Table of Contents

Causes of dark colour
Can dark colour be avoided?
Does colour matter?


People are accustomed to foods having their own characteristic colours, and sometimes find an unconventional colour unattractive or even repulsive; roast meat is brown, eggs are yellow and salmon is pink. The flesh of white fish such as cod and haddock is expected to be white, and even when it is only slightly darkened or coloured it may be rejected.

Processors know that fillets of white fish can sometimes be dark coloured and therefore less suitable for use in certain products. This note explains some of the causes of dark flesh, tells the catcher and processor what to look out for, and points out that where dark colour is unavoidable, there is nevertheless the possibility of changing consumer attitudes provided quality is consistently good.

Causes of dark colour

Intrinsic dark colour

The entire flesh of some species of white fish is intrinsically darker than others; saithe in particular is noticeably darker than cod or haddock, and some farmed flatfish have slightly darker coloration of the flesh than their counterparts in the wild state. Some kinds of deboned material recovered from fresh white fish in the form of a mince also are dark in colour.


When newly caught white fish are gutted and stowed in ice they remain good to eat for several days, but eventually they acquire an undesirable taste and smell, and in advanced spoilage a red colour develops in the flesh along the backbone. This form of discoloration is not discussed further in this note, since it is a well known feature of spoilage, and the fish are at this stage generally unsuitable for food.

Insufficient bleeding

When white fish are gutted at sea, washed and iced, they have white flesh on landing because they have had time to bleed. If they are filleted immediately after capture, however, the flesh oozes with blood and is quite red; if fillets from such unbled fish are frozen and thawed, they will be brown. When newly gutted whole fish are frozen, the blood similarly does not have time to drain completely, with the result that the thawed fish yields fillets that are noticeably brown in colour. Brown discoloration of the flesh from inadequately bled fish is thus not necessarily a sign of poor quality; provided the fish have been properly cold stored, their flavour should be excellent. When a fish is left ungutted, however, for some hours in the holding pounds of a freezer trawler, it is impossible to bleed it, because the blood clots in the blood vessels and will not run out after gutting; again the flesh after freezing and thawing can acquire a brown colour, and in this instance the fish might be of inferior quality.

Thus residual blood in fish flesh can result in darkening and browning, but this is not necessarily an indication of inferior eating quality.


If whole fish soon after capture are knocked against a hard surface, bruising of the flesh can occur in the form of a dark coloured patch in the fillet; this is caused by rupture of fine blood vessels in the flesh with consequent release of blood which does not drain away during gutting and icing.


The flesh of white fish can become dark as the result of either heat processing, for example canning, or storage in the frozen state under poor conditions. Neither instance comes within the scope of this note.

Brown muscle

Along each side of many species of fish, just under the skin, is a strip of dark brown muscle, figure 1, the colour of which remains unaffected by washing or draining in ice. It contains more fat than the white muscle and, because it is richer in certain chemical substances, is more tasty. When the fish is cold stored, the brown muscle goes rancid or acquires a cold store flavour before the white muscle and, if the flavour permeates the whole fish, it is strongest in the brown muscle. Variation in the amount of colour of brown muscle affects the appearance of the skinned flesh, and hence its value.

FIGURE 1. Cod with skin removed to show dark muscle.

The amount of dark muscle varies between species, depending on genetic factors which determine their habitual activity. Flatfish are not strong swimmers and do not regularly travel over long distances; they have little brown muscle and invariably yield a white fillet when properly bled. Cod, haddock and whiting are sluggish and have some brown muscle, but saithe, probably the most active fish of the cod family, has considerably more. Fish that never stop swimming, herring and mackerel for example, are richest in brown muscle, to the extent that there is no really white flesh at all. The depth of brown muscle in several species of fish is shown in figure 2 as ranging from almost nothing to about a third of the fish.

In a fish like cod the thickness of the brown strip does not vary much from head to tail but, because the whole fish tapers towards the tail, the brown muscle forms an increasing proportion of the total from head to tail; the front half of the fillet therefore contains the most white muscle.

The proportion of brown muscle varies little within one species, but the pigmentation can change; this is why some batches of cod, for example, may be judged to be too dark. There are two reasons for this. First, some stocks of a species swim more than others; cod at Bear Island and Spitzbergen, for example, which swim to the Norwegian coast to spawn and back again, have much darker brown muscle than North Sea, Faroe or Iceland cod. Secondly, there is a pronounced seasonal variation in the dark colour of brown muscle; in North Sea cod, for example, it is darkest in July and August, possible because the fish are then most active in their search for food.

FIGURE 2. Section through the body of several species showing the depth of dark muscle: A herring; B mackerel; C haddock; D cod; E whiting.

Can dark colour be avoided?

Intrinsically dark flesh cannot be made white, but good handling and stowage practice helps to ensure that the degree of coloration is kept to a minimum. The colour of saithe flesh, for example, although never white, can be controlled or modified to some extent; advice on handling saithe is given in Advisory Note 47. Minces of fish flesh that are too dark can sometimes be whitened by washing, by better control of the raw material, or by masking the colour; experimental work on control of colour is continuing in this growing area offish processing. Red or brown discoloration resulting from spoilage can be avoided simply by excluding stale fish from the processing line. Careful handling can help to reduce the incidence of bruising.

Brown discoloration of thawed frozen flesh results principally from inadequate bleeding of the whole fish before freezing, and occurs almost entirely in sea-frozen whole fish and fillets; it can be avoided by ensuring that the fish are gutted soon after capture, that they arc given plenty of time to bleed after gutting, and that they are kept chilled to prevent the blood clotting before it drains from the system. Excessive amounts of brown muscle in cod can be avoided to some extent by excluding fish from Arctic grounds, but since cod landings in the UK from Bear Island, Barents Sea and the Norwegian coast are likely to decline, the problem of brownbacks or redbacks, as they are sometimes called, may in any event diminish. Where brown muscle is totally unacceptable in a particular product, it can be trimmed from the fillet with a knife, or a skinning machine can be set to take off a thicker slice and so remove brown muscle from the fillet with the skin.

Does colour matter?

Colour is important to the processor who wants to present the consumer with a product of standard appearance. White has been, and still is to a large extent, the accepted colour for the flesh of white fish but, provided the product is not stale or spoiled in any way, there is no reason why darker flesh should not become acceptable; consumer resistance to change can be overcome if a colour is consistently linked with good quality, as happened for example with scampi meats when they changed from pink to white as a result of improved quality. Dark coloured flesh has been used successfully for example in some frozen coated products such as fish fingers, where the batter gives improved protection against rancidity in cold storage.

If you have any queries, write, phone or call at either of the addresses given below:

The Director

The Officer in Charge

Torry Research Station

Humber Laboratory

PO Box 31

Wassand Street

135 Abbey Road





Tel: 0224 877071

Tel: 0482 27879

Other recent Notes in this series, which are available free of charge in the UK from the above addresses are:

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.

Earlier notes in the series, most of which are still available, are summarized in 60. Key to Advisory Notes 1-59, by J. J. WATERMAN.

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