Scientific names
Common names
Distinguishing features
Geographical distribution
Length and weight
Composition
Handling and storing wet fish
Grading
Hand and machine processing
Skinning
Freezing and cold storage
Smoking
Canning
Other products
The scientific name for scad is Trachurus trachurus L. It is the most common species of the trachurid family found in British waters. Two other species from the Mediterranean Sea, Trachurus mediterraneus and Trachurus picturatus may be confused with scad and are occasionally found in the Bay of Biscay and Southern Celtic Sea. Both species grow to greater lengths (65 cm) than the scad of the North East Atlantic (maximum length 45 cm).
The name scad must be applied when the species is sold at the retail level in the UK, and is used in this Note. The alternative name horse mackerel is commonly used for the species when caught but can be misleading, as the Atlantic mackerel, and mackerel-like fishes generally, belong to a separate group, the scombrid family.
Some of the names used in other countries are given below. The maasbanker of South Africa is sometimes regarded as a separate species, Trachurus capensis, or as a sub-species.
|
Denmark |
Hestemakrel |
|
Egypt |
Seif |
|
France |
Saurel |
|
|
Chinchard |
|
Germany |
Stöcker |
|
|
Bastardmakrele |
|
Holland |
Horsmakreel |
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Italy |
Suro |
|
Morocco |
Chrène |
|
North America |
Jack mackerel |
|
Norway |
Taggmakrell |
|
Portugal |
Carapau |
|
Republic of South Africa |
Maasbanker |
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Russia |
Stavrida |
|
Spain |
Jurel |
|
Tunisia |
Chourou |
|
Turkey |
Istavrit |
|
Yugoslavia |
Sarun |
The head of the scad is large and is approximately one quarter of the total length. A distinguishing feature is the row of raised hooked scales, called scutes, along the lateral line, which slopes down noticeably above the anal opening, behind which is a pair of sharp spines. The scutes become more prominent, hard and spiny towards the tail. When freshly caught the upper part of the body and head are dark grey-blue with greenish tints; the lower part of the body and head are silvery white with a metallic violet gleam; the belly is white. A dark spot on the edge of the gill cover is another distinctive marking.
Scad are found in waters from around the Cape Verde Islands to the Straits of Gibraltar, extending into the Mediterranean and Black Seas and northwards along the Atlantic and North Sea coasts as far as Icelandic and Norwegian waters. There appear to be two distinct spawning populations in the North East Atlantic. One to the west of the British Isles follows the continental shelf edge as far as the north-west of Ireland; the other lies mainly in the central and southern North Sea and English Channel. Spawning takes place earlier in the southern part of each particular area. Around the British Isles the main spawning period is June and July. Scad are at their most difficult to catch during the spawning season as they tend either to be scattered or in small groups. Migrating scad, mainly larger fish, travel northwards during summer and may reach areas around the Shetland Islands, Norwegian coast and northern North Sea by September. The fish then return to their over-wintering areas along the continental shelf edge from north-west Ireland to the Bay of Biscay, the Celtic Sea area and the English Channel. The over-wintering shoals tend to congregate during November and begin to disperse again in March. Further details of the life history and fisheries can be found in Laboratory Leaflet No 56 'Scad in the North-east Atlantic' , 1983 (CEFAS Lowestoft Laboratory, Suffolk, NR33 OHT).
Relationship between length and weight
The diagram shows the relationship between total length and weight, the total length being measured from the tip of the snout to the tip of the closed tail fork. Most of the fish measured were from the English Channel and off the south-west coast of Ireland. The initial rate of growth in length is fairly rapid up to the age of sexual maturity (3-4 years) but thereafter slows markedly, while the weight continues to increase. Scad is a long-lived species, individuals probably attaining the age of 15 years.
In scad, as in other pelagic species such as herring and mackerel, the fat content is different at different times of the year, as seen in Table 1. The fat content appears to reach a maximum during October and November after the feeding period, and declines to a minimum during the spawning season. The fat content can vary greatly between individual fish caught in the same haul, and this could cause problems for processors requiring fish with a uniform fat level. As the fat content increases the moisture content decreases, the total of the two being approximately 80-81%. The protein content of scad flesh is 17-19%.
TABLE 1 FAT CONTENT OF SCAD FLESH THROUGHOUT THE YEAR
|
|
Mean fat contents of samples caught over a period of
years |
Average |
Range of individual fish |
|
January |
6.7,7.2, 9.1, 9.2 |
8.1 |
2.3-13.7 |
|
February |
6.3 |
6.3 |
2.9-10.5 |
|
March |
4.8, 6.1, 7.1 |
6.0 |
3.6-9.4 |
|
April |
4.3, 5.6, 11.3* |
7.1 |
2.4-12.0 |
|
May |
6.9, 11.3* |
9.1 |
2.1-13.5 |
|
June |
4.6, 5.4, 6.1, 8.0*, 9.3* |
6.7 |
2.2-13.5 |
|
July |
11.9 |
11.9 |
5.7-20.2 |
|
August |
7.6* |
7.6 |
4.8-10.3 |
|
September |
6.7* |
6.7 |
6.4-7.3 |
|
October |
7.0, 11.1, 11.4*, 11.9, 15.2* |
11.3 |
5.5-16.2 |
|
November |
11.6*, 15.7* |
13.7* |
9.4-16.7 |
|
Nov/Dec |
11.6 |
11.6 |
6.6-20.4 |
|
December |
6.3,7.9 |
7.1 |
2.5-14.8 |
* Data from Fisheries Laboratory, Lowestoft: other data from Torry Research Station.
Special care must be taken in manual handling of scad to avoid injury from the scutes and the sharp spines. Gloves do not afford complete protection. The scutes and spines tend to snag on one another and on other surfaces, interfering with smooth flow along conveyors, on the operating channels of machines, and on ramps. Fluming with water can help to keep the fish moving. Scad can be pumped out of the net or out of the hold.
Scad, like herring and mackerel, are usually stored on board ship as whole, ungutted fish. If they are kept at an ambient temperature of 10 to 15°C the fish become unacceptable to consumers after 12 to 24 hours. At these temperatures the flesh softens and filleting yields diminish; autolysis, or self-digestion, can lead to burst bellies after about 30 hours, later than in herring and mackerel. Fresh fish should therefore be chilled immediately after catching by storage in ice, or in chilled or refrigerated sea water. Taste panel assessment shows that whole fish held in ice or chilled sea water maintain good quality for 6 to 10 days, considerably longer than expected for herring or mackerel. Seasonal differences do not seem to affect spoilage rates.
Chemical tests are often used as additional criteria of the degree of spoilage of fish. Of the tests commonly applied, only measurement of hypoxanthine concentration is useful for scad stored at chill temperatures. For whole fish stored in ice or chilled sea water, an average value of 0.25 mmole of hypoxanthine per gram of fish is reached after about 6 days.
Grading trials have been carried out on a tapered roller grader. Although the grade overlap was greater than that found for other species, grading could sort the fish for machine processing and reduce the amount of unwanted species, but did not achieve accurate weight grading.
Scad can be prepared in fillet or other form either by hand, taking care to avoid injury by scutes, or by a suitable machine. Table 2 gives yields obtained for both hand and machine processed whole ungutted fish.
TABLE 2 YIELD FROM HAND AND MACHINE PROCESSED WHOLE FISH
|
Form
|
Yield from whole fish |
|
|
by hand (25-35 cm fish) |
by machine (20-28 cm fish) |
|
|
Single fillets, untrimmed, skin on |
40-45 |
- |
|
Single fillets, trimmed, skin on |
34-38 |
- |
|
Block, butterfly or cutlet fillets |
36-38 |
40-42 |
|
Nobbed (headed and gutted) |
66-68 |
66-70 |
|
Split as for kippering (heads on) |
80-83 |
- |
TABLE 3 YIELD OF MINCE
|
Form |
Yield of mince from whole fish % |
|
Single fillets, untrimmed, skin on |
32-36 |
|
Single fillets, trimmed, skin on |
26-30 |
|
Nobbed |
40-48 |
|
Kipper split (heads on) |
49-54 |
The lateral scutes, described earlier, are attached very securely to the fish muscle, particularly near the tail. They are not amenable to traditional methods of descaling, but should be removed at some stage in the preparation of a number of possible products including canned and marinated ones. Small single fillets, less than 15 cm long and suitable for canning, can be skinned reasonably well by machine, but the scutes interfere with conventional machine skinning of larger fish. Alkali treatment of nobbed fish, as in the lye-peel process, has been found to be successful in removing skin and scutes prior to canning. Short exposure to steam or hot water under the right conditions also weakens the connective tissue securing the scutes so that both skin and scutes can be removed quite easily without affecting the properties of the fish for further processing. This latter approach is relatively simple and could be developed for commercial application.
Preservation of fresh scad by freezing is best carried out using a vertical plate freezer to freeze the whole fish in blocks (25 or 50 kg) in plastic sacks topped up with water prior to freezing. After freezing, the blocks can be packed inside cartons. Storage in this form at - 30°C provides at least one year's shelf life, since dehydration, oxidative changes and the ensuing development of rancid flavours are reduced. The blocks are also well protected from mechanical damage. Plastic laminate bags with a discontinuous, rough, outer layer can be used: this layer increases the friction between blocks and helps to prevent stacks of blocks slipping due to the movement of the ship or when stacked on a pallet. As in chill storage, seasonal differences do not appear to affect the storage life at -30°C.
Scad fillets can be frozen in blocks in a horizontal plate freezer. They should be placed in layers, with interleaving, in moulds or trays with the cut surfaces together and the skins to the outside of the block. To reduce oxidation, the blocks should be glazed and packed in high density polyethylene bags. Alternatively, as the scutes may pierce the bags, blocks may be stored in well-fitting waxed cartons. Scad fillets may also be individually frozen, glazed and packed. Vacuum packing of single fillets will help to extend the frozen storage life but in this case skinned fillets should be used.
Minced fish should be frozen in blocks, protected by waxed cartons in typical commercial fashion; it should then have a shelf life at - 30°C of up to 9 months. Care must be taken during the packing of the mince to ensure that there are no air pockets trapped in the block and that all round contact between mince and carton is achieved during and after freezing in order to minimise dehydration and oxidation.
Scad fillets are on the whole smaller and less oily than say mackerel or herring fillets from fish of a similar size, so the brine strength and brining time before smoking need careful control. The 80° brine normally used is too concentrated and gives a rather salty product even for short brining times. It is therefore recommended that a 60° brine (188 g salt added to one litre of water) be used for scad. Table 4 gives a guide to brining times in 60° brine to give acceptable salt contents in cold and hot smoked products.
TABLE 4 BRINING TIMES FOR VARIOUS COLD AND HOT SMOKED PRODUCTS
|
Product
|
Brining time (minutes) |
|
|
Cold smoked |
Hot smoked |
|
|
Block fillets (cutlets) |
3 |
3 |
|
Unskinned fillets |
3 |
3 |
|
Skinned fillets |
1½-2 |
1½-2 |
|
Kipper split |
5-6 |
- |
|
Nobbed |
- |
10 |
Some products prepared from scad
A very acceptable product is made by canning smoked scad fillets. Block fillets (unskinned) or single fillets (skinned) are immersed in a 60° brine for 2 minutes and allowed to drain for 1 hour. They are smoked for 1½ hours at 27°C, followed by 1 hour at 50°C; this procedure restricts further darkening of the fillets during sterilisation in the can. After cooling, skinned single fillets can be cut from the block fillets. The smoked fillets are packed into a suitable can, such as a 200 g flat ring-pull can, steamed for 3 minutes at 100°C and drained; sufficient hot vegetable oil (maize oil) is added to cover the fillets, and the cans are sealed and heat processed at 115°C for 55 minutes.
Unsmoked fillets can be canned in a tomato sauce in a similar manner. The skinned single fillets are brined as before and allowed to drain. After packing, the open cans are precooked in a steam cabinet at 100°C for 10 minutes, drained, filled with hot tomato sauce to cover the fillets, sealed and heat processed for 1 hour at 115°C. Unsmoked scad fillets canned in vegetable oil were not considered acceptable by taste panel assessment.
Breaded and flash fried fingers can be prepared from minced scad. When stored at -30°C in waxed cartons, they have a shelf life of around 4 to 5 months. Double enrobing of the fingers followed by flash frying extends this to 8 to 9 months. Development of rancidity in frozen mince blocks and fish fingers, as in other fatty fish products, is easily controlled by vacuum packaging in a material having good oxygen barrier properties. Unfortunately, closely packed flash fried scad fingers tend to bind together under vacuum, making them dificult to separate. Use of interleaving or, preferably, sectioned tray inserts overcomes this problem.
Bone-free scad mince, spiced and flavoured, can be combined with rice or potato and coated with batter and crumb to provide a range of attractive frozen products. Various other products such as pate, and both sweet and sour marinades can be prepared from scad.
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Further information Further information can be obtained by contacting: The Director Tel. Aberdeen (0224) 877071 This Note is one of a new series, prepared by staff at Torry Research Station; the principal author this Note was J G M Smith. New Notes are: 91 Sensory assessment of fish quality Most Notes in the original series (numbered 1 to 90) are still in print, but are being replaced: a list is available, free of charge. Copies of all Notes are obtainable, for a small handling charge, from the above address. BL. 5811 |
