pacific oysters in the UK may result in large numbers becoming available commercially for potential use in a wide range of frozen convenience foods. The White Fish Authority's Market Development Unit has developed a range of possible products, including breaded meals, meats in flavoured batter, boil-in-the-bag dishes such as oyster mornay and sweet-and-sour oysters, oysters in the half shell, au naturel or with added sauce, and oyster flans. Several of these products have been test marketed, and detailed recipes can be obtained from the White Fish Authority (now Sea Fish Industry Authority).
Smoked oysters
Whole oysters are steamed for 20-30 minutes, after which the meats are removed, cooled and washed. The meats are immersed in 50° brine for 4 - 5 minutes, drained, brushed with or dipped in edible oil, spread in a single layer on plastics-coated wire mesh trays, drained again and loaded into a mechanical smoking kiln. They are held in a dense smoke at 80°C for 30-45 minutes depending on size, being turned over once after 15 minutes to ensure uniform smoking. The smoked meats can be distributed chilled or frozen, but are more usually packed in small glass jars or cans, covered with edible oil and heat sterilized at 121 °C; a 200 g jar requires about 15 minutes. Glass jars should be processed under water to prevent breakage.
Canned oysters
Oyster meats are canned in the USA and Japan. The whole oysters are steamed open, meats are removed and washed to remove grit and other debris. After draining, inspection and grading they are packed in cans with 1 - 2 per cent by weight of hot brine, and the cans sealed. The cans are heat processed for 20-42 minutes, depending on size, at 116°C.
Oyster shell contains a high proportion of calcium carbonate, and crushed shells are used as a supplement to poultry food. Finely ground shell is also used as agricultural lime and as a constituent of concrete blocks and other building materials. Oyster shells also make an excellent culch, the bed of material laid by planters in oyster rearing areas on which oyster spat can settle.
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If you have any queries, write, phone or call at either of the addresses below:
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The Director |
The Officer in Charge |
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Torry Research Station |
Humber Laboratory |
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PO Box 31 |
Wassand Street |
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135 Abbey Road |
Hull |
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Aberdeen |
HU3 4AR |
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AB9 8DG |
Tel: 0482 27879 |
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Tel: 0224 877071 |
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61 Gaping of fillets, by R. M. LOVE.
62 The freezing time of fish, by F. J. NICHOLSON.
63 Fishing ports in the UK by J. J. WATERMAN.
64 Fish silage, by I. TATTERSON and M. L. WINDSOR.
65 Fishworking machinery, by S. MAIR.
66 Handling and processing mackerel, by J. N. KEAY.
67 The haddock, by J. J. WATERMAN.
68 Icemaking plant, by J. GRAHAM.
69 Cook-freeze fish products, by J. N. KEAY.
70 Advice for the fish industry; who does what, by J. J. WATERMAN.
71 Processing cod; the influence of season and fishing ground, by R. M. LOVE.
72 Reducing odour in fish meal production.
73 Stowage of fish in chilled sea water, by J. H. KELMAN.
74 Handling and processing rainbow trout, by A. MILLS.
75 Freezing small pelagic fish, by I. MCDONALD.
76 Dark colour in white fish flesh, by R. M. LOVE.
77 Squid, by G. D. STROUD.
78 Health hazards of handling industrial fish, by A. WARD.
79 Minced fish, by J. N. KEAY.
80 Round worms in fish, by R. WOOTTEN and D. C. CANN.
81 Handling and processing blue whiling.
82 Hot smoking of fish, by A. MCK. BANNERMAN.
83 Fish smoking: a dictionary, by J. J. WATERMAN.
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