Introduction
What is meant by gaping?
What holds the fillet together?
When does gaping occur?
Causes of gaping
Do freezing and cold storage affect gaping?
How to reduce gaping
This note explains what gaping is, when and how it occurs, and what can be done to reduce its occurrence; the effects of size and condition of the fish, the manner of handling, and the time and temperature of stowage are discussed. The advice on how to reduce gaping applies particularly to handling on board freezer trawlers.
When fish are filleted, the cut surface is normally smooth and glossy. Sometimes, however, the flakes separate from one another so that slits or holes appear in the fillet and, in bad cases, it may even drop to pieces when it is skinned; fillets like this are said to gape. Badly gaping fillets cannot be sold from the fishmonger's slab or used in products where appearance is important; some are used for fish fingers but, when they have to be used in cheaper products like fish cakes or fish meal, they represent a loss to the industry.
There are about fifty flakes in a cod fillet, and each flake is separated from the next by a thin membrane, which can usually be seen as a bluish shiny surface where the flakes have parted. The flesh is joined to this membrane by little threads of connective tissue which run through the flakes and merge with the membrane at the join; when the threads break, the fillet gapes.
Fig. 1. A badly gaping fillet compared with a normal one.
Fillets sometimes gape when they are taken from fish that have been stowed in ice, particularly when the fish are rather stale, but the problem is more serious when whole fish are frozen, at sea on freezer trawlers for example; fillets taken from the thawed whole fish can gape very badly indeed. When the fillets are still on the bone, the connective tissue is under some tension, so that the extra mechanical strain of freezing is enough to break it and force the flakes apart; this does not happen if the connective tissue is relaxed in a cut fillet.
Thus fish will sometimes gape if they are frozen whole for thawing and filleting later. However, they do not gape every time and, with reasonable care during handling and processing, the incidence of gaping can be greatly reduced.
Handling
An hour or two after catching, the fish become stiff in rigor because the muscles pull against each other. This puts the connective tissue under some strain and, when a fish that has stiffened in a bent position is wrenched straight before it is put into the freezer, the connective tissue tears and causes gaping. A fuller account of the effects of rigor is given in Advisory Note 36.
Similarly, any rough handling of the fish, for example by throwing them against the washer or standing on them as they lie in the pound, can cause damage that may result in gaping.
Time after death
When whole fish are frozen before the onset of rigor, fillets cut from them after thawing are least likely to gape. When whole fish are frozen during rigor, even though they are handled gently, the fillets will gape to some extent, and when the fish are frozen after they have passed through rigor, fillets taken from them will gape still more; the staler the fish are before freezing, the more the fillets will gape.
Temperature
The connective tissue of newly caught fish is very sensitive to small rises in temperature; it is weakened after only 4 hours at 17°C, the temperature of a mild day. At 20°C it weakens more quickly, and fish kept at 25°C before freezing will almost always gape severely.
When a newly killed fish is warmed by lying on a warm deck for half an hour, it quickly stiffens in rigor, and the strong muscle contraction breaks the weakened connective tissue, so that at 20°C or more severe gaping results from the fish going into rigor even before it is frozen; freezing and thawing make matters worse still.
When the fish are warm, any handling such as gutting, washing or moving them about almost invariably results in severe gaping. However, when warm fish are cooled again in ice or cold sea water, the connective tissue recovers most of its strength. Provided the warm fish did not go into rigor, rapid chilling will enable the fish to be handled in the usual way without increasing the risk of gaping. If the temperature reaches 30°C, as it quickly can on a warm deck, then subsequent cooling will not help much; the connective tissue will have been damaged too badly.
Size
For fish in the same biological condition, and given the same treatment on board, small ones gape more than big ones; the explanation is not known for certain, but probably the thicker connective tissue of large fish gives them a structural advantage.
Condition
Fish in good condition are smooth and firm to the touch, and kick furiously about the deck after capture, whereas fish in poor condition are lean, perhaps loose skinned, and lifeless; the cut surface of a fillet from a fish in bad condition feels watery.
Fish lose condition at the spawning season. Since most white fish species spawn in the spring and thus make their eggs or sperm in winter when food is scarce, they rob their own body resources and so cause the fillets to become soft and watery. This softness does not cause gaping; watery fish often have a completely smooth fillet surface, because the connective tissue becomes stronger at the spawning period. However, there is a change in the chemistry of the muscle after spawning, when the fish begin to feed heavily again at the beginning of the summer, and the fish are then very liable to gape; in North Sea cod this occurs in late May or early June. This is the time, more than any other, when the fish must be kept cool and handled gently in order to keep gaping at a minimum.
Do all fish gape the same?
Round fish gape more than flatfish, and each species has its own characteristic degree of gaping; among the principal UK commercial species, haddock gapes by far the worst whereas plaice gapes very little. The following species are listed in decreasing order of liability to gape; haddock, cod, saithe, whiting, redfish, halibut, lemon sole, plaice. Some species never seem to gape at all, for example catfish, ling and skate.
Fig. 2. Variation of gaping in cod with season.
Freezing and cold storage conditions do not affect gaping to any marked extent The rate of freezing seems to make no difference, except that very slow freezing times, 3 days or more, do cause a slight increase. Length of time in cold storage has no effect on gaping, and neither docs the method of thawing, unless the fish temperature is higher than the recommended maximum of 18°C.
The following advice applies mainly to round while fish frozen whole, since flatfish apart from halibut gape very little, and fish frozen as fillets are not a problem in this respect. Good handling practice above and below deck will help to reduce the incidence of gaping.
Chill the fish as soon as they are caught, and handle them gently at all times. Any fish that have warmed should be cooled in cold sea water or in ice before being further handled.
Freeze fish as quickly as possible after death; fish frozen before the onset of rigor will gape least of all. This is especially important with haddock, which are difficult to freeze without causing some gaping.
When the catch is too big for all of it to go into the freezer before rigor, freeze the smaller fish first in summer and autumn; size is not a relevant factor in winter and spring.
Do not force bent fish straight again while they are still stiff.
Do not thaw frozen whole fish at a temperature higher than 18°C.
Treat fish that are feeding after spawning with particular care; some gaping is almost inevitable.
Fish caught at or just before spawning time are least likely to gape.
When fish have to be filleted before they are completely
thawed, cut them very carefully and never use a blunt knife.
